Vacuum reservoir for power brakes

TRWRacing said

I’m not sure how the pedal should feel. I’ve only driven modern cars with power brakes. My first car (‘71 Demon had manual drums).

I do know the master cylinder and power booster were replaced a few years ago with NAPA parts. I just assumed 15” of vacuum was low.

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15” of vacuum at idle really isn’t low. I mean, it’s probably a couple points lower than a 100% bone stock engine in good tune but it shouldn’t cause any brake booster issues. Typically you can run down to about 12” before the booster starts having issues.

And even then it wouldn’t be constant. During low vacuum situations it might come through as a lack of assist, but during high vacuum situations it would be normal. So, if you constantly have low boost assist you probably have something else going on.