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I have a 75 F-250 that has been idling rough and stumbling periodically while driving, from what seemed to be a valve sticking open so today i opened up the suspected valve cover but all looked clean and in good operation. Then later i noticed that the brake booster check valve is busted allowing the vacuum line from the manifold to suck in air from the atmosphere. I didn't have time to go get a new check valve but could this be what is causing the problems, or should i keep looking into a bad valve?
Sounds like the check valve would be the problem.
The only thing I will add is to pick up a vacuum gauge if you can afford it.
One of these can point out very specific things.
Stuck valve, bad rings, bad valve guides, etc.
Life is so much easier when you can diagnose problems with some kind of tool.