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.