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Hello I will help you with your question,
The discs require a larger volume of fluid to fill and bring the pads into contact with the rotors than the drums do.
You installed as a kit or is this system pieced together?
So the rod to the pedal was left in the same place?
Was the rod between the master and the booster adjusted or just left the same?
is there a kit number?
Is there a kit number? I don't see anything on their site for this vehicle.
At the least you need to adjust the rod out of the booster so it has just a thread or two of slack but under no circumstance should it press on the piston without you pressing the brake. The other thing is the leverage you have to operate the drum master. The rod may be 1/3 of the way down so you get a 3x force multiplication when you press the pedal. Great if you are trying to stop with drums but for the discs they have better clamping and you can move the rod down to the next hole (some B bodies had discs so the holes may be there) and get further stroke with less pressure applied to the booster.
Let me know if you can tell me what was installed so I can see where the problem might be.
Did you use the steps from the Right Stuff site to bleed the system and check pedal height?
If you can pump up the brakes and they feel normal then the pistons are being kicked back from rotor run out or you still have air in the system.
Click here for link
Let me know what happens when you clamp the rear away from the front brakes.