Hey guys, background info:
95 Formula, 135k miles, a4, 4 wheel disc abs system with line lock installed on front brakes.

When in gear either in the air, in snow, or on the dyno, my rear brakes can not hold the wheels still (you can hold the brake pedal to the floor, and wheels still spin.) On dry ground this problem does not exist. When driving on the dyno it can slow down and stop the wheels from 60 mph (you have to shift into neutral after 5 mph, this is on an inspection dyno btw.) This was a problem last year in the snow, I would have to shift to neutral everytime or the car would just continue to slide- with the pedal on the floor, the rear wheels would spin and the fronts would slide while locked on the slippery ground.

My thoughts: given that the rear brakes work, just do not have enough pressure to hold the torque of the car being in gear, I used the brake flush machine at work (on all 4 wheels.) Got no major bubbles out of the rear, no obvious dirt particles, and seemed to have a good flow. Problem still exists. Rear pads were changed, did not cut the rotors at that time, but I can not see the rotors causing what seems to be a pressure-related problem.

I'm stumped on where to work next without just throwing money at the car. Do I replace brake hoses, calipers, or the master cylinder? Not sure where to mention this but the pedal has a somewhat good feel to it, could be a little stiffer, but nothing feels so terrible about it. Could the abs have gone bad (I have no lights on?) Are there any other proportioning valves or metering valves in the system? (I know one of those valves are used on disc/drum systems but not sure which one.)

Looking for any ideas to help me save some money that could be better spent before I replace unnecessary parts. Thanks in advance

 

Did you ever figure this out?

 

Nope .... the other day I changed the pads and rotors though, I can try to test it in the air again. Other than that I'm working on putting an m6 in the car which will solve the problem