Ok, I know when running a performance cam, you may have vacuum issues....hasnt become a problem but something I notice why sitting at lights at idle, is my pedal will slowly loose pressure. They still work well, I just loose some pedal. As soon as I increase the rpm's the pedal becomes solid again....I dont have the specs on my cam, But i can tell ya it is very lumpy. It wants to Idle around 1000rpm in neutral and in gear lopes hard at 700rpm.
I was thinking master cylinder, but the condition has not changed since my swap....my brake fluid level stays full, Ive never had to fill it. Any Ideas.
Also my cheap uncoated headers are very close to my brake lines..(due to v-8 swap) could this cause the problem?

 

You may want to inspect the vacuum check valve on the brake booster. Replacements are only a few bucks, and a leaking valve can cause loss of assist as you describe.

 

thanks Vader, I'll do just that...