on really hot days, my brakes act really strange. especially in stop and go freeway traffic, and even more so when i have the AC on in these condition... when i push on the brake sometimes, it will go almost all the way to the floor before slowing the car down, if i pick my foot up, then push it again, it will usually engage at a higher point... is this a dying master cylinder? why does it only happen when it's hot? and what does it have to do with the AC?

 

Brake fluid is only effective in a certain temperature range. Hot weather will make the fluid hotter, as will running the a/c. If your brake master cylinder is on its way out, hotter brake fluid will make the problem more noticeable.

 

so should i try replacing the brake fluid first, or should i just go ahead and replace the master cyclinder?

EDIT: and would i need just to replace the cylinder, or the brake booster too? (i'm not too sure how the braking system works)

 

Fresh brake fluid might work, but I wouldn't bet on it. When it happened to me, in only helped for a few weeks then the problem came back.

If the MC leaks alongside it's shaft, the brake fluid gets into the booster. That's not good for the booster, but USUALLY the MC doesn't leak that way. You won't know for sure until you take the MC off of the booster.

 

I had mine replaced when the same symptoms occured. was great afterwards. Stopped on a dime...well mayde a quater.

 

you replaced just the master cylinder? not the brake boosteR?

 

brake booster.

Pain in the butt to swap. I had a 10mm socket disappear, then one of the four retaining nuts. Gotta lay under the pedals. Frickin bermuda triangle for parts and tools.....
Not to mention you gotta know the secret trick to get it out of the engine bay. Hint: the cutout along the right after moving the booster away from the firewall is the exact size of the plunger that connects to the pedal. Those tricky Japanese....

 

so you think this is more likely a problem with the booster and not the master cylinder?