My son was trying to help me work on my '68 Chevelle last night. While the car was running, he pulled off the master cylinder cap to check the brake fluid. After replacing the master cylinder cap, I now have a very high pitched noise that seems to be coming from the master cylinder - booster area. Any ideas? Could the gasket be faulty?

Thanks.

Sounds like a vacuum leak. Lightly spraying the connections, seals or wherever you think the sound is coming from with water might help you find it.

Check the brake line vacuum check valve where it pushes into the booster.

Thanks guys. It sounds like a vacuum leak. I will check the vacuum check valve. I need to change out the booster/master cylinder anyway since I have a leak between the two. It also gives me the opportunity to put a chrome one in!