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Brake Master Cylinder Help

I have an older car ('71 Nova). I “upgraded” from manual brakes to a brake booster, and now cannot get pressure to the rear calipers. Front brakes work fine, no no rear brakes at all.

The master cylinder is a standard GM 2 reservoir (a “Corvette” style) that bench bleeds just fine. The lines are open and I get brake fluid to both rear disk brakes. When I pump the pedal to bleed the brakes, I do get fluid out but no real pressure. On the bench the MC squirts just fine out of both ports. I have set the booster/MC rod to spec (3/32" as I remember).

I’m really frustrated! :frowning: None of the brake shops around here have any ideas and don’t want to work on an aftermarket system. I’d like to keep the booster but am about to give up and revert to manual brakes!

Please…if you have any ideas, pass them along!


Maybe clogged rear break line?

Usually when converting from manual brakes to power assisted brakes, the proportionating valve requires replacement. Was this done?


i believe tester has jogged my memory.
there is one proportioning valve for manual brakes, and one other for power assist brakes. but you have changed everything, changed drum to disk, and changed MCs too. you may need to find a couple of chevy proportioning valves to find one that works.

as an alternative, remove the proportioning valve, to see if it changes the way the cylinders. (but put it back when done)
you said rear calipers?? disc on the rear? on that year? or you changed that too? this may have required changing the proportioning valve too.

A 37 year old car should NOT be using brake hoses more than 10 years old. Old brake hoses deteriorate inside and won’t let brake fluid pass. Replace.

Thanks for your suggestions! Yes, I have tried it with and without a proportioning valve. Doesn’t make any difference. The brake lines are clear, and all of them are hard lines (steel tubing) except the flex to the front calipers (new) and to the rear axle (10 years old at the max). You can see why I am so frustrated.

When you step on the brakes, do the front brakes go on first or do they just apply more pressure? I’m still wondering is there could be a mismatch between the booster and the MC.

If you had manual brakes, then you must have had all drum brakes to begin with. Your post implies that you now have disk brakes on the rear. If thats so, what is on the front? If you have drums on the front and discs on the rear, you should change the fronts to disc as well. At the very least, you will have to have a proportioning valve on the front brakes, not the rear.