squishy disc brakes

[oneIwilly]

Hi, I recently came into a AAM 11.5 rear disc brake axle from a 2008 chevy duramax. The wheel mounting surface dimension was perfect for my 1976 chevy scottsdale, and the lug pattern is the same 8 lug. So I pulled out the 14 bolt full floater, and moved the perches and shock mounts on the 11.5, to match. It is under the truck now. I just finished the brake lines, and im waiting for my new driveline. Heres the problem. I have bled the brakes three times, always starting farthest from the MC. I did it rite. When I start the truck and hit the pedal, it feels like im stepping on a marshmallow. I cant find any info on the bore and stroke of the duramax MC (I was going to try to calculate the volume it would push). Or do you think I need to steal the proportioning valve off the duramax? Any help?

[Captkaos]

Did you get all the air out?  The discs generally require a different combination valve, but they shouldn't be squishy.

[Fairlane514]

I agree with the combination valve. There is also a difference between a master cylinder for a disc/drum and disc/disc.