VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc 55 brake problem

hello, i am looking for help with my 55 chevy 210. i have a 1981 pontiac t-am rear with factory disc brakes. same in the front. of course power and an LT-1 350. my problem is i have no real braking power. i push the pedal and it travels 5"(or so) before the brakes apply, then it dosent stop(it feels like very poor manual brakes). i have been a mechanic for about 10 years and this has me very stumped. i have replaced the master/p-valve/all four calipers/pads and turned rotors/and lines.most not for above reason i.e. lines pads rotors etc... but still. i have adjusted the travel rod to 1/8" from the (also new) booster. and checked the vacuum supply and check valve. also all parts are for a 81 pontiac to assure compatability. the car is allmost perfect in all other aspects. PLEASE help any answers or comments would be a great deal of help. thank you

today i found that the rod should be .025 from the master piston so i readjusted and no go. BUT i did put a preasure gauge on the master and found i only have 200psi apposed to the needed 800psi. so i have narrowed it down to master/booster. i pluged the master and applied the brake, the pedal traveled two inches and hit the master and stopped. no leak down, engine off. where is the two(or so)inches coming from????? i checked the geometry of the pedal and found a respectfull 4.1 ratio. and i double checked the linkage and rod adjustments. also measureed a 1" sweep of the linkage a 1"sweep of the booster rod and a 1" depth of the master piston. argggggg!!! so two questions i need two answer but cant.....

where does the two inches come from?????

why only 200 psi??????????

please help

Has the m/cyl been detatched from the booster and brake pedal pressed? If so a shim inside the booster may have fallen out and therefore vacuum not being applied. Also check the depth of hole in m/cyl piston and length of pin from booster. If excess clearance there will give excess travel at pedal.
Only silly thought but can happen

i did press the booster without the master, but i can defently tell vacuum is applied with engine running. all though i do not recall any shims. ???? and i have a very spicific .015" between the booster rod and the master piston as recemended by p.b. specialist for my current aplication. thank you for the help thou.

sorry a spec of .025" from rod to piston. my bad

the shim I mentioned is inside the booster between pedal rod and the output rod and falls down inside the booster. Cannot be seen from outside.