Clutch Slave Cylinder Losing Pressure

I tried doing a clutch fluid flush a week ago and have been fighting to get the clutch working properly to engage and disengage for the past few days. At first I thought I had air inside of the system, and so I bled the system of air at least 5-6 times. Yesterday I took this video that shows my clutch slave cylinder losing pressure while the clutch pedal is depressed:

https://www.youtube.com/watch?v=skS10jAx8DM

From the 20 to 30 second mark you can clearly see the issue. The clutch master/slave cylinder are original and so they have 163k miles on them. Not opposed to buying a new set but I'm wondering why the slave wouldn't be holding pressure. Appreciate any insight.

 

Probably the clutch master cylinder is bypassing. did you bleed the air out of they system after you changed the fluid?

 

I have attempted to bleed the air out of the system no less than 5 times. The past two times I felt confident that I got all of the air out, but I didn't see any improvement in clutch engagement.

Can you explain what you mean with the "clutch master cylinder is bypassing"?

 

I'd say he means the fluid is getting by the seals in the cylinder, losing pressure.

 

I'd say your leak is in the master.
If a slave "leaks", fluid is getting by the piston seal. When fluid gets past the piston seal it will drip out the boot and be obvious.
Study the diagram below, ignore red arrow.

 

a master cylinder is just a piston inside a cylinder that pushes fluid when you push the clutch pedal. The fluid that is pushed by the master cylinder in turn pushes on a piston in the clutch cylinder causing it to extend thus extending the clutch rod. If the fluid leaks back around the master cylinder piston the clutch cylinder will contract because it does not have fluid to hold it out.

 

Was it working before you changed the fluid? If it was can you explain how you bleed the system after the fluid change?

 

Thanks for the clarification. I wasn't sure how to diagnose a leak in the master vs the slave, but if one has gone bad I'll be replacing both at the same time. This was all working properly before I put new fluid in. Any idea how I might have damaged the master cylinder piston?

[edit] My flush procedure was a gravity bleed. I cracked the slave bleeder and filled the reservoir as the fluid level dropped.

 

if it was working before hand, short of running the master cylinder totally dry, i doubt anything you did would cause the master to start bypassing. I still think you have air in the system. Did you bleed the system like you would a break system? Push the clutch in and hold it, crack the line going into the slave cylinder, close the line, then release the clutch pedal, repeat..... Air in a hydraulic line can sometimes take a while to get it removed.

 

That's how I've been bleeding them, and every time I do it I feel like more air has entered the system somehow. Makes me wonder if the slave bleeder valve (or something else) is letting air in.

Would air in the hydraulic lines lead to the master or slave not being able to maintain pressure? I've read that air in the lines can lead to a mushy pedal, but my pedal feels good. The system just isn't holding pressure.

 

What is the master cylinder level? if you allow it do get to low when bleeding you can reintroduce air back into the lines. Everything points to your master cylinder bypassing but things to normally just stop working. It was working fine, you changed the fluid now it doesn't, so i would have to go back to that. make sure all your lines are tight before bleeding and specially before you release the clutch pedal, if they are not it will suck air back into the lines.

 

I went to flush all my fluids about a month ago and when I did the clutch the master cylinder gave up when I started bleeding it, so I had to replace mine and went ahead and did the slave at the same time. I've been taking my time doing all this,,, old and slow, 72yrs old and hot as can be outside in Houston weather,, so I got back at it to do the coolant next, went to start the truck, now it won't start , crank but no start, so now I'm diagnosing that issue.
Moral of the story is, don't drag your feet like I did, good thing I'm re-tarded and don't have a job to have to go to.

 

I don't think I ever let the master run dry, I've been keeping a close eye on it. I do wonder if maybe I'm not tightening the slave bleeder all the way before I release the pedal. How many cycles of (pedal down, bleeder open, bleeder closed, pedal up) would you expect to have to do to bleed all of the air out?

 

I was trying to be proactive here, I saw dark and dirty clutch fluid and figured I was doing the right thing by replacing it. Now I can barely get my truck out of the driveway. If I knew it was going to be this much of a problem I would have left it alone.

 

My DD 02 has 460k miles and i have never changed the clutch fluid, so i really dont know, lol. But it should not take many <10 i would assume. But sometimes when you bleed a hydraulic system it helps to bleed it, pump the clutch quite a few times before you bleed it again. And then sometimes let it sit for 10 minutes or so before you do anything and then bleed again. If you get really small air bubbles in the system it will work but over time the little bubbles get together and make big bubbles and big in this case is not what you want...

 

I had similar symptoms and replaced the master. I took the old one out and the seals were pretty beat up. On another truck I used Toyota’s rebuild kit and it works great! If you opt for total replacement, get the Denso brand. Sounds like you’ve spent way too much time on this, better to just replace the part eventually.

One other thing- if you have the linkage at the pedal incorrectly set, where it’s pushing on the push rod slightly with pedal released, you will have problems with bleeding. The piston has to fully retract to let the hole between the reservoir and cylinder clear, which allows fluid into the system, especially when bleeding.

 

That's the smart play here. Keep it simple and refreshed.

Doubtful you did. But the rubber cup the piston rides in is likely worn (one way the fluid darkens, wearing rubber). The fresh fluid/bleeding may have just washed out enough debris to set off the internal leak (what a bypass really is)

I've changed / bled a few slaves/masters on my autox ride. In that marquee the slave tended to fail first. Pull back the boot and wet.

For the maintenance oriented, changing the clutch fluid on the same schedule as brake fluid (about every 2 years) is a good practice. And for the same reasons, as it's actually a very similar hydraulic system that uses the same type fluid.

 

Just wanted to update this thread with the outcome.

After my last update I had my neighbor help me bleed the clutch, and things worked well enough for a few days. Then this past weekend the clutch started acting up again, so I bought a new Aisin master and slave cylinder to rule out those parts. I installed them yesterday, and while trying to gravity bleed I wasn't getting any fluid moving to the slave cylinder. Then I tried to build up pressure by pumping the clutch and got no fluid moving to the slave. I figured the fluid should move downhill to the slave, but the air in the hard lines seemed to prevent that from happening.

Today I picked up the Mityvac Vacuum Pump from Harbor Freight, and within 10 minutes I had fluid moving out of the slave bleeder, and had purged the air from the lines. Clutch feels great now. In retrospect, I doubt I needed a new master and slave, but the parts cannon is hard to resist when I get frustrated.

In short, if you plan on replacing your clutch or brake fluid, I would recommend picking up a vacuum bleeder. Neither a gravity bleed or normal pressure bleed were able to remove all of the air in my situation.

 

Thanks for updating this thread. Im having similar symptoms and I think I need to get a Vacuum pump to help bleed the slave cylinder. Can you explain how it works? Are you attaching the pump down at the slave cylinder and sucking it out with vacuum or up at the resivoir and pushing the fluid with pressure?

Thanks

 

I had the vacuum bleeder attached to the slave cylinder bleeder valve. It takes a few pumps to get fluid flowing and get the air purged.