Anyone loose their clutch pedal pressure from overheating then have it work fine again afterward? I raced my car in an SCCA Autocross event last weekend and after one of my last laps the clutch hydraulics did not have enough pressure to disengage the clutch and shift out of neutral into gear. I had to have help pushing my car out of line because I could not get it to move on it's own. Then I let it sit and cool down for a while and now the clutch works fine again. I tried going WOT and there was no noticeable slipping. I have 23k miles on a GMPP Clutch Kit and about 7k miles since I bled the clutch hydraulics when I installed a new LDK engine. I don't like the uncertainty of not knowing if something is wrong. I don't see any fluid lost or leaking. Any theories? Maybe I boiled the DOT 4 brake fluid?

 

I would find it hard to believe that you would boil the clutch fluid and not also the brake fluid. It wouldnt hurt to bleed the clutch and see how the fluid looks coming out though.

 

Clutch master cylinder seal might be getting weak. The cup seal may have folded due to the intense pressure. Or there is a tiny bit of air in the system that wouldn't cause an issue under normal conditions but since it was getting driven hard it worked its way down the the throughout bearing/slave and had no where else to go and once the car sat the air rose back up to a spot where it wasn't an issue

 

Thanks I'll check the master cylinder carefully when I get a chance. I thought I had bled all the air out of the system but that would make sense that a tiny air bubble got shifted during autocross driving. I haven't really driven hard in the last 7k miles until that Autocross event. I was squealing tires for half of all my time spend on the course haha

So this may be totally unrelated but to add to the mystery... over the last 2k miles the car developed a rattle. Specifically like an old school telephone ring that occurred mainly off throttle cruising with a warm engine. I was paranoid it was a bad bearing. But now after the Autocross event and the clutch issue the rattle is gone. So it may had been just about any object that shifted position during racing, idk man

 

I've personally never ran into that with mine. I guess with it comes back you may have to do process of elimination. Is it rpm related or speed related and then go from there. Is it there when the trans is in gear.

 

RInging noise was pretty constant. Not directly related to engine rpm or wheel speed. It would happen constantly whenever i let off the throttle or sometimes during light throttle, but not so much with a cold engine. Didn't matter what gear or in neutral or with the clutch engaged or disengaged, which tells me it's not the throwout bearing.

 

I agree with checking/replacing the clutch master. They're so damned finicky and not very reliable. I would also suggest looking into a solid clutch line and insulating it as well.

Very possible that the fluid did get cooked and is creating air bubbles in the slave, considering where the slave sits and the amount of heat is in that area.

This is truly the bane of GM - their shitty hydraulics.

 

Since the clutch and brakes use the same reservoir it's likely you boiled the fluid. Were you hard on the brakes? How long were the runs?

Also, how old was the fluid? Brake fluid cap new? Don't forget any little bit of water in there lowers the boiling point significantly.

 

Brake fluid is usually only good for about 3 years. Bleeding the clutch is only getting part of the fluid. I've never had this issue but I also don't push mine hard because I don't have allot of spare money to fix it lol. I flush mine every 2 years but I have access to a B.G. brake fluid flush machine that pressurized the fluid. You can use a simple vacuum bleeder from harbor freight and bleed each caliper along with the clutch to ensure the fluid Is good

 

I have never Autocrossed, but have had a similar issue prior to a slave cylinder failure. Clutch would lose pressure on extended wot pulls. Never did fail to shift, but did lose pressure. I think it only lasted a month or so like that though.

a master cylinder would be a more preferred failure point though. And I agree it could be that also

 

Brake fluid is probably 3 years old. I have bled the brake's many times in the past. I've never replaced the fluid resivoir cap. I used my brakes hard briefly on the Autocross course but not super hard. Autocross courses are relatively short and low speed with tight corners. I'd do a single 38 second lap with about a 10 minute break in between for a total of 4 laps. I usually did the whole course topping out in 1st gear, so very little shifting going on. I have a harbor freight vacuum bleeder, which I've never used successfully. I always end up having an assistant pump the pedal to get all the air out. Has anyone tried a less expensive aftermarket clutch Master Cylinder? They seem to run nearly $100 cheaper than the OEM ACDelco ones I've seen for sale.

 

I’ve gotten a master cylinder from oreillys and it was a lifetime warranty. Replaced it once for free. Kind of crazy, but they did.

i used a power bleeder with success. You don’t want to pump too much pressure though. I think I would keep it around 6-8 psi. Those kits cost about 60$ if I recall correctly

 

So the master cylinder from Oreillys failed on you?

 

Factory one, do it once and done.

 

Yes, it failed a year later when I was bleeding the fluid. I was finishing out a clutch install. Kind of a pain, but was free

 

I'd suggest either AC Delco or a LuK unit. I've had the best luck (no pun intended) with both. Dorman is junk.

While you're in there, I'd suggest replacing the factory clutch line with a solid piece from OTTP.