another disc brake question

I just replaced my d27 with a d30 with the thin rotors. I was running a 1970 chevy 1/2 ton truck booster and mc for drum brakes. i changed the mc to a disc brake mc. I have brakes with no drag, my problem is I have the spongiest pedals ever. I've bled them and bled them, no air. thanks

 

is it possable the calp. are on the wrong sides, upside down. if they are all the air wont come out when bled. just a thought

 

they are on correctly. I'm thinking that the master cylinder is the wrong size piston.

 

What MC do you have on it now? I read that you put a disk brake MC on it? Chevy truck disk brake or jeep disk brake? Wish I could tell you for sure piston sizes but I can only assume. Did you bleed the rears also? Do you have an equalizing valve installed? Residual valves are in the disk brake master cylinders right? Geez, guess I have more questions than answers.

 

A few things come to mind. First, improperly bled brake system. You start with the furthest brake from the MC and work your way to the nearest. Second, no 2lb. residual valve installed in either MC or front brake line. Also, rear brakes not properly adjusted. There should be some light drag on the disc brakes.

 

It's a mc for a 1970 chevy truck with disc brakes, I've bled and bled them, I don't have any drag on them, I guess I might not have a 2lb residual valve, the brakes work good they just go way down to work.

 

Double check what Posimoto said about the rears not adjusted correctly. If they aren't you may never get all the air out or may continue to get a spongy pedal. Nickmil

 

I'm not familiar with the Chevy MC you are talking about, but it is possible that Chevy used an inline residual valve for the fronts and that there isn't one installed in the MC. Just guessing though.

 

Did you bench bleed the new disc drum master cylinder? Also check that the pushrod is the right lenght.