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Hydraulic Clutch keeps getting air in it

oharris

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Having trouble with a hydraulic clutch on a '94 Explorer Sport. The throwout bearing went out so I pulled the trans and replaced the clutch and slave cylinder. Put it all back together, bled it and it seemed to work fine. Couple days later, it wasn't releasing, so I bled it again. Again it seemed fine, but again it started not releasing. I decided to replace the master cylinder, bled it again, and it seemed good once again. A few days later, it's still not releasing again! Where could the air be coming from?
 
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410Fortune

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They O ring that goes from the line into the master cylinder, did you replace that?
Also these master cylinders are known to trap air in them, and the small bubbles of air can collect into a larger bubble of air right at the top of the master cylinder. It can be VERY difficult to get this bubble out.
You can try to bleed all day long, or you can simply remove the C clip in the top of the master cylinder (Inside the truck under the pedal) then pull the master cylinder plunger out of its bore VERY CAREFULLY until you see fluid and then put it back together. This will remove ALL OF THE AIR

Also what brand of slave cylinder you use matters a great deal here, some of the cheaper auto store ones (Power Torque and the like) do not last!!! I would never install one again
Let me know if you want more info on the bleeding technique if you think it will help your situation
 
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oharris

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Thanks for the reply. I tried your trick and it didn't really work. I ended up taking it out and bleeding it like this. I did forget to replace the o ring, but this time I got it. How much should I be able to push in the push rod? With the line disconnected I have a little less than 1/8" play, does that sound right?

20180926_194809.jpg
 
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massacre

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That may be the coolest bench bleeding setup I have seen
 
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oharris

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410Fortune

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yes 1/8" sounds about right

If you did "my trick" there would be no air in the master cylinder at all.
Then you just have to bleed the slave and the line in the truck only takes a few cycles of the pedal and bleeder

I like your setup however its setup incorrectly in the picture
The master cyl needs to be above the hyd line and the fluid reservoir at the top of the system.
The hydraulic line/slave fitting needs to be at the bottom, trapped air needs to make its way up the line through the master and into the reservoir.

I have fought these over the years, I have bled 15-20 of these systems easily if not more.
Now days it takes me 10 minutes to bleed a NEW (dry) master, line and slave
You can tap on the line, press the plunger and fight air bubbles for an hour, or you can setup the system, and bench bleed the master in 5 minutes by pulling the plunger until fluid reaches the top. There is no physical way air can still be in there if you do it this way.
It is not my trick, I actually learned it online years ago and am just trying to spread the word to help save people from crushing trucks because they cannot get the clutch to work properly due to a pea size air bubble

here you go:
 
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oharris

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yes 1/8" sounds about right

410, thanks again for the reply, I appreciate you talking this over with me. Is that 1/8" of play still about right for when the slave is connected or will it be a bit more?

I understand the theory, and I pretty much did exactly what the video showed and it didn't help. That tells me the air was not in the master, but in the slave or possibly in the line, would you agree? Any tips on the best way to get all the air out of the slave? After taking the plunger out I tried pumping the pedal up and opening the bleeder screw several times without much success.

As for my bench bleeding setup, the way I was doing it was to open the fitting at the end by pushing in the spring loaded stopper while pressing the push rod. My theory was that by holding the line as vertically as I could, any air would work it's way towards the fitting at the top moving in the same direction as I was pushing fluid. It seemed to work quite well. As for setting it up with the reservoir at the top and the line down, how does the air make it's way through the check valve? I understand that the valve should open up with no pressure on the pushrod, but it seems like you're just hoping the air finds its way out rather than forcing it out. Am I missing something?
 
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410Fortune

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Air was likely in the line because it was not at the bottom.

Once the master and the line are bench bled it should seriously take about 5 bleeds on the slave (usually 3 does it) and you are done, there is not much air in a slave cylinder to begin with
I have my helper pump the clutch pedal SLOWLY 5 times, on the 5th press I open the bleeder SLIGHTLY and let the last 3-4" of his foot/pedal travel force all the air and then fluid out.
The next time I wait for helper to press clutch 5 times, hold pedal, then open bleeder. Repeat this 4 more times = done

This is the proper way to bench bleed the master cyl and hydraulic line:
It can take FOREVER

Seriously we have spent 45 minutes to an hour doing this method, its a pita but it does work., You can watch those little air bubbles come out for a long time and its really hard to depress the master cyl plunger by hand
Now I just set it all up just like in the video, above except I just open the master cyl and pull the plunger until brake fluid comes out all the way to the top (Split second)
Then press the plunger back in, put master back together and done
For me this has worked every time, new parts right out of the box shipped dry.

If you have air trapped in the line you need to get the master above the line, reservoir above that. Now wrap on the line with screwdriver to release the bubbles.You can watch the air make its way up and out of the fluid in the reservoir, some bubbles are the size of a needle point and some are size of a green pea.

You do not need to open the quick release/spring loaded fitting on the end of the hydraulic line. If you do, it should be in a jar of fluid so no air can go back up inside.
The hydraulic line will be fully purged of air when you hook it to the slave and bleed the slave. Getting all of the air out of the master cylinder before it is installed is the key here, once installed that air bubble is trapped.

A leaky slave cylinder is usually the problem child with these systems, or a bad seal in the master. People get in deep when they try to re use an old slave or TO bearing, or keep the same master and add a new slave, or again when they use crappy defective parts right out of the box.
Never use a cheap slave! I know I said this before but I have dropped a trans, installed a customer supplied slave and weeks later we are doing it again.
This happened a couple of times and now I will no longer even mess with cheap auto parts store crap here.
Use Ford or Luk (OEM).
Also if I am going to drop a trans I am going to install a new clutch (pilot, driven disc, pressure plate, TO bearing), new slave and new master pretty much every time....
 
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oharris

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Thank you so much! I will give that all a try when i get home tonight.
 
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oharris

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I followed your tips and got it bleed last night. Hopefully it stays vlad this time. Thanks again for the help!
 
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Centaurious

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I had to pull the master cylinder and line out of my explorer and fasten it to a board with the line stretched down and the master cylinder tilted so the reservoir line was at the top. After tapping on the line with a screwdriver handle for about 30 min it worked all the air up and into the reservoir. Then the piston had almost no play.

A lot of aftermarket slave cylinders have the bleed line exit from the top and loop down to the side of the transmission, causing an air pocket. The factory style units have the line exit just above the inlet line and rise up to the side of the transmission, a much better design.
 
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