DOT 4 -> DOT 5.1

Yvan gurgled happily, sounding much like they were saying:

Umm, if you're changing the brake master, then you're going to need to bleed everything through completely anyway - so it will be a full fluid change.

But, yes, if it's DOT 5_.1_ that you used, it's miscible with DOT 3 and DOT 4. If it's silicone DOT 5, then - no.

What d'you mean by "not retract fully"? You mean that the brakes are staying on when you release the pedal? Sure it's the master, not a sticky caliper or shagged flexi?

Unless you're thoroughly abusing that Audi on track days, there's no way you're going to need more than DOT 4.

Nedavno Adrian napisa:

Why do I need full fluid change when I change master cylinder?

All four wheels get hot after few miles, brake pedal becomes hard. And last time changing master cylinder fixed it.

On this car radiator is at the left side (not center of the car), so the fan blows hot air directly at the master cylinder. So I thought if I put the best brake fluid I can find it might help. That's why I used DOT 5.1

But I guess that it must be something to do with the rubber seals or something, perhaps fluid is eating it, I don't know. The original cylinder, and the one I used as a replacement was OE Ate, this time I'll fit aftermarket Metelli, maybe I'll have more luck :-)

Yvan gurgled happily, sounding much like they were saying:

I'd love to know how you're planning to change the master cylinder without emptying the reservoir and needing to bleed the air through...

Does sound like the master, but it's a surprise for two to fail in exactly the same way so close together.

New DOT 4 boils at 230degC. New DOT 5.1 boils at 270degC.

If you're getting air anywhere near that temp off the rad, then you might like to have a look at the cooling system, too...

From new, the car would almost certainly have had DOT 3 in it.

Nedavno Adrian napisa:

By "full fluid change" I thought you mean change all of the brake fluid, in reservoir, brake lines, calipers...

:-)

Most likely cause of your fault is that the servo operating rod protrusion is too far. Easy way to find out: use the vehicle till all the brakes stick on and pedal is high, slacken master cylinder mount bolts, if the brakes release then the operating rod is too far out.

Yvan gurgled happily, sounding much like they were saying:

I did.

How else were you planning to bleed out the air that got introduced to the lines, right up next to the master?

I have changed dozens of master cylinders without going near a wheel, just bleed to the connections at the master cylinder, try it, it works.

Nedavno Mrcheerful napisa:

That's how I did it last time.

Nedavno Mrcheerful napisa:

So why when I changed master cylinder all was OK for a few years? Perhaps I can also wrap damp cloth around the cylinder when it gets hot, that might cool it down to see if it will it will release the brakes...

Actually, I did not use the car for a couple of years, and I plan to get it back on the road again, but I do not believe that the fault disappeared :-) I'll start the car, get it up to the temperature, and check what happens.

Interesting.

I has a similar problem after I first bled the brakes in my 1997 VW Transporter. Then every time I went for a drive, things started out OK but the pedal soon became harder. Then the wheels became hot and I needed more accelerator just to maintain speed.

I started reading. A Nissan workshop manual I have emphasised the importance of releasing the brake pedal slowly when pumping as part of the bleeding process.

I bleed the brakes again using the altered technique, and that fixed the problem completely and permanently. That was many years ago, and I've had no further problems.

I have no explanation.

John

I have never had a problem with overhot master cylinder on any vehicle, including non standard things where the master cylinder is over the exhaust !! The only real failure mode of master cylinders is leakage, either internal or external. They don't tend to jam on, which is what your fault would appear to be.

Seized wheel cylinders or calipers can cause the brake fluid to boil which might cause a sort of blow back effect to the master cylinder (but I have never seen it).

Drive it till the brakes are hot and sticking on, then try releasing the master cylinder bolts, if that cures it then it is the operating rod problem. If it doesn't release then slacken each pipe union at the master cylinder and see if that releases the brakes, if it does then it would likely be a master cylinder fault, if not then the problem is further on in the system.

Mrcheerful

Nedavno John Henderson napisa:

Your problem might be corroded master cylinder - the part where the piston does not get to during normal operation. When you bleed the brakes you piston travels further, and if cylinder is corroded you might damage the rubber seal.

But why could that be avoided by slowly releasing the pedal... I am probably wrong.

But my problem started with the failed clutch master cylinder. At the first shop they changed rubber seals only. It failed again. I bought aftermarket one. It failed after 500 miles.

About the same time brake master cylinder failed. So i changed both with OE parts. And filled the system with Castrol 5.1 fluid.

A couple of years ago brake master failed again, but I decided not to use the car for a while. Now I want to get it back on the road again.

Nedavno Mrcheerful napisa:

OK, I'll try that. I'll report back how it goes. But I have to get license plates first.

Yvan gurgled happily, sounding much like they were saying:

Hold on a mo... So this fluid that you want to keep is a few years old and been sat around unused for most of that time?

It's due for changing completely, anyway!

This is a common(ish) problems on Audis such as yours, but specifically the ones with the hydraulic brake booster and not the regular vacuum servo. The

100 (and 200) turbo had the hydraulic booster and cars like my ur quattro, but I'm not sure about your 2.3E? Anyhow the booster cylinder can cause the problem of binding brakes.

I think you are wrong to suspect that heat is causing the problem with the master cylinder.

Don't tell anyone that you can bleed a master cylinder without going near a bleed nipple - many chaps here struggle to open a bonnet and it's beyond their understanding :-)

Julian.

Nedavno Julian napisa:

Yes, it has hydraulic brake booster.

So can hydraulic brake booster be adjusted somehow, or does it have to be replaced? Or can I just put thin dividing plate between the booster and the master cylinder?

I think the problem normally turns out to be an internal within the booster cylinder itself, I have heard of one or two folk successfully managing to reseal leaking boosters, but I can't help you with seals etc. If you pop along to quattroforum you may find some more advise from people who have dealt successfully with the booster problem, but it is a pretty rare fault.

A new booster may possibly be sourced from Pete Reeve at quattro corner:

Many Audi parts are common across the range. You'll be really struggling these days at the dealers, stuff for your car is obsolete and getting tricky to find! Julian.

Nedavno Julian napisa:

Here?

Thanks for the inf, I'll check further.

Dunno about modern things, but it's perfectly possible to change the master cylinder on a Herald and then bleed just one front brake - everything after the master cylinder stays full during the change. Of course it's a bit daft not to bleed eveything while you're there.

Ian