power brake t/s

[chevyhouse]

i just installed a disc and power brake conversion on my 69' and we are having trouble with the booster we think. with the engine off, the pedal is firm and returns like normal, with the engine running (17-20in vacumm) press the pedal and it stays down, has to be pulled up. the entire system has been bleed and checked, the disc brakes worked fine before we did the power booster. any ideas, i need to get this truck back on the road soon. thanks

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Kurt
94' Camaro
85'Plymouth Horizon
69' C10, 350/4spd, lwb, fleetside long box, 3.5"/5" drop, power and disc conversion
early 80's Schwinn 18 spd