Try bleeding it out again and just catch and reuse the fluid. Use a bleeder bottle and it will go MUCH faster.

If it is due to a bubble, the reasoning goes something like this: A small bubble is in the line and will allow the line to get enough pressure to get the pressure plate up, disengaging the clutch. But when you spin it fast enough, the speed (I believe centrifugal force) will make the line have to provide more pressure to lift it, and the bubble absorbs enough of your foot's force to not get enough force to the clutch assembly, making the clutch stick until the RPM's go down.

I had this happen to my Prelude on my first motor I built for it; I ended up having to run at least 5 of the small bottles of fluid through it before it got it out. IIRC, old mustangs used to have similar problems, only it was due to their pressure plate from the factory......

 

are you just gravity bleeding it, or have you been pressure bleeding it?

 

I've been bleeding similar to brakes, give the clutch pedal five or ten real hard pumps then hold down and turn the bleeder screw. This is with the vehicle on level ground.

Will

 

There should be a heat shield protecting the slave cylinder,if it's missing the fluid in the slave cylinder may be getting boiled when you drive the car hard!

 

I would suggest doing something to determine whether the clutch pivot rod is moving, or if you are just losing hydraulic pressure. It does seem strange that RPMs could alter hydraulic pressure.

 

my 2 does the same thing if i drive on the interstate for more then 20 or 30 minutes my clutch will go completely to the floor and once i start driving around town again its fine and i never have i problem with it i have changed the clutch to an act since i have had it and obviously bled the new clutch i have been puzzled by it for a while and would also like to have an answer

 

Going off of what Sonicsilver said, I read on one of the threads about someone who lost hydraulic pressure in their brakes. He realized he lost pressure cuz he was missing a heat shield and his exhaust manifold was overheating the master cylinder. (This car wasnt an mr2 since the master cylinder is in the frunk, i think it was an ae86)

 

So does brake fluid contract when heated?

 

So does brake fluid contract when heated?

I think the main problem is that it boils

 

And it contracts when it boils?

 

And it contracts when it boils?

That's a good question. I wouldn't think so. Doesn't boiling entail bubbles? I have no clue, at this point I'm just pulling stuff outta my ___

 

Normally. But it seems to me that heating, or boiling, would increase the hydraulic pressure, push fluid back through the master cylinder, and leave you with no pedal after it cooled. I assumed, possibly incorrectly, that he had pedal once it came back down below 4000 RPM.

 

Well, I'm 100% sure taht the boiling does not increase pressure. I used to drive my old MRII n/a pretty hard on some twisties where i used to live in Cali, and towards the end of the drive the brake fluid was boiling and I got alot of the typical fading brake symptoms. Initially I thought it was my pads or rotors that were just heating up. But later when I flushed and filled my brake fluid with synthetic it fixed the problem. Go figure, I really don't know how or why though?

 

It could still increase pressure, push some fluid back into the reservoir, and leave some bubbles in the line. When you stepped on the brakes again, and sealed off the bleed hole in the master cylinder, you would have some bubbles in the line. As soon as it cools back down, brake fluid could be pulled back into the lines. I suppose the same could be happening to the clutch. Possibly it could be self-correcting as it cools.

I really don't know either. I don't think I have ever seen two people explain so much of what they don't know! Silly huh!

 

I don't think I have ever seen two people explain so much of what they don't know! Silly huh!

Haha.. true... Okay. I'll be quiet now so that maybe someone with actual knowledge on this top can chime in.

 

The main problem of brake fluid boiling is the bubbles it entails. Liquids tend to be more incompressible than gasses and when gasses get into the brake lines, the effectiveness of the hydraulic system is lessened because gasses compress more than liquid. Thats why we bleed our brakes and hope theirs no air pockets because air pockets compress.

 

Alright, so after some more modifications/repairs (-6AN fuel lines, alternator, o2 sensor) I'm revisiting this same problem. I'm picking up a mightyvac this afternoon, and I may try going to a synthetic higher temp fluid.

The heat shield is something I never considered. But now that you have mentioned it, I don't know why I didn't think of it sooner. Of course, that slave cylinder is in a pretty hot spot, and I don't think there's anything over it right now. I'll see if I don't have a bit of sheetmetal I can fab a heatshield out of. Because if I ever had one, I sure don't have one now.

The concearn of the hydraulic fluid boiling is vapor lock.. also the bubble produced will compress.. and compressability in a hydraulic system is exactly what you don't want..

Looks like I have my work cut out for me today. Thanks alot everyone for all your feedback. I'll see soon if it helps out, I imagine that most of it will..

Will

 

good luck. keep us updated.

 

So long story short, I did all of the above.

I unscrewed the hardline from the master cylinder, and used the mightyvac to "bench prime" or "bench bleed" it. Found a small bubble. really small.

Reattached the hardline and vacuum bled the system from the back with the car sitting on level ground. Found a couple of small bubbles, but nothing really obvious.

Extended my clutch master cyl pushrod out about 1cm, as it seemed a little too close in for my liking.

I then bled the clutch system "the old fashioned way" with my friend Brian pumping the clutch 7 to 10 times and me bleeding it out. I suggest doing this with the bleeder bottle on the cam cover and the line running down to the bleeder screw. remember that bubbles like to go up.

Fabricated a heat shield out of 12ga galvanized sheet steel that covers the lines going into the clutch slave cyl, and most of the cyl.

Replaced the fluid with DOT 4.

After doing all of this, my clutch feel was much more "correct". After taking it for a romp around the "test track" (a series of roads near my house that's about a 15 mile loop) it's feeling really good and I can actually achieve 5500+rpm now if I choose.

Thank you everyone for your help and suggestions. Viva MR2OC!

Will

 

There should be a heat shield protecting the slave cylinder,if it's missing the fluid in the slave cylinder may be getting boiled when you drive the car hard!

No. Doesn't happen.

I have had to remove that shield on countless projects cars and everyone has plenty of pedal pressure. His setup just needed to be properly bled.

 

Man, NoShoes, that would have been nice to know before I fabricated a heat-shield for it :smile:

So the exposure to the turbo hot-side and downpipe right there doesn't raise the fluid above 380*F? That's good to know..

It didn't end up being the bleeding so much as the pushrod adjustment.. sure I got a couple of miniscule bubbles out of the bleeding process, but those could be attributed to the connection of the hose to the bleeder.. but it's good to know the system's bled right.

Looks like I now have to decide whether to keep the shield or not now.

Thanks,
Will

 

It's right next to the cold side of the turbo. It still gets hot, but it's not a problem. Sorry I missed this thread until tonite.

 

I boiled my clutch fluid at a track day once. Lost the clutch on the cool down lap. Fortunately, it came back after 20-30 minutes.

Used to just bleed the brakes. Now I make it a point to bleed the clutch once in awhile too.

 

well, at least you lost it on the cooldown lap and not in the middle of a corner or something..

I think that I may have to buy an extra liter or two of Motul 600 when I do the brakes and change out the clutch fluid as well..

Anyone have any suggestions on replacing mineral fluid with synthetic? I was told you mustn't run the two together, and you have to flush the whole system out. Do you just run fluid through and bleed it forever, or is there something you need to wash through before you put the other fluid in?

Thanks again all.

Will

 

twoina2, had you cracked open your clutch line previously, meaning was there the minute possibility that there could have been air in there beforehand?