i have to say that i HIGHLY doubt the problem is air in your brake system. even with air in the system the brakes still should perform very well at least much better than what you are describing. I have a honda crx which currently has a problem where the brakes engage on me on their own and the only way i can drive it is by loosening one of my brake lines either at the master cylinder or at the proportioning valve as to let brake fluid leak of and alleviate some of the pressure in the system. now even when i loosen the the brake line and leave it like that for a week my braking is still great and im sure that it acquires lots of both air and moisture. so for your problem to be caused by a slightly loose master cylinder cap i find very to believe. my bet is on the proportioning valve. you are supposed to buy them new from the dealer but i would just go to the junk yard and pluck one.