So check this out i bought this 04 G35 Sedan 2 years ago and its always had a brake problem the guy that i purchased it from said that the brakes just need to be bleeded which i thought wasnt gonna be a problem i bleed the brakes the next day and the brakes were nice solid for about 2 weeks then went back to spongey i check the pads rotors brake fluid everything is good so i bleed them again with a bleeding kit and once again only lasted about 2 weeks then back to spongey at this time i got desperate as my wife uses it as a daily driver with my kid so i take it to a local dealer and sure enough they check it out and tell me i have to change the master cylinder $500 for everything my dumbass pays it and again only last me 2 weeks went back to spongey so i go back and of course they tell me now that its my brake booster and it will be a $600 fix i leave the place laughing these dealers are ridiculous so i get a family friend mechanic i buy the booster get him to replace it i paid no more than $200 and the brakes once again go bad in 2 weeks and now im seriously starting to get really annoyed i dont wanna get rid of the car but im not gonna keep burning money on brakes so if anyone has experienced this and solved the problem i would really appreciate it thanks!!

 

Does your brake fluid get lower when it gets spongy?
is there air in the line when you bleed them?

 

what pads/rotors do you have?

is the brake fluid level always the same?

are the slide bolts greased (replaced my brakes the first time a few weeks ago, so just throwing out things i thnk)

 

Brake fluid stays the same i get little air bubbles when i bleed them.
OEM Nissan pads and rotors

 

Do you live near hills? Maybe your wife is riding the brakes excessively going down the hills? My gf had this problem with her envoy plus its auto so i told her use "3" instead of "D" and braking and waala no more fade. Or maybe one or more of the calipers could be bad causing to much pressure on the pads and heating up the fluid? This seems like a hit or miss problem much like with ac systems. You could try using dot4 or rbf as It can withstand higher temps which im thinking is causing the bubbles since the fluid isnt getting low. And you replaced the master and the booster so the only things left are the calipers and proportioning valves.

 

Upgrade to dot4. It sounds like you are boiling the fluid!