When I release the brakes it is air between the disk and outside pad on the little diameter end, but full contact on the big diameter end.
I checked the push-rod length, and it seems to be ok.
HOW did you check pushrod length? There are TWO pushrods--one from pedal to booster, one from booster to master cylinder. The one from pedal to booster should be in the LOWER hole in the brake pedal.
When you have a helper push the brake pedal (less than one inch) there should be TWO "geysers" --one in each of the reservoirs--as fluid is pumped out of the compensating ports of the master cylinder.
That assures that the pushrods are not too LONG.
If the pushrod(s) were too SHORT, one inch of brake pedal travel might not provide any movement at the master cylinder, and you'd have no geysers until the pedal was pushed farther. Or, you'd have problems adjusting the brake light switch because the pedal was already too low.
Any chance you've got a master cylinder with a deep pushrod socket, and a booster/booster pushrod that's intended for a shallow pushrod socket master cylinder? You'd maybe have an inch or three of brake pedal travel that provided NO stopping power, then you might feel the brakes engage as the pedal dropped farther. What brand of booster do you have, and what brand of master cylinder? Delco master cylinder goes with Delco booster, Bendix master cylinder goes with Bendix booster. PHOTOS would be welcome.
Schurkey's Rule Of Thumb for brake pedal height: The brake pedal, on a NORMAL, ORDINARY GRADUAL STOP should STILL be HIGHER than the gas pedal in it's "idle" position. Level with the gas pedal is somewhat acceptable. Lower than the gas pedal is unacceptable. The difficulty here is describing a normal, ordinary stop.
I have read a trouble-shooting section suggesting a bad power brake can give excessive pedal travel on Mopar cars.
Is this possible on GM cars too?
Not that I've ever witnessed. I'm prepared to learn something here, but so far as I know, a defective booster can cause a HARD pedal, but not a LOW pedal. The pedal may drop a little bit as vacuum is applied to a booster that's been drained of vacuum--but the pedal travel isn't that much.