I purchased a 1990 20valve sedan for a winter cruiser, it needed some work, but that's what I'm good at, buying junk, dumping an a$$load of money into it then selling it cheap. anyhow, I had a brake issue where the pedal would hold a little pressure then go all the way to the floor. I figured air in the line, bled all the lines, still the same. now I figured maybe the prop valve was bad (it's mounted in front of the master on this car), bled that, no difference. next step swap a master in, bled all the brakes (master and prop valve) still the same. now I'm at a loss, I take a look at the factory plastic master cap and to my disbelief, it's blown out, cracked all the way around the top. I put the o.g. master back on with a cap from my //s4 and voila I had brakes. anyone else ever encounter this with older audi's?

$2 cap FTW. never seen it before, but i'll remember to check it now. i've seen a leaky gas cap throw a CEL though.

So a little round piece of plastic is now an integral part of your braking system? That doesn't sound good.

do you know something that every car manufacturer doesn't?

Maybe I am misunderstanding something, but I don't think the reservoir is supposed to be pressurized during brake application. It certainly doesn't appear to be designed to hold pressure!

every time you depress the pedal?

And no air should come in or go out.

You are talking about the screw-on cap for the reservoir, right? Where you fill the fluid?