Lizard928

Ok so the car is an 1984 Euro 5 speed.

A little while ago the owner was down in seattle and the pedal started to get spongy. Tom bled it for him and he drove home, then picked me up and I replaced the Master cylinder. The problem came back. I found that there was alot of air in the system. So I replaced the blue hose. There was no fluid leaking from the slave nor was there any fluid loss from the resevior.

I bled the system (top and bottom) and pedal felt perfect! He drove it that day and then the following day he got a couple miles from his house and the pedal fell right to the floor! I went out bled the system and took the car home. The pedal would be good, then soft then good. I inspected the system and did a search and one other person had an issue similar on another board and it turned out to be the slave. Now I should also note that I have one of the goodyear hoses from 928intl in this car too.

So I replaced the slave, bled the system. Clutch pedal was very firm, felt perfect. I then kept the car and tested it for 5 days or so. Every day I would take it for a drive. The issue never came back.
The owner then decided to take the car on a cross country trip. He got about 12 hours away or so and the top of the pedal has gone soft on him. The fluid level has not dropped at all! All the componants are new, well short of the hard lines. But somehow air is being allowed to enter into the system without letting any fluid out!

Does anyone have any siggestions as I am atm.

TIA

Fabio421

Theres not much left in the system. There is the flex hose that goes between the body and the bell housing and theres the short hard line. That flex hose usually leaks. I am assuming thats the Goodyear hose you mention. Is it dry? Is it possibly allowing air into the system? Are you using a power bleeder or the pump method. I have found that the power bleeder is a must but I also use the pump method in conjunction with the power bleeder.

Does the car still have the heat shield installed between the exhaust and the flex hose/ If not, maybe the heat from the exhaust is boiling the fluid. I'm sure with your level of skill you will figure it out soon. Please let us know what you find.

Mrmerlin

Did you modify the new master cylinder?
The mod is to cut off about 1/4 in. of the inside tip and then re slot it.
This gives the master more travel that will match the old one.
You have to remove the piston to do this dont cut off too much though as you can unport the rear piston seal

GlenL

I'm confused by why this mod is done. Isn't there enough adjustment at the clevis on the pedal?

This is odd behavior, to be sure. Bad fitting at the master?

Enough air to make the system behave badly will not be noticeable in the brake reservoir. The perplexing questing is where the air comes from and why there's no fluid leaking. Is there an obstruction in the blue hose to the reservoir? Just guessing.

Fabio421

The master cyl. mod will not correct an intermitant spongey pedal. The symptoms of this problem is a dragging or non engaging clutch

Lizard928

Fabio,
The goodyear hose is the one that connects the hardline on the body to the hardline on the engine. This is the hose you are asking about. It is new.
As well everything is BONE dry, meaning there is not ONE drop of fluid anywhere on the componants! This is why it is so baffeling. How can air get in and no fluid out????
I have not removed any heat shields from the car and before it happened and I replaced the componants the car ran perfectly for a long time. So nothing has changed here. As well the fluid being used is VERY high temp. The mixture would have to be way off causing cherry red manifolds and alot of heat before that would ever effect it.

Stan,
When replacing the MC, I pulled the guts out of both the old (origonal) and new and confirmed that the metal post you are talking about was the same length on both of them. As well when I bleed the clutch there is absolutly no dragging and it has perfect disengagement.

Glen,
If it was a bad fitting at the master cylinder either from the hard line or the blue line and plastic fitting I would see a fluid weeping around the area. Or even some dampness. But as I said, everything is completely dry, and the resevior is not going down at all.

I like Fabio's thinking with the heat boiling it, but I simply dont see that as being an issue.

Thanks for all the input and thoughts thusfar though!

heinrich

Colin I had a Mercedes years ago that would suck air in at the clutch pedal past the plunger.

Sailmed

+1 on a '78 Euro!
I think the "new" master sat around on a shelf and the plunger hardened long before I bought it.
$16 rebuild kit from Rock Auto into the "new" master cylinder = problem solved.

Lizard928

I am really hoping that isnt what is happening. Though would we call that a defective master cylinder?

I find it odd that the new one would do the same as the old as well.

Does anyone know if vasoline or something similar is compatible with brake fluid. That would help me rule that issue out by placing a gob of it up at the top. None would really get in, just prevent that air from being able to backfeed in through those seals.

Lizard928

Thank you for your input too Mike.

Though all the seals on the old plunger, and new plunger were soft and pliable. Not overly so, but like a proper seal.

Sailmed

Colin - if it was air - there would be a consistency to the problem... Is there any chance the brake fluid has contaminants or moisture?

Lizard928

Brake fluid in the entire system is roughly 4 months old, however most of the fuild in the resevior is brand new. So I would say no.

Before I replaced the slave, everytime it would bleed it I would get air out of the MC. I thought that was odd. But after installing the slave cylinder it was rock solid and before giving it back to the fellow I did another check and there was no air in the system. But from how it is acting now there is.

Tampa 928s

Leaky caliper seal once it heats up ?

Lizard928

The problem is in the clutch circuit, no calipers there.

And if the problem were a leaky seal I would see fluid weepage, and a loss of fluid. Neither of which are present.

dprantl

The most plausible scenario IMO is air entering the system where the clutch pedal meets the master cylinder. If it was high enough in the system, you probably wouldn't see any leaking fluid.

Dan
'91 928GT S/C 475hp/460lb.ft