Sometimes, if my '83, 4 wheel disc, Crossfire sits for a while, I have a mushy brake pedal right after start up. It's like I need to push the pedal to the floor to get any braking power. After a few minutes it's goes back to normal. I suspect that this is vacuum related. Could a vacuum leak cause this? Should I be suspicious of my booster?

What do you guys think?

What is your vacuum at idle? Normally, it should be over 10" of vacuum. Any lower than that the vacuum brake booster will probably not work very well.

How to test a vacuum booster:

With the engine off, pump the brakes till you bleed off all of the vacuum in the booster (usually six or eight pumps will bleed it all off). Pump the brakes up and hold them. Start the engine. The brake pedal should immediately be pulled down towards the floor. If not, you either have a bad brake booster or not enough vacuum.

Did it work? Was that the problem?

I sold my house and haven't moved into my new house yet, and the Camaros are in storage. Haven't tried it yet, but it's on my list when we get to our new home.

Just tested it as you instructed. Normal.