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Hi All,


I have a 76 280z with turbo and a mild cam and recently have had a problem with power braking.  About 1/4 of the time, I will step on the brakes and they are rock hard.  This happens while cruising at light throttle, or after I've let off the gas and have ~15" vacuum.  I checked and replace the check valve, but still have the problem. 


If the brake booster failed, would it fail all the time or sporadically?  What else could cause loss of power braking - sometimes?



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A bad or stuck check valve in the booster vacuum supply hose will let the booster vacuum vary with intake manifold vacuum.  It would also allow boost in to the booster.  A boosted booster probably won't last long. 


A leaking booster will typically cause idle speed to change when the brakes are applied.

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My brake pedal was also rock hard some times and it seemed like it was only the front wheels braking. The problem on my car was the rear brake proportioning valve that was more or less blocked. I removed the valve function so it is just a connection point now and installed a external proportioning valve. That solved my problem, but perhaps your problem are different.


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