I want a PRXB for my jalopy and already have air onboard for the air bags. It's just the standard cheapo compressor that comes in the kit along with a 1 gallon tank. Will I be able to run the brake from this?

 

I do not see why, air is air under compressed condition it all blows.

 

Find out how much air the brake consumes per actuation then estimate how many times in a given time period you will use the brake. You should be able to figure out if you have enough air. The other thing to take into account after you figure out the above, is how much will that consumption make your existing compressor run. I would be willing to bet that the compressor for your suspension is only rated to run a very small duty cycle. If you run it a lot more often it will wear out quickly. The numbers will tell the story. Post up what you figure out.

 

I have a PRXB, it comes with a small Viair compressor and a little tank, I'm thinking you'll be okay.
How much are you saving buying the ex brk without a compressor? Where are you getting it? I live 15 minutes from Pac Brake and got mine cheaper from Lary through the DTR store, saved over $400.