I didn't have this problem with my unit, but I followed Paul's bleeding instructions to the "T." I recall that they were pretty involved, having to start the car, shut it off, check the fluid level, start it, turn the wheels slightly, pump the pedal, etc. It was quite extensive, but it seemed to work in my case. My guess is that if one tries to do this too quickly that you'll introduce air into the P/S fluid. If this is the case, you'll have to wait quite a while to let it settle out before you can go at it again. Call Paul at Hydroboost if you can't figure it out, he is a HUGE help.
69 SS, ZZ430 Look-alike, Tremec TKO, Hotchkis Suspension