jmdirk
Well-Known Member
- Joined
- July 22, 2004
- Messages
- 238
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- City, State
- Ottawa, Canada
- Year, Model & Trim Level
- '93
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:
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)
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"
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:
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.
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.
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.
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:
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)
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"
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:
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.
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.
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.