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Brake Master Cylinder Differences Explained (with Pictures and Math!)

Discussion in 'Under the Hood' started by jmdirk, March 16, 2015.

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  1. jmdirk

    jmdirk Active Member

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    I've been working on a SAS for my 1st Gen including swapping in a disc brake rear axle. There's all sorts of info on the board about whether you can retain the 1st Gen master cylinder, or if you need to use the 2nd Gen master. there's not really a consensus about what should be done.

    For my swap, I'm using a HP44 from a 79 Bronco with '79 T-bird calipers. since there's nothing stock left in my brake system, I decided to do a bunch of research including dismantling my stock 1993 master and a 1999 Ex Sport master. In this first post I'm going to address only the swap from drums to discs. A second post will cover the front brakes.

    First of all, both master cylinders are a dual or tandem type. This means that both front and rear circuit are along the same axis in a single master cylinder bore. A typical exploded view of a master cylinder is shown below:
    [​IMG]

    There are primary and secondary pistons, and as a result you also have primary and secondary chambers. The critical details of the master cylinders are the diameter (or bore) and the stroke. The bore determines the amount of pressure multiplication, the stroke determines the amount of volume the master cylinder is capable of moving.

    A couple of things we know for sure:
    - Discs require more line pressure to operate than drums as there is no mechanical advantage like drums have where the shoes pivot. And drums are also self energizing (one they start to engage, the rotation of the drums actually helps to keep the shoe engaged on the drum)
    - Discs also require more volume than drums

    The first picture is the internals of both my master cylinders. The top is the 2nd Gen (1999), the bottom is the 1st Gen (1993)
    [​IMG]

    A couple of things to note about the differences. The secondary chambers are on the left in the area the springs occupy. The primary chambers are on the right, again in the area the springs occupy. It should be noted that the springs, the stops (the shafts extending into the chambers) all reduce the actual volume of the chamber. This eventually becomes relevant.

    There are no internal metering valve, residual valve or brake proportioning valve inside either of these. the 1st Gens have all this inside either the 4WABS or RABS control HCUs. The second Gen (at least my master) had a proportioning valve threaded onto the rear brake outlet (primary) of the master. Not all of the 2nd Gens are like this, some do not have the proportional valve.

    Next up is a comparison of the diameter of the pistons. 2nd Gen on the left, 1st Gen on the right. The thing to note here is the difference in diameter. The 1st Gen is 1.000" diameter and the 2nd Gen is 1.050"
    [​IMG]

    So what does this mean for the operation of the brakes? Using a standard formula of Pressure = Force / Area we can calculate the brake lines. this is neglecting any reduction in pressure you will see from expansion of the flexible brake hoses etc. This is also assuming that the pads have engaged the rotors and the caliper piston can extend no further. The Input Pressure (which I realize now I've named incorrectly - it should be Input Force) would be the amount of force applied by the push rod of the brake booster. The results are below:
    [​IMG]

    You can see to start that the 1st Gen master actually results in a higher line pressure than the 2nd Gen. What does this mean for swapping a rear disc axle into a 1st Gen?

    Absolutely Nothing!! Recall that the brake proportioning is done by a external module. So the pressure created by the master never reaches the rear axle The proportioning is the critical part for rear brake operation. You should use the proportioning mechanism that matches the donor rear axle. A drum proportioning valve is going to start limiting at a much lower pressure than one for a disc brake axle.

    If you try and use the proportioning valve from a drum brake axle for discs brakes, you're not going to get enough pressure to the calipers for them to operate effectively - which defeats the purpose of installing a disc brake axle to begin with. So again, you need the proper proportioning valve to match your donor axle.

    So that covers the pressure needed. The 1st Gen master actually can create more pressure than the 2nd Gen. Though, this doesn't gain you any advantage over the 2nd Gen for the rear brakes, since the proportioning valve limits the pressure to the rear.

    But what about the extra volume needed? For this we need to consider the relative volume of the master cylinder chambers. the following picture compares the pistons inside the master for the rear brakes. The bottom one is the 1st Gen, top is the 2nd. Initially, it looks like the 1st Gen has the larger chamber of the two since it appears longer. What you need to consider is the actual stroke for that chamber. See that piece on the left inside the spring? That's a stop that's going to limit the stroke of the piston. Those stops are there so that if one of your brake circuits fails, the other circuit can still operate.

    [​IMG]

    Based on simple calculations of the volume of a cylinder you get the following results. Note that I'm ignoring the volume around the stops as that is constant regardless of the stroke. I'm also ignoring the volume occupied by the springs. In the case of the 2ng Gen, the spring for the primary piston sits is a large cup on the secondary piston, this part is actually the limit of the travel for this master.
    [​IMG]

    You can see that even though the 2nd Gen has a shorter stroke, it has a larger bore, but in total, the volumes are very similar.

    So as it pertains to swapping a rear disc into a drum equipped truck, here's my conclusions:
    - It's perfectly okay to use either the 1st Gen or 2nd Gen master cylinder. You will actually get slightly better braking performance from the 1st Gen - but it will be at the expense of pedal travel (you will have to press the pedal about 5% further)
    - You must use either the proportioning valve from the donor vehicle, or you could install an aftermarket proportioning valve.
    - Since the proportioning is done by the ABS systems in the 1st gens, you cannot swap to discs and retain ABS while maintaining optimal operation of the rear discs. The only way yo get the best of both worlds would be to swap over the entire ABS system (HCU, controller etc.)

    If your original equipment is a 91-92 or any truck with RABS only, you could easily use the master cylinder I have with the proportioning valve installed and bypass your stock RABS module.

    On 93-94 and all trucks with 4AWBS, you can either bypass the entire ABS system, or you could try bypassing only the circuit for the rear wheels - though I expect this would cause also sorts of trouble codes in the ABS system itself.
     
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  3. jmdirk

    jmdirk Active Member

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    Part 2 - Front Brakes

    The second part of this has to do with the compatibility of the master cylinder to the front brakes.

    I'll start with comparing the differences in the 1st Gen and Second Gen master cylinder as hos it relates to the operation of the front brakes. Then I'll go over the details for my specific setup on my SAS.

    In terms of line pressures, since the bore of the master cylinder is consistent, we are going to have the same line pressure for the front as calculated for the rears. However, in this case, since there's no proportioning valve interfering with how much pressure reaches the front calipers, we can now calculate how much force the respective calipers are able to exert on the rotors. to do this, we use the same calculation, but have to rearrange the variable a bit:

    Pressure = Force / Area

    Since we know the line pressure (the we calculated before), and the area (based on the diameter of the caliper pistons) we can then rearrange the formula to calculate the clamping force on the rotor. the equation becomes:

    Force = Pressure * Area

    One very important thing is the diameter of the calipers as this determines how much force is put on the pads. The 1st gen calipers are a single piston 2.625" diameter. The 2ng Gen are dual piston calipers with each piston having a diameter of 1.811" But since they are dual piston, we have to use the total area of both pistons in the calculation.

    [​IMG]

    You can see that the 2nd Gen calipers have a slightly larger surface area, but due to the larger bore master cylinder resulting in a lower line pressure, the 2nd Gen calipers exert less force on the rotors than the first gen. Now there are advantages to dual/multi piston calipers that this analysis doesn't address such as more even distribution of force on the pads/rotors etc.

    The Alternate Front Caliper Pressure line shows the difference if you used a 1st Gen master on a 2nd Gen and vice versa. You can see the highest pressures are achieved by using a 1st Gen master on a 2nd Gen.

    The other part that needs to be considered is volume. you don;t get this extra force for nothing. the caliper pistons have to move in order to engage the rotors. In order for them to move, some fluid has to be displaced. Larger diameter calipers (such as the 2nd Gen) require more fluid to move the same distance compared to a small diameter caliper. this fluid originates in the master cylinder. The smaller diameter master cylinder in the 1st Gen, provides more pressure, but has to move further than a larger diameter master. Which means the pedal has to travel further before the brakes engage. This can have a very negative effect on pedal feel.

    In our case, you can see the difference in the secondary chambers of the two master cylinders here. The top is the 2nd Gen and the bottom is the 1st Gen. The 2nd Gen has a much longer stroke than the 1st Gen, ~0.5"
    [​IMG]

    The danger in having a master cylinder that is too small is that you could run out of pedal travel before the calipers have a change to fully engage on the rotors. The calculations of overall volume are shown below

    [​IMG]

    you can see that with the larger bore master, and the longer stroke the 2nd Gen master has a much higher volume. Now the big question is, does this really matter? Consider that the caliper piston only has to move far enough from the resting position to be fully seat the pad on the rotor. It's a very short distance. Once seated, force is transferred through the system, but fluid is no longer flowing.

    So the amount of fluid displaced by the master per inch of travel is based on the diameter of the bore. Here we can just use the ratios of the master cylinder bores to determine how much more (or less) pedal travel is required. Since the 2nd Gen master 10.25% (based on area, NOT diameter) larger than the 1st Gen, it would require 10.25 less pedal travel than the 1st Gen. Conversely, the 1st Gen master requires about 10% more pedal travel than the 2nd Gen.

    Using a 2nd Gen master on a 1st Gen would work, there's certainly enough volume, though the pedal throw will be shorter making the brakes feel firmer and more responsive. This is probably a contributing factor to the sensation of improved braking when swapping from to rear discs along with the 2nd Gen master. I've never seen anyone actually measure their stopping distance after a rear disc swap and I'd be curious to see the results. the rear discs will provide more stopping power certainly, but it's the fronts that do most of the work. Using a 2nd Gen master will make the brakes feel more responsive, but the lower pressure will reduce the effectiveness of the front brakes, possibly resulting in a decrease in overall stopping distance.

    We've already seen that the 1st Gen master provides higher pressure, but can it push enough volume to operate a 2nd Gens front brakes given the smaller diameter bore and the larger diameter pistons areas? To try and figure that out, I've calculated the maximum linear displacement of the caliper pistons based on a maximum stroke of the master cylinder
    [​IMG]

    Again, Alternate is using 1st Gen with 2nd Gen etc. with the first column being a 1st Gen Master with a 2nd Gen calipers.

    A stock 2nd Gen would in theory be able to extend its caliper pistons up to 0.26" Switching to a 1st Gen master on a 2nd Gen would reduce the maximum linear displacement down to 0.170" That's a big drop compared to stock, but compared to stock 1st Gen numbers, it's a fairly small decrease, only about 0.010"

    So to summarize the detail about whether it's better to use a 1st or 2nd Gen master when doing a rear disc swap. My conclusion is that using the 1st Gen master provides more pressure than the 2nd Gen one. And has enough volume to operate the rear calipers. You must use the correct proportioning valve however. Based on this my recommendations are:
    Best option: 1st Gen master cylinder with an after market adjustable proportioning valve. this give you the benefits of the higher line pressure over a 2nd Gen master and allows you to tune your brakes for optimal performance.
    Better option: Use a 2nd Gen master cylinder with the proportioning valve. this will give you the right brake balance and a firmer pedal feel. Though the actual clamping force on the front brakes will be reduced

    This kind of opened up a alternative idea and that's whether a 1st Gen master could be considered an upgrade for a 2nd Gen. this will depend on the type of master your 2nd Gen has. Some have the proportioning valve built in, other don't. Some have a switch for the cruise control, etc. In the 2nd Gen I had, since it had the cruise control switch built in, you would have to find a alternate location to pick up the brake signal, likely the brake pedal itself would work. I estimate that there is enough volume in the 1st Gen master to operate the front brakes, but the pedal travel will be longer so the brakes would be feel less responsive. With the right proportioning I think it could work. the longer pedal throw may make uncomfortable for some drivers, others may not notice that much of a difference.
     
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  4. jmdirk

    jmdirk Active Member

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    Part 3 - SAS brakes

    I said I'd cover this in part two, but didn't want the post to get too lengthy. So I'll cover it here.

    Taking all this stuff into account for my SAS which is a HP D44 from a 79 Bronco. I wanted to know if either one of the master cylinders that I had was suitable for the brakes on the D44. I had to consider the that I was also doing a rear disc swap at the same time. So basically, the stock 1st Gen master cylinder was not intended for use with either the front or rear brakes that were going to be in the truck.

    to complicate things, I'm not using the stock calipers from a 79 Bronco. The calipers from a 79 T-bird are a direct mechanical swap to the D44 with the exception of the flex line attachment. Some custom brake lines and 7/16" banjo bolts and you're good to go. The reason for using the T-bird calipers is that they have huge 3.125" caliper pistons compared to the 2.903" ones in Bronco.

    I've actually done the swap, plumbed all the brakes, bled the system several times and while the brakes worked, they didn't feel great. that led me to question if the master was up to the task of all these new brakes. Based on the info above, the 1st Gen master can work with rear disc brakes if biased properly. But is it enough to run the fronts? Or is the 2nd Gen MC that I have a better option, or alternatively again is a master from an F-250 with it's 1-1/8" bore better?

    Rather than going through all the details again, I'll just show the results. To get a feeling for the difference in pedal feel etc, I also calculated how much stroke would be required in the MC to achieve 1/16" of linear displacement of the calipers. 1/6" was an arbitrary number and only to be used to compared the various options together.

    [​IMG]

    As you can see, the 1st Gen with the T-bird calipers makes great pressure at the calipers, but at the expense of maximum linear displacement and pedal feel. with the 1st Gen caliper in the truck, the brakes worked when moving around the yard, but you almost had to put you foot to the floor. This is just due to how much more fluid you had to push out of the master to get those big 3.125" pistons to seat the pads on the rotors. As a result, I would only have about 1/8" of caliper travel before I bottom out the master cylinder. Also, the amount of MC stroke need is about 42% higher than a stock configuration. It might be enough, but certainly didn't feel very good. It should be noted that the stock T-bird, also has a 1.0" MC. The difference is likely that there's a shorter pivot on the pedal itself, making it feel like a shorter throw.

    Using the 2nd Gen MC, I would sacrifice a bit of line pressure, but the volume comes back up, but the the pedal is still going to feel quite a bit softer than a stock.

    Using a F250 MC (I chose this because apparently the 1st Gen Ex and the 89 F250 use the same booster - so this would be a direct fit), the pressure has dropped even further, but it's still higher than a stock 1st Gen setup. Furthermore, the pedal feel will now be very similar to a stock 1st Gen.

    To this point I'm still not sure which route I'm going to go as there is also the booster and pedal ratio to account for as well. There are a lot of tradeoffs to consider. I could start by thinking about modifying the brake pedal to change the ratio which would make the feel better with a shorter throw, but would require more effort to get the same force on the master.

    Sorry for the long posts, I just thought some of this analysis could help other as they go about different brake and axle swaps
     
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  5. BKennedy

    BKennedy Elite Loser Staff Member Moderator Elite Explorer

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    I wish you had posted all this a year ago when I was doing my SAS. I have the same front brake calipers as yours, and 99 rear discs. I have had brake issues since the swap. I tried the 94 master cylinder and could not get good pedal feel, tried a 99 and it was a little better, but still poor. 99 with an in line rear proportioning valve was better, but still not as good as those big front brakes should be. The 99 master started leaking, so I got another one and the brake line fittings are reversed (larger diameter fitting on rear instead of front).

    Does this mean the primary piston is used by the front brakes, or does it just mean the fittings need to be reversed on the lines? Or, does it matter since they should be making equal pressure to each line?

    Also, a bunch of others have done First Gen SAS's and installed a 99 master cylinder. Its supposed to be the best fit for someone who does not want to change out the booster.
     
  6. 5spdman

    5spdman Elite Explorer

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    Great write-up! While I haven't gone SAS, I did go with the axle swap/disc brake upgrade, and haven't had really good brake pressure since.

    I'll be reading this again after work, but it looks like you mentioned that in order for the rear calipers to get the volume needed, a proportioning valve should be added? Where would be the best place to get one online? Do you have part #'s?
     
  7. jmdirk

    jmdirk Active Member

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    Pressure wise, it doesn't really matter whether you use the primary or the secondary as long as you have the proportioning valve on the rear. For the volume, I would put the piston with the longer stroke to the front. In both cases that means the Primary is for the rears and the secondary is for the fronts. So the most rearward port goes to the rear etc.
     
  8. jmdirk

    jmdirk Active Member

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    For the rears the bigger thing is the pressure. The 1st Gen MC has enough volume, but the proportioning valve built in to the RABS module would reduce the pressure too much.

    I bought a Wilwood lever actuated valve (260-8420) which I will mount in the cab. this way I can find the setting that works best for the street and then easily switch between that setting and allowing full pressure to the rears for off road use.

    There's a number of different models available that you adjust by turning a knob. Wilwood, Titton, SSBC are all reputable companies.

    Again, all this is assuming that you're bypassing the factory ABS HCUs which contain the factory proportioning valves. You will lose ABS
     
  9. jmdirk

    jmdirk Active Member

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    BTW, the next step for me is a F250 booster and MC. I intend to do a detailed on the how and why
     
  10. BKennedy

    BKennedy Elite Loser Staff Member Moderator Elite Explorer

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    Thanks.

    I have deleted the ABS stuff, and installed a rear in-line proportioning valve. I think I had a bad MC to start with. The pressure in the new same year MC is much better.

    For now I have:
    76 Thunderbird front brakes
    99 rear brakes
    99 master cylinder
    Summit Racing rear proportioning valve.

    I will be waiting to hear about the F250 MC.
     
  11. jmdirk

    jmdirk Active Member

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    Basically I ran through some of the numbers based on the original T-bird MC and booster. The T-bird has the same diameter MC as the 1st Gen Ex, but it was a 11.75: single diaphragm booster. Old cars always tended to have spongy feeling brakes.

    Our 8.75 dual diaphragm wouldn't be able to create the same pressure as the original T-bird.

    By going to the F250 MC 1-1/8" MC should make the pedal feel nice and firm. And the matching 10.75" dual diaphragm booster should get the line pressure right up to the same as the original t-bird. My calculation come out to ~1700 psi for 100 lbs of force put on the pedal.

    I can test fit the F250 MC on the Explorer booster, but I don;t think I would recommend that as there's too much pressure lost with the larger MC. Getting the F250 booster to fir might be a bit of a challenge - it's huge. Looks like I might have some interference issues with the wiper motor.

    I'm hoping to at least get it mounted up this weekend
     
  12. BKennedy

    BKennedy Elite Loser Staff Member Moderator Elite Explorer

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    That sentence killed it for me. I have coil over hoops that barely let me get the Explorer booster in and out.
     
  13. jmdirk

    jmdirk Active Member

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    You could still possibly use the F250 MC with the stock booster. The pedal would feel very firm, but to over come this you can play with the pedal ratio to increase your mechanical advantage. Stock is ~4:1 by my measurements.

    12" overall length and 3" from the pivot to where the pushrod connects. If you modified the pedal to move where the pushrod connects up by 0.5" and then lengthened the pedal overall by 2" you would recover the line pressure lost by using the larger MC
     
  14. BKennedy

    BKennedy Elite Loser Staff Member Moderator Elite Explorer

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    Proportioning valve question: Does the pressure created by the pistons remain equal with the line pressure restricted on one piston by a proportioning valve?
     
  15. jmdirk

    jmdirk Active Member

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    Yes, since the pistons are equal diameter, they both create the same pressure. The pressure in the front and rear lines are the same right up until to the inlet of the proportioning valve. And at low pressures, up to the cracking point of the proportioning valve, they are equal all the way through.

    Once the cracking pressure of the proportioning valve is reached, the pressure past the valve increase at a much lower rate than the input line pressure.

    A graph of inlet vs outlet pressure will useually look like this:
    [​IMG]
     
  16. BKennedy

    BKennedy Elite Loser Staff Member Moderator Elite Explorer

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    Thanks
    I changed out the 99 MC for a better quality unit. The pedal pressure is much better. I can feel the MC is struggling to push the T-Bird calipers upon hard breaking applications, but it stops well. The pedal can't be pushed to the floor with the engine running like before.

    Would a Wilwood Tandem MC work? Some of them have a 2:1 ratio for primary and secondary pistons, which might work for T-Bird calipers.
     
    Last edited: April 3, 2015
  17. BKennedy

    BKennedy Elite Loser Staff Member Moderator Elite Explorer

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    Back to master cylinders. I have at the house, a 99 Explorer MC, and a 94 F-150 MC. Both have the same two-bolt mounting and bolt hole width:

    Here is the break down

    99 Explorer MC:
    Cylinder Bore: 1.063" wide, 7-5/8" depth
    Outer cylinder mounting surface: 1.571

    94 F-150 MC:
    Cylinder Bore: 1.056" wide, 8-5/16" depth
    Outer cylinder mounting surface: 1.471

    They are nearly the same width, but the F-150 MC has quite a bit more stroke. You can see in the picture, that it is taken up with the secondary piston (primary pistons are same length). 99 Explorer on the left, 94 F-150 on the right

    [​IMG]

    A recap on what I am working with:

    94 Explorer
    1976 F-150 HP Dana 44, narrowed to Early Bronco width
    T-Bird calipers with slotted/dimpled rotors
    99 Explorer rear disc brakes
    99 Explorer MC
    94 Explorer booster
    Everything is/was new at the time of the swap.

    I was looking on some other off road and car forums about soft pedal feel after doing a disc brake swap, or upgrade. Several threads made mention of gutting the proportioning valve attached to the MC for the rear drum brakes and it working out for them. I am limited to a booster no larger than OEM due to my coil over hoops and the simple fact that I don't want to redo them. Winter (a engineer type genius guy who used to come out with us on runs before he got married, etc..) told me many years ago that the F-150 MC would help with soft pedal feel with a SAS.

    Do you think the longer stroke of the F-150 MC will help with pedal firmness? And, I know it will fit onto my 94 Explorer booster, but will the pedal work with the longer stroke?
     
  18. jmdirk

    jmdirk Active Member

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    The lack of firmness is a result of the larger diameter calipers. In order to move the caliper piston the same distance, you need to move more fluid. Which means you need to push the pedal (and ultimately the MC piston) further. Or, you need to increase the volume of fluid that the MC displaces per inch inch of stroke.

    Based on the specs you have for that '94, that's a smaller diameter bore than the '99, which means while you'll have more pressure in the line, you'll have to push the pedal even further which would give you even worse pedal feel.

    The stroke of the MC really only matters to make sure you can displace enough fluid, but it doesn't affect the performance or feeling of the brakes.

    In summary, it's not how long it is, it's the width that matters :)

    There's two ways you can improve pedal feel. Go to a larger diameter MC, but you'll lose line pressure. So while the brakes may feel better, you'll lose performance. Or you can reduce the mechanical advantage of the pedal itself by shortening it. The trade off here is that you now need to push harder to get the same pressure in the line. I don't think it's a big deal since I think most people have enough leg strength to engage the ABS under most circumstances anyway.

    Is it the diameter of the booster that interferes with your shock hoops? Is is it the depth of it? You could always consider a hydroboost system.

    The other thing you could look in to to offset the loss of pressure from a larger MC would be to use an electric vacuum pump which would increase the boost of your booster. I've seen a few that are supposed to be able to supply up to 30 inches of vacuum - which I think is more than what the stock engine vacuum is
     
  19. BKennedy

    BKennedy Elite Loser Staff Member Moderator Elite Explorer

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    Thanks for the reply. The MC I have in there now is for a 99 Explorer, but it has a 1-1/8" bore, which is larger than both of the MC's I have on the shelf. I thought the longer stroke of the F150 MC would push more fluid, helping the t-bird calipers work, but that's not the case.
     
    Last edited: November 10, 2015
  20. mounty71

    mounty71 It's green, not gray. Elite Explorer

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    Wow, great thread! I need to reread it to fully grasp all the info. I'm going through my brake setup now, 2000 Mountaineer with ABS removed, stock booster and master cylinder, and currently stock D44 Bronco calipers. One thing I'd like to clear up, I read that the REAR MC port is for the front brakes and FRONT MC port is for the rear brakes, but not until getting new lines made and eliminating the ABS module did I noticed that there's an F and R cast into the MC over the ports. I'm assuming that F denotes front and R denotes rear, but that contradicts what I read and how the brake lines are currently hooked up. Above it was stated that the primary and secondary create the same pressure on the 2nd gen master cylinder, so does that mean that how I have my brake lines hooked up currently is okay?
     
  21. jmdirk

    jmdirk Active Member

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    I think you should be fine. The stock units with ABS (as far as I know) have the pressure reduction in the ABS module itself. As long as there's no pressure reducer on the MC itself, it really shouldn't matter which port goes where. The pressure reducer often looks like a large cylinder thingy screwed into one of the ports.

    You'll still want a brake bias put in there otherwise you're likely to lock up the rear brakes fairly easily
     
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