Need to Pump the Clutch - Master or Slave?

[BeerBudgetRacing]

Situation:

RF01.
I need to pump the clutch to get it to disengage.
If I pump the clutch, depress it, and put it in gear, in less than 1 minute the clutch will lose pressure and start to engage.
No fluid loss.

My thought is the master is leaking (slave would lose fluid)

Thoughts?

[Steve Davis]

Either one could cause that issue. Easiest way to check it would be to close off the line at the 'easy point' near the slave. With it blocked, the pedal should stay rock hard for as long as you want to push it. If it doesn't - master cylinder.

Steve, FV80

[bob darcey]

If there's no fluid loss anywhere, the pressure is relieving back into the reservoir--bad cut-off valve seal in the master.

[BeerBudgetRacing]

If there's no fluid loss anywhere, the pressure is relieving back into the reservoir--bad cut-off valve seal in the master.

Thanks Bob.

[mark elder]

If the line from the master cylinder to the gearbox is dash 3 (I would have thought 95% are), then a simple -3 plug can block off the line at the gearbox end and you can pressure test the master cylinder alone. We should all carry such a plug.... Sometimes there is a banjo at the gearbox end but this can be unscrewed from the fitting and the plug installed.

Better still you have a dry break in the line. Just disconnect it and test.

Note to self, reconnect the dry break afterwards (don't ask why I know this).

[Gary Godula]

I had a clutch bleeder screw have a very small leak in it....small enough to let air in, but prevent fluid from coming up. The clutch would bleed out just fine and work properly in the paddock, but by the time I drove to grid, I lost the clutch. There was a slight groove worn into the sealing face of the bleeder screw from 30+ years of use. Replacing the bleeder screw fixed the problem.

It probably is not your issue, but it is worth checking and changing bleeder screws as needed.

[bob darcey]

Probably worth noting (not that BBR was doing so) that holding the clutch in for a minute with the motor running is torture for the crank thrust bearings.

[DaveW]

Probably worth noting (not that BBR was doing so) that holding the clutch in for a minute with the motor running is torture for the crank thrust bearings.

That is especially bad during cold-engine starts since the thrust washer gets pretty dry after not running for a while.

[BeerBudgetRacing]

Probably worth noting (not that BBR was doing so) that holding the clutch in for a minute with the motor running is torture for the crank thrust bearings.

Great point! Heavy/strong pressure plates - they ain't road cars.

Thankfully I'm not. I only use the clutch for:
- Getting into gear when moving from a stop.
- Getting into gear when missing a shift. (which currently doesn't work because I have to pump it)
- Trying not to kill the engine on a stop (which also doesn't work because I have to pump it)

On Sunday I spun in the entry to T8 at Laguna and I probably lost 2 places because I couldn't keep the engine running and I had to sit and pump the clutch before I could move. Had to do a 3 point turn. Stalled, etc.
Just not working like it should!

On a side note, my start from a stop technique is to keep the revs low and feather the clutch.
A friends technique was to rev the engine and drop the clutch.

My logic is it's less stress on everything.

What is the better way?

[Lotus7]

Great point! Heavy/strong pressure plates - they ain't road cars.

On a side note, my start from a stop technique is to keep the revs low and feather the clutch.
A friends technique was to rev the engine and drop the clutch.

My logic is it's less stress on everything.

What is the better way?

No hard evidence, but intuitively I think I'd prefer a heavier load on the crankshaft bearing for a fraction of a second rather than a partial load for many seconds.... YMMV

edit: also less chance to glaze the clutch?

[DaveW]

...On a side note, my start from a stop technique is to keep the revs low and feather the clutch.
A friends technique was to rev the engine and drop the clutch.

My logic is it's less stress on everything.

What is the better way?

I prefer the gentle method - one can break stuff like CV joints and half-shafts dropping the clutch.

[BeerBudgetRacing]

No hard evidence, but intuitively I think I'd prefer a heavier load on the crankshaft bearing for a fraction of a second rather than a partial load for many seconds.... YMMV

edit: also less chance to glaze the clutch?

My anecdotal evidence is: he shattered his clutch.
I've also seen axles break and I'd suspect the hit exceeds the LD200 specs.
To me (since they have no springs on the disc) it's like hitting everything (crank, rods, etc. though to the hubs) with a hammer...
Clutch disc is cheap compared to everything else....

[DaveW]

My anecdotal evidence is: he shattered his clutch.
I've also seen axles break and I'd suspect the hit exceeds the LD200 specs.
To me (since they have no springs on the disc) it's like hitting everything (crank, rods, etc. though to the hubs) with a hammer...
Clutch disc is cheap compared to everything else....

Even with gently slipping the clutch (QM sintered), and using the clutch for every shift, up and down, I have never worn one out in one season. I usually replace them when they start to crack at the segment stress relief radii.

The key is not using high revs while slipping the clutch to get going or driving through the paddock. That, IMO, is what wears them out. Generating excessive heat is the culprit.

[EACIII]

In all the responses I have not seen anyone suggest that he bleed the clutch. Would have been my first move. Guess assumed that was done?

Ed

[BeerBudgetRacing]

In all the responses I have not seen anyone suggest that he bleed the clutch. Would have been my first move. Guess assumed that was done?

Ed

Yes, but what I didn't say is I "used the crush box" last year. Front left, nose/wing/crushbox/mounts, etc. I bought a new master and then we decided the original didn't get damaged. I'm thinking there is a crack internally.

Going to replace the master and bleed everything.

[DaveW]

Yes, but what I didn't say is I "used the crush box" last year. Front left, nose/wing/crushbox/mounts, etc. I bought a new master and then we decided the original didn't get damaged. I'm thinking there is a crack internally.

Going to replace the master and bleed everything.

That'd be interesting if there is an internal crack. I can't think of where that might be to cause this. Maybe it's just deformed a bit from the wreck, making a seal not seat correctly. If you had your foot on the clutch pedal, maybe excessive piston travel damaged or deformed the check valve seat, because as mentioned previously, that fits the symptoms of no external leakage.

[bob darcey]

Keeping one's foot on the brakes in a crash can damage the master cylinders as well...

[DaveW]

That'd be interesting if there is an internal crack. I can't think of where that might be to cause this. Maybe it's just deformed a bit from the wreck, making a seal not seat correctly. If you had your foot on the clutch pedal, maybe excessive piston travel damaged or deformed the check valve seat, because as mentioned previously, that fits the symptoms of no external leakage.

Or it could have torn the check-valve seal and the metal parts may be fine.

[BeerBudgetRacing]

Or it could have torn the check-valve seal and the metal parts may be fine.

It'll be next weekend before I can get to it.

[DaveW]

Did you resolve the issue ?

[BeerBudgetRacing]

I'll let you know. Planning to work on it today.

[BeerBudgetRacing]

Finally got to some testing this evening with the help of my son.

Pulled the line off the bellhousing and put a plug in it. Bled it.

Pedal was really solid and not moving. Damn.

Reconnected everything and re-bled the clutch system. Took quite a while - didn't seem to make progress.
Pedal did get stiffer. Tightened everything up.

Press the pedal and I could hear gurgling in the bell housing!

Crap.

What's surprising is there wasn't any fluid loss....

Looks like a slave rebuild -

[mark elder]

At least you have progress, Depending on the design, slave throw out bearing units can leak

forwards; and therefore dribble down the inside of the bell housing and this is seen quickly and with little fluid used.

backwards; and there can be a big collection area (maybe 50-100ml) that has to fill up before fluid dribbles onto the ground. Sounds like your unit was blowing bubbles in the well of fluid. If you lift the rear of the car up lots, then the well empties (tips out) confirming the seal has gone.

ask me how I know this............

sounds like a posterior leak. The rear O ring can be nicked by the hole that brings fluid to the unit. The front one avoids trouble because it is always in front of the hole. Check the hole is smooth on its edges. You can see the nick in the O ring where the hole would have caught it.

Report your homework back to the panel.

[ejsprint30]

I noticed no fluid loss at all in my situation, and my slave was toast. When mine finally went, the engine would stall when I would push in the clutch. Don't forget to set your pedal stop. Might also be a good idea to check your crank end play when apart.

[JHerscher]

When you replace the O rings, use rubber lube. In my experience it works better to avoid the little nick that can happen if you just use brake fluid. Any flavor rubber lube seems to work - the stuff from Pegasus or the red stuff you get in rebuild kits both work equally well.

John

[Rick Kirchner]

The high probability of getting nicked on that hole is why I disregard the conventional wisdom of changing the clutch slave o-rings whenever the car is apart.

I figure, if it ain't broke, don't fix it in this situation.

Been going something like 7 years on this set but i am having issues in the winter. It seems that they are a bit harder and don't seal well when cold.

Next time I have it apart though I'm going to take a serious look at where the hole is WRT the aft o-ring position. If there's a safe distance, a dremel with a polishing wheel might be used to smooth that orifice so I don't have to worry about it any more.

I'd also say that building a fixture with a crossbar that bplts across the bell housing and a target with a die spring for the slave to work against, and an old master cylinder rigged as a tester is another good way to make sure everything is GTG before going through the pain of buttoning it all up.

[Lyngengr]

I'd also say that building a fixture with a crossbar that bolts across the bell housing and a target with a die spring for the slave to work against, and an old master cylinder rigged as a tester is another good way to make sure everything is GTG before going through the pain of buttoning it all up.

I bought a 4" aluminum channel and spring from McMaster-Carr to do this test before putting the back end on the car. I push the bell housing close to the rear of the engine, connect the clutch line, bleed it, then test the slave cylinder for actuation and leaks.

[BeerBudgetRacing]

I figure, if it ain't broke, don't fix it in this situation.
....
I'd also say that building a fixture with a crossbar that bplts across the bell housing and a target with a die spring for the slave to work against, and an old master cylinder rigged as a tester is another good way to make sure everything is GTG before going through the pain of buttoning it all up.

As many times as I split my 94 I never touched the slave. Never had a problem.

I bought a 4" aluminum channel and spring from McMaster-Carr to do this test before putting the back end on the car. I push the bell housing close to the rear of the engine, connect the clutch line, bleed it, then test the slave cylinder for actuation and leaks.

Any pictures?


So talked to PO car engineer. Apparently when the cutch was changed the pedal had been over adjusted and the slave 'popped out'. Function was restored without splitting the car 'popped it back in'. I suspect a tear that has just gotten progressively worse.

New seals on order.

[DaveW]

That'd do it. Glad you found an explanation.