I just finished up re-building the rear brakes on my 82 z28, the problem in the bigging was it pulling really hard to one side and upon romveing the drum i notice one wheel cylender was side ways almost pointing up, well i think the backing plate was rusted out so i replaced some stuff....

new lines from the split in the center of the rear end, new backing plate on one side new wheel cylenders on both sides, and then shoes and hard wear...

I get a firm brake pedal when the cars off but when i start it gets all soft and brake light comes on.

 

no one can help me? how should i go about checking the brake system to find out whats wrong?

 

Bleed the brakes?
Got fluid?
Master cylinder get air in it? (even once?)

 

Will i have to bleed the brakes while the cars on or off? We did it with it off but it didn't seem to help

 

No, it doesn't need to be running. Sounds like you got air in the master cylinder, that's why your'e getting the brake light. You will need to "bench" bleed the MC first, then the rest of the system. Do a search, it was addressed here recently. And buy a book, they come in handy.