hydro boost brake system on 1991 Audi Coupe Quattro

I have a problem with the brake system I replaced the brake accumulator and the master cylinder, but the brake pedal is hard and difficult to stop, I haved to press hard and lock the brakes up, so vibration from the anti-lock system occurs. This is a hydro boost brake system so the boost comes from the power steering.

by in Clinton Township, MI on June 11, 2010
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1 answer 1 comment
ANSWER by , June 11, 2010
I would have a pressure output test done of the hydraulic pump, just to be sure that there is enough pressure being generated. Does the power steering boost feel like it takes the normal amount of effort? How are the proportioning valves and metering valves. There should be a step by step inspection procedure for this system in this manual site, http://bit.ly/mitchell_repair_manuals_online
COMMENT by , June 11, 2010
I bleed the entire brake system including the proportioning valve everything seems right, my next thought was the hydraulic pump. But what if its a booster problem, Im use to a vaccum booster or check valve causing a hard pedal with very little stopping power, would it be the same for a hydro boost system. I checked the manual there are test procdure for the hydraulic pump and the accumulator, but I am new to this system and no to confident on my ability to perform these test. My first reaction is to through parts at it.