I order a front disc brake kit from CPP the install was easy. I'm using a brake booster from a 78 K5 blazer and the same MC. When the engine is off I have good pressure in the pedal.When I started up the truck the pedal goes soft and all the way down to the floor.:confused: Need help....can't drive without brakes.

My guess would be to do more bleeding.

If it is using a booster, i would check the vaccum lines for a leak or check what kind of vaccum your cam uses.

A bad booster will make the brakes hard to apply.A pedal to the floor is air in line,bad MC,leak in the brake lines,wheel cylinder,caliper.If you know the MC is good (did you bench bleed it ?),I would bleed some more.Tap your calipers to bring air bubbles to the top.There is a pin on the front of the MC that needs depressed while bleeding.

You need a MC with a larger bore. If you converted from manual it would have a approx. 7/8 bore so the pedal force on the wheel cylinders would be sufficient. With power disc you need a larger bore to push more fluid into the disc cylinder. You will need a MC for Disc front / drum rear one from a 73 -87 should do fine. You may also have to adjust the pushrods. One on the booster and one from the pedal. The one in the booster should be ok if it from a 78 because it had Disc from and drum rear. There is also a dimensional difference on the inside of drum and disc MC's where the push rod goes in. one is deeper that the other. I don't remember which is which. But mate the 78 booster with a 78 MC and you should have nothing to worry about with exception of adjusting the pedal rod.

Thanks for the help guys, that will be my project for the weekend.