Clutch Slave Cylinder disassembly - Ford Bronco Forum
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post #1 of 1 (permalink) Old 02-28-2017, 01:54 AM Thread Starter
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Join Date: May 2007
Location: Colorado
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Bronco Info: 95 XL 302-M5R2-1356-3L55-31's
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Clutch Slave Cylinder disassembly

Last year around this time I did a clutch job on the Bronco. Rear seal, clutch, pressure plate, clutch release bearing, slave cylinder (and master). Unfortunately, the slave cylinder didn't last so I had to do the job over again. What follows is not an instructional about taking it out, but instead the disassembly and a theory about why the part failed in the first place.

This is the reason for the wonderful time I spent on Saturday. What is depicted here is the cheap, Chinese piece of cr@p clutch slave cylinder that I replaced almost exactly a year ago. It was a slave cylinder from O'Reilley Auto parts. Can't remember whether it was the PowerTorque or the BrakeBest. I believe it was the PowerTorque.


Because I was in a hurry, I left the transfer case attached to the transmission for removal. Because its heavy and hard to stab back in place, I don't recommend this method. If you ate your can of spinach, feel free. The globs in the lower part of the picture are clumps of slush that continued to melt off of the truck as I was working on it. Lovely.


What is depicted here is the new slave cylinder placed in the bellhousing of the transmission. Notice that the old slave cylinder leaked fluid all over the inside of the bellhousing. Its hard to tell, but there was literally a puddle of Prestone Dot3 brake fluid in the bottom.


I've cleaned up the old slave cylinder a bit for disassembly. I'm holding it together, as the snap ring that normally does that job got lost.


Depicted are the constituent parts of the clutch slave cylinder. I'm not sure about the proper terminology for the parts, but from left to right we see the following:
Clutch release bearing
Bearing retainer/piston "follower"
Upper spring retainer
Dust seal
Piston seal


To remove the piston seal, I used a syringe with some mineral oil in it to force the piston seal out using hydraulic pressure. Despite the lack of spring pressure, the oil poured out from the seal when using the syringe. Fluid loss was worse with the Dot 3 brake fluid.

Here's the funk that came out with it. Keep in mind that both the clutch slave cylinder and the clutch master cylinder were replaced about a year ago. I also bled the system until clean fluid came out when I replaced the set.


This is the piston. Well, its less of a piston than an extension of the piston. I'm going to call it a "piston follower". The actual piston is a rubber ring. This piece sits on top of the rubber ring and transfers force to the clutch release bearing, which in turn presses against the fingers in the pressure plate. The bottom portion rests against the rubber piston seal, and the whole assembly is pushed out using hydraulic pressure when you step on the clutch. Its hard to tell from these pictures, but notice that the bottom portion is not smooth, and instead is covered with burrs. These burrs are solid, and are present around the entire base. I've cut many pieces of plastic pipe in my day, and if I had to guess, these burrs are from the (lack of) machining process during construction. The burrs are fused to the unit, which makes me assume that the factory uses (inadequate) heat to smooth the surface after cutting.


Again, its hard to tell from the pictures, but one can see the texture of the bottom portion of this piece.


One of the burrs can be plainly seen in the upper "corner" of the piece. Lack of proper surface preparation and poor quality control at the factory. These burrs have caused gouges in the seal, which is in the next image.


This is the piston seal. The outer surface is supposed to provide a seal on the outer part of the housing, and the inner surface is supposed to provide a seal on the inner housing surface. As fluid pressure builds up behind this seal (stepping on the clutch), the seal is forced out and consequently moves the rest of the assembly out. The part with all the burrs on it rests on the flat surface on top.


The seal is quite flexible.


Peering down between the outer housing and inner housing. This is where the seal is installed, and it moves back and forth (up and down in this pic) inside this gap. Notice all the funk at the bottom. I can only imagine that this stuff was originally part of the seal. My assumption here is that a lack of chemical resistance to the fluid caused a breakdown of the seal.

Additionally, I'm not so sure that the outer and inner parts of the housing, which are made out of some sort of plastic, are strong enough to resist flexing when pressure is applied.


As one can see in this picture, the burrs have cut into parts of this piston/seal. I also removed quite a bit of what I can only describe as dissolved rubber during cleaning. Remember that this seal handles all of the force applied when the clutch is depressed.

I think that by design this seal is intended to expand (get fatter) as pressure is applied, which again is designed to provide a better seal. Hydraulic pressure is applied from one side of it, and pressure from the spring mechanism is applied from the other direction by the finger springs in the pressure plate.

My theory for the failure of this part is as follows:
Chemical deterioration of this seal, coupled with the gouges caused by the burrs in the "piston follower", coupled with flexing of the inner and outer housing caused fluid to get past the seal. As the chemical deterioration progressed, fluid continued to gain the ability to get past the seal at the weakest point, which were the gouges.

Bottom line is that I'll never buy a cheap Chinese part again, especially when it takes so much labor to replace.
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