whats going on with my brakes

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ok so ive been having a bit of a problem with my brakes lately. identical problems in two different cars.

first car is my 91 crx si. i used to have stock brakes with just a 15/16 MC, stainless braided lines, hawk hps pads, and valvoline synpower brake fluid. now the problem i was having was primarily only seen at the racetrack (1/4 mile). after a few runs the brakes would feel like they were fading. while sitting still or moving the brake pedal would just sink to the floor. it was slow, and i still had some pressure, but i would just have to keep pushing the pedal further til it hit the floor and lost almost all pressure. im losing absolutely no fluid anywhere, ive checked and ive kept an eye on my fluid level. my 95 integra LS is having this EXACT same problem with the same symptoms, again no fluid loss.

my first instinct tells me that im just overheating the brakes and its causeing fade and the fluid to boil. so 2 days ago i upgraded the crx's brakes to 10.2" rotor with type r calipers and hawk hps pads again. it was my understanding that this would reduce the amount of heat being generated and help fade (i really should have zero fade with this setup). so i did that, bled the brakes and i thought everything was fine. but today i took a trip to the bank and sure enough after about 30 mins of driving the pedal started to sink again. i have yet to bleed the brakes on the integra cause my brother has it.

since i have no fluid loss to me this says its the fluid boiling again, but 1 i have bigger brakes now, and 2 i JUST bled the brakes and i cant see the fluid boiling this fast. im not even braking really hard or really slamming on them, just driving normal. this really only seems to happen when ive been driving for a while and when im braking frequently. if ive been driving on the highway with very little braking its fine. so do i just need better fluid? or what? the valvoline fluid has like a 500 degree boiling point and i dont think ive gotten the temps up that high since when i get out of my car i can touch the brakes without severley burning myself (it is slightly hot though, as expected). or could this be something else going wrong in the brake system?

remember im having the exact same problem with 2 different cars on 2 different brake setups. both under the same conditions, with the same symptoms. maybe the MC?

any help is greatly appreciated.
my first guess would be your master cylinder or brake booster....either one of those situations or you have air in your lines somewhere..have you ever lost all fluid in mastercylinder i.e. replaced with a new one...if so you need to bench bleed it or these same symptoms will occur.
yea i replaced the MC in the crx. (it doesnt come with a 15/16 stock) and i did bench bleed it. i know theres no air in the lines. cause when the pedal isnt sinking, its very solid, no mushiness.
If you are boiling the fluid and it doesn't sound like it. There would be air in the lines on a re- bleed.
I am a rookie but what about the vacuum to the booster. There is a check vavle to hold the vacuum in the booster. Try checking the vacuum?

Leaking orings in the master cylinder will also let the pedal go to mush....

can you pump the pedal back up?
the only reason i dont suspect the booster is because the pedal would be stiff as hell if it wasnt getting enough vaccuum, and my problem is the complete opposite of that. im starting to think its the MC