71 442 build. Brand new correct master cylinder (NAPA) and booster (Parts Place). When bolted in place using original short push rod (wire brushed it and lubed it) the front brakes are stuck. If I back the mounting nuts off a few turns on the master cylinder they release. Bench bled master and bled wheels after install. I had a brand new CCP master for my 69 Chevelle laying around so I swapped out the master cylinders just to try something different and with the CCP unit brakes work fine ??? Can't figure out why short push rod seems to be too long on the correct Olds unit ? Any ideas ? Thanks

 

I'm going to guess that the "correct" M/C isn't.

Clearly the recess in the piston in that new M/C is too shallow. The first thing to do is to compare the depth measurements on the two M/Cs. To be really correct, you need the dimension from the M/C mounting flange to the bottom of the recess in the piston. That's a little difficult to get, so you'll need to measure up from the flange to the face of the rear boss then down from there into the piston recess.

 

Thanks Mr P. I have the original MC and have measured the depth of both recesses and they appear to be the same. The aftermarket is a delco remey, well at least its marked as such and appears to be a duplicate of the original. Push rod about 1/8" too long. Could I have been sent the wrong GM booster ?

 

If the new M/C is brand new and not rebuilt, then the only other possibility is that the compensation port inside the M/C was drilled in the wrong place or the piston seals were assembled incorrectly such that the piston for the forward brakes doesn't fully uncover the port when you release the brakes. The 69 M/C and the 71 M/C should be interchangeable as far as the power booster is concerned, so if the pushrod works with the 69 it should be fine with the 71. Can you tell if the pushrod is actually moving the piston in the M/C when you bolt it in place? Is the pushrod to the brake pedal properly adjusted so there's no preload in the booster?

 

The MC is new from NAPA but I need to be sure on that so I will check today. I had previously rebuilt the original MC that came on the car. (OEM with casting #'s). This same scenario occurred so therefore I purchased a new unit from NAPA. Yes the pedal is in what i call a neutral position (no preload). I assume the push rod is engaging the piston when I tighten the bolts. I have the front end slightly off the floor so I can check to see if front wheels turn freely or not. When nuts are tightened on the studs for MC the front wheels are locked tight. As I said when i loosen them a few turns they free up. I'm beginning to thing the "new" booster is the issue. I'm headed to the garage now to work on it some more

 

BUT doesn't the 69 Chevy unit require the long push rod? If so why is it working on the Olds with the short rod ?