Im having trouble bleeding the bakes on m 1970 3/4 ton chevy pickup. i have pedal till i start the truck?

(2011-01-26 15:15:10) 1. Wookie's Mama Says: i just replaced my master cylinder and i bench bled it to the best of my abilities, i have new drums and shoes all the way around it has power brakes2. Kenny D Says: You still have air in the system, either in the master cylinder or in the brake lines. When you start the engine the brake booster can apply more pressure and compress that air than when the engine is not running and for that reason you can have a rock hard pedal with the engine off but then the pedal goes to the floor with the engine running.3. clayton Says: Bleed from the longest to the shortest brake lines. Get help, have some one pump the brakes while you work the bottom side.4. WilliamG Says: You did something wrong. Let a brake man do the job correctly. Tag: Im having trouble bleeding the bakes on m 1970 3/4 ton chevy pickup. i have pedal till i start the truck?