Master cylinder shot?

Brake pedal super mushy, almost to the floor. Endless bleeding and no air coming out. We think the master cylinder is shot. Can’t find any replacements in NorCal. Is it OK to substitute the RS5 master cylinder on a 6 speed S4?

EDIT: looks like the RS5 is not a direct replacement, as it has a different pump.

I found a salvage 2012 S4 with a master cylinder for $50. Then a few minutes later we located a new replacement master cylinder from ATE. I assume this is not OEM as it was only like $140.

My takeaway is that the big brake kit, really hot fluid over 6 years of track driving (52 days on track), and stainless steel lines contributed to the blowout of the seals in the master cylinder. This resulted in a feeling of mushy brake pedal at low speeds even in a cold car. The brake bleeding on the pad change last month may have been what finally made the seals give out.

I know you are not a fan of vendorzine but check this out:

I’m a fan of AZ. I think the problem was they were not a fan of me?

I think it’s a mistake to change the piston diameter of the master, if you have not changed the diameter of the brake pistons. Even if you change the fronts on a B8 to RS5 calipers, the rear remains the same.

What a cluster. Part number -012B is not the same cylinder diameter as 012A. I needed the ATE built part; some are built by TRW. Massive delay. Trying to get on track this weekend.

Hello westwest888,

I know this is the S4 Side i recently bought a complete Audi S8, V10 2007 Model from an auction to use for spares
I am in the process of extracting the engine from it after that i have a full car to use for spares.

please check out this link

It has big brembo brake calipers, brake pads, rotors, brake cylinder, alloy wheels (The whole shebang).

I don’t need them my car has ceramic brakes. so how about you take the whole shebang and you Frankenstein.

your ride. i would like to help you get your ride up and running, and offcourse i will give you a very good low price.

Send me a message if your interested.


I’m all set, thanks. The car is back in operation with 3 hours of labor and a $140 part.

FYI the RS5 master does work but you need the booster as well. I had the RS5 front brakes on my B8 along with the master and booster. The rear calipers are the same minus the RS removal badge. Brakes worked like a dream.

Great information add. member bump for you