wvudelsol

I've been having a problem with my 94 del sol si on and off for a while now, and it is finally starting to really bug me. When the weather is hot(above 80 or so), my brake pedal slowly goes to the floor when i'm sitting at a stoplight. I think it could be the master cylinder, but i don't understand why it only does it on hot days. Is there a seal somewhere in the brake system that could be expanding when it's warm out? The brake fluid is full, so it's not leaking any fluid. Does anyone have any ideas/explanations as to what is going on with my car? Thanks.