just replaced my master cylinder today and the brakes are still very poor, they need to be slammed all the way down and still only slowly stop. Also when the car is off, pressure never fully builds up, whats else could be wrong? Im guessing power booster or proportioning valve. Help please.

info on car- 1984 l69 trans am, 700r-4 and rear discs/3.73's
110k on car

Sometimes depending on the crankshaft u got in your car.
If its a big time performance crankshaft it takes away brake power.

But I dont think thats your problem, but ive heard about this.

the 305 is still stock and its perfomance camshafts that will rob your vacuum, any other ideas? Anyone?

Well...
First things first

1. Make sure the calipers are assembled correctly
- The pins should be clean straight smooth and lubed
- the o rings need to be fresh (and lubed)
- There needs tobe at least .005 in clearance
between the spindle ears and the caliper itself

2. The brake fluid should look like a pilsner not a stout
( like miller light not like guiness )

3. Bleeding
This is what I think may be the problem.
I am a "true believer" with respect to power bleeding.
My suggestion is that it is difficult to adequately gravity
bleed an assembled system. I would either get one of
the power bleeding tools (phoenix, mity vac or some such)
or take it to someone that has one.

Sounds like your brake booster is failing. When you first get in the car is there any reserve? The breake pedal should operate about 2 or 3 times with assist without the engine running. If the pedal is stiff the booster needs to be replaced.

Here is a simple test you can perform to test your brake booster function.

1) With the engine off. Pump the brake pedal several 5 or 6 times.

2) Hold the brake pedal on the floor.

3) Start the engine.

4) If the brake booster is working properly the pedal should move slightly closed to the floor.

The newest of our cars is already over 10 years old. So replaceing the brake booster and master cylinder is cheap insurance.

Thanx,ANDYZ28