I have a brake problem which I thought was a vacuum leak,but can't find one.I have power brakes yet every so often the brake light goes on and the brake pedal goes further down before engaging. Is this a sign that my booster is on it's way out then? If it was anything else I would leave it for a winter project, but brakes are too important to mess with. Can anyone help me with this? Thanks.

could be a fluid leak somewhere or a bad master cylinder. booster wont cause the light to come on.

When I had that it was the piston leaking in the master cylinder. Sometimes conditions would cause it to leak internally between reservoirs when the brake pedal was applied and sometimes not.

Has the master cylinder's been emptied at any time, or the level of fluid been low enough to permit air into the system? From my own experience I know that it's almost impossible to get the air out of it without bench bleeding it.

I'm with Scott & Jim c on this one. It's a master cylinder problem. Good news, they are cheap and easy.
Jim

Thanks for all the advice guys. I will check all of your suggestions and if need be buy a new cylinder then.
AL