Trying to bleed the clutch system....with no success

[FFRWRX]

A while back I had to make a small change to the clutch line; installing A/C and needed to move the line where it interfered with the compressor. So I cut the line, flared it, and reconnected it to the rear flex line. While doing this the clutch master cylinder basically emptied. The clutch system was working fine before any of this.

So now I am trying to bleed the system and getting nowhere. No effort on the clutch pedal, just goes full travel without any effort. Master cylinder is full. Pedal is moving the rod into the cylinder.

No fluid coming from the slave cylinder bleed fitting. Tried my vacuum bleeder on it and it doesn't pull any fluid through.

Next step. Undid the flex line from the slave cylinder. No fluid coming through with pumping the pedal.

Next step. Put my pressure bleeder on the master cylinder and same thing, no fluid coming through.

Undid the fitting I flared and connected near the rear flex line. The flex line is clear (no blockage). Hooked a tube up to the end of the line I flared (the other end of which basically goes to the master cylinder). No fluid coming through.

So with my pressure bleeder on the master cylinder (at about 10 psi) there is no fluid flow through the system. The pedal moves with basically no effort through its full travel. I can't imagine that bench bleeding the master cylinder would be needed (but maybe I'm wrong) as it is full of fluid under pressure, but it is not going through it. It would be quite a coincidence if the cylinder somehow went bad during this process and need replacing.

I assume the knob-like thing on the bottom cannot be used to somehow get fluid flowing through the system, but I don't really know what it does.

Suggestions?

Thank you,

Rick

[NAZ]

Rick, what you're describing sounds like the replenishing port in the M/C is blocked. If you're using the Wilwood type single M/C, there is a port that connects the reservoir with the cylinder. That port is normally open when the piston is fully retracted and allows fluid to fill the cylinder. When you press on the pedal the piston moves and covers the port to make a seal and as the piston moves down the bore fluid is transferred to the slave cylinder. If the port is blocked the fluid in the reservoir cannot fill the cylinder. To use pressure bleeding (or vacuum) this port has to be open. The two most common reasons for blockage is foreign material or the piston not returning fully. Note, just because the pedal returns full does not guarantee the piston does. The other style M/C uses a check valve instead of a replenishing port but it's the same basic operation. The fluid is allowed to flow from the reservoir to the cylinder but not the other direction.

I looked for an animation of cutaway drawing of this but couldn't find one I liked. You can Google How a Master Cylinder Works if my description is not clear.

[RPGs818SNA]

NAZ correctly described my clutch bleed issue. I had to release the push rod from the clutch pedal and pull hard on it to get the master cylinder to fill with fluid. Then I pushed on the rod to fill the slave. After several repetitions, no more bubbles from the slave and the clutch worked fine.

[FFRWRX]

I appreciate the help. Here's what I did since I posted.

Did a bench bleed; on the car, not actually on a bench. Hooked a line up from the outlet port on the M/C back into the reservoir. Pumped the pedal a few times and got a couple glorps of air. Then the line was full of clear fluid. The level in the reservoir would go up and down as I pumped the pedal.

While I had the line off it, I cobbled together an arrangement to fill the line with fluid. Put it under some pressure at the front and had it coming out the bleed at the back. So the M/C was full with no air and same with the line. Hooked it all back up and no change.

If the port you are describing is plugged and the air was out of the system, wouldn't you expect it to still build up some pressure? There is no pressure in the system when the pedal is pushed. And the fluid level was going up and down when bench bleeding it.

The only thing I can think of now is the M/C is no good. Strange coincidence to have it go the same time I modify the line. I'll disconnect the pedal and try pushing and pulling the rod to see what that does.

thanks again

Edit: with the rod disconnected from the pedal, it doesn't move any more than it does when connected. Can't pull it out any further and can't push it in any further. I pulled very hard on it.

[STiPWRD]

Typically, disconnecting the MC rod from the pedal and pumping it directly has done the trick for me. I found that I was able to push the rod further in when it wasn't hooked up to the pedal. This at least got the fluid pumping to the slave cylinder. I also had issues bleeding the clutch. What worked for me was to unbolt the slave cylinder and hang it up as high as I could (it's on a SS flex line). I used a large C-clamp to keep the slave piston from pumping out and a vacuum bleeder. That seemed to work.

[FFRWRX]

I removed the clutch master cylinder and took it apart. The only thing I can figure is that I had a double failure. This is what it looks like:

133354

The spring is there to shoot the brake fluid covered parts out and onto your shirt. I hate working with this stuff. I manage to get brake fluid all over...on my tools, the bench, the floor, myself. Hate it. But I digress....

There is a small hole that feeds the fluid from the reservoir into the cylinder; that is the one on the right in the counter-bore. The center one doesn't seem to be a through-hole. When assembled, the first seal is just to the right of that hole.

So with the pedal not depressed, the fluid has a path from the reservoir into the cylinder and into the line. I had the pressure bleeder on the reservoir and couldn't get any fluid to flow out to the slave cylinder, so that small hole must have been plugged. It all looked pretty clean when I took it apart so it is hard to see how it was plugged, but can't think of any other explanation. That would be failure #1.

Bench bleeding it, even with that hole plugged, would take fluid from the reservoir and suck it into the cylinder from where the line would normally attach, and fill it with fluid. So the cylinder was filled with fluid, but when the line was hooked up and full of fluid, the pedal did not have any resistance to it. So failure #2 has to be the seal was bad. The seal looks fine, but if you've ever rebuilt one of these, the seals rarely look bad even when they aren't sealing.

I took the spring off and put the piston back in. I held my finger over the line connection port and pressed the piston in. There was a little pressure building up, but the piston bottomed out and it shouldn't do that if the seal was good.

I've ordered a new master cylinder. I'm tired of playing around with it and I've got every surface of my workbench nicely covered with brake fluid now, so my job is done. Even if you could get a rebuild kit I wouldn't bother at this stage.

I appreciate the quick replies to try and help me with this.

Rick

edit: I never did conclude this short thread. A new clutch master cylinder solved the problem.

[RPGs818SNA]

Upon hearing an occasional grinding noise during shifting, I figured I would start troubleshooting by simply bleeding the clutch to be sure it was properly disengaging. I tried the Subaru manual instructions shown below:

Slowly depress the clutch pedal and keep it depressed. Then open the air bleeder to discharge air together with the fluid. Release the air bleeder for 1 or 2 seconds. Next, with the bleeder closed, slowly release the clutch pedal.
Repeat the procedure, until there are no more air bubbles in the vinyl tube. Tighten the air bleeder.

After completing the above procedure once, the pedal moved with no resistance and the level in the reservoir hadn’t changed at all. I tried connecting a vacuum bleeder to the slave but got a never ending stream of air bubbles when the nipple was open. Putting Teflon tape around the nipple threads solved that, but no amount of vacuum pulled any fluid through the system. That seemed weird.

I studied Rick’s disassembled master photo and the information provided by NAZ, and drew how I believe the system should work.
213816

Considering my earlier experience and NAZ’s comment, I disconnected the clevis from the pedal and pulled a vacuum on the slave and it worked! I think I had failed to adjust the clevis so the rod was fully extended when the pedal was up, so that slight push from the pedal covered the reservoir inlet.
213817

So when I followed the procedure, I ended up with this.
213818

And when I released the clutch, the master piston didn’t retract, so there was no resistance to pushing the pedal, and applying a vacuum at the slave didn’t do anything.
213819

Shortening the push rod by turning the clevis 2 full turns correct my problem. After raising the slave cylinder per STiPWRD and using a vacuum to pull all the air out, the shifting noise vanished.

My one remaining question is how the Subaru procedure is supposed to work. How does the master piston retract enough to let in more fluid if there’s no air to expand? Does the right hand seal allow fluid to pass one way?

RPG

[Bicyclops]

The pistons move in tandem in both compression and suction. When the pedal is depressed, the fluid displaced under pressure from the master cylinder into the line moves the clutch cylinder against the spring pressure of the clutch pressure plate. The Subaru method supposes that the clutch cylinder is still mounted during the bleeding procedure which would allow the pressure plate to put pressure back towards the MC. When you let up, the pressure plate moves the slave cylinder back to its original position displacing the fluid back into the master cylinder and moving the piston to clear the inlet. There is a return spring on the pedal to pull it back enough to clear the inlet and also keep it from riding on the throwout bearing.

If the slave is removed from its mount, the pressure plate can't push the fluid back into the MC after you bleed the slave. Normally, the suction of the returning MC piston would be enough to pull the fluid back out of the SC. But because your rod was adjusted too long, the pedal return spring wasn't able to pull the MC piston back to clear the inlet.

I pressure bled my brakes and clutch. I did the clutch with the slave dismounted. I have a remote reservoir. I drilled a spare filler cap and installed an air fitting on it. I turned my compressor way down and put a little pressure into the reservoir and then opened the bleed nipple (with a piece of tubing on it) to dump fluid and bubbles into a can. Works great. Just have to close it up and put more fluid in the reservoir before it runs out is all.

Ed

[RPGs818SNA]

Thanks Ed! I agree that the best way to bleed the clutch is to dismount and raise the slave and then push or pull fluid through the system without depressing the clutch pedal (assuming the clevis is adjusted to allow the master piston to fully return and open the reservoir inlet).

I also agree that “The Subaru method supposes that the clutch cylinder is still mounted during the bleeding procedure which would allow the pressure plate to put pressure back towards the MC.” But when I open the nipple and allow the fluid and pressure to drain out, and then close the nipple, there’s no pressure left to push the master piston back and not enough fluid left to allow the master spring to push it back. How does the master cylinder refill with fluid before it returns and opens the reservoir inlet? Is the master cylinder spring strong enough to create a vacuum between the master piston and slave piston so the master piston can return and open the reservoir inlet? Is the master piston seal designed to allow fluid to pass from the reservoir past the seal but not in the other direction? Do the seals stiffen over time such that they don’t allow fluid to pass either way on my 19 year old cylinder?

Curious,
RPG

[Bicyclops]

Howdy RPG,

If the seals were bypassing, you wouldn't get much action on the other end. This is how master cylinders sometimes fail. The pedal is soft but there are no puddles. The seals are just O-rings and yes they will prevent fluid passage both directions.

The pressure is always equal throughout the system. Push the pedal and it could be in the many hundreds. The piston on the receiving end is going to feel the same psi and will move. Has to, unless it is seized. When you open the bleeder you reduce the hydraulic pressure everywhere to atmospheric. The is no hydraulic impetuous for either piston to move either direction. The pedal return spring pulls the MC piston back creating suction in the line. It's not going to be hundreds of psi, but it will be enough to move the clutch piston. Nature abhors a vacuum. If the bleeder is open, you'll suck air back in. Closed, the slave piston is pulled back. The master cylinder piston sucks the fluid back through the line - just as much as it pushed before, less perhaps some that squirted out with the air. You can't squeeze all the fluid out. If there is still air in the line, the clutch piston won't be pulled as hard and you'll have to keep trying til all the air is out. The reservoir will provide more fluid as needed when the piston pulls back past the port and if there isn't any more fluid to be sucked out of the line. This will keep the system full if there is a small amount of leakage somewhere in the system or in this case, loss from bleeding. If the assembly were complete, the pressure plate spring will provide plenty of pressure until the MC piston passes the refill port. The reservoir is vented to atmosphere. Guess what - no more pressure in the system and if there were excess fluid in the line, it would be returned to the reservoir.

I'm a fan of pressure bleeding. That's how you do it in light aircraft - from the bottom up using a pressurized fluid bottle. It carries the bubbles on up to a remote reservoir high on the firewall. As you found out, when pulling vacuum, any tiny leak will introduce more air bubbles. Yes, many have successfully vacuum bled. I just have had better results with pressure. It also shows you where the leaks are so you can correct them.

Ain't hydraulics fun? Because the pressure is the same throughout, you can create mechanical advantage by using larger diameter pistons at the receiving end. However much psi there is works on a larger area over a shorter throw than at the MC. Or vise versa. Reduce the diameter at the MC and you don't have to push the pedal as hard, tho you will have to push it further. It's another way to adjust brake pedal feel.

Ed

[RPGs818SNA]

Hey Ed,

Thanks for all the additional information about how hydraulics work. I especially like the explanation of the advantages of pressure bleeding. I can see why the pressure bleeding kits can command a higher price than the vacuum kits. I may try to find an extra cap to make my own.

I’m still puzzled as to how the Subaru bleed process is supposed to work. If the pedal is depressed, the master piston pushes the fluid to the slave cylinder which pushes the clutch release lever to release the clutch. When the bleeder nipple is opened, the release lever pushes the slave piston back to the non-extended position and pushes fluid (and air) out. When the nipple is then closed and the clutch pedal released, there’s not enough fluid left in the slave cylinder to allow the master piston to return and uncover the reservoir inlet.

Looking at Rick’s photo of the disassembled master cylinder, it appears that the cylinder diameter is about a half inch with an area of about 0.2 sq in. That would take a pull of 3 pounds to overcome nature’s resistance to a vacuum. The vapor pressure of brake fluid is only around 0.0002 psi, so it’s no help in forming a brake fluid vapor bubble. The clutch pedal spring doesn’t help either, because the push rod isn’t attached to the piston. The spring in the photo doesn’t look that strong, but maybe it is.

Looking closely at Rick’s photo, it appears that the piston seal is wedge shaped. Would that let fluid pass in the reverse direction to the low pressure side?

I’d just let this question drop, but I miss seeing a new post or two most every day on this forum. Thanks for the learning opportunity.
RPG

[Bicyclops]

Don't forget about all the fluid in the line. The fluid in the clutch cylinder probably never makes it all the back to the MC anyway. I've been assuming that the pedal is hooked up, but maybe I'm wrong. The spring in the MC must be doing the job. You really don't want fluid being able to pass the seal. If it could, it wouldn't hold pressure. It looks to me like the wedge on those seals is toward the outlet and would expand under pressure making a better seal.

Ed