I recently replaced my brake boosters on two cars for the following reasons.
1. Brake pedal was rock hard.
2. When I applied the brake I could hear a vacuum leak
3. With the brake applied, I had an engine misfire and the RPM would increase slightly.

After I installed the other booster, purchased from one of our reputable members, everything was fine for about a week. Now problems 1,2,and 3 stated above is now happening again on the carburetted vehicle. Is there something else that could be wrong? Does the check valve have anything to do with this? AFAIK the check valve is there to maintain vacuum at the booster. All vacuum lines are tight. No leaks there. Do I need another booster or is there something else I should check first. The second car is ok so far
Thanks, Frank

 

Sounds like you have another booster failure. All your assumptions seem correct. Check the seal in the back of your master cylinder. If it's weeping brake fluid it may be contributing to the booster failures.