well I just want to know some signs because I just change my master cylinder, after that my brakes where nice and stiff, after two weeks or so I was going 65 on the highway (not street racing, street was just emty) then I had to take my exit so I press the brakes to slow down and I felt my brakes loose preassure, then after I started driving on a 25mph street the brakes just sank with no preasure at all I pulled over and pumped the pedal and nothing. I waited like 5 minutes and then I slowly pumped my brakes again and it got the preassure back. other two weeks or so passed and same situation driving on a highway brake for slower traffic, again my brake lost preasure I dont know why it does this, it seems like it does the same thing everytime Im going fast and slow down 2 weeks or so pass by and again it happens thank you for your help.

I bench bled, bled the brakes individually in order like the manual shows so I dont know what else to do and I dont want to replace the booster if its not bad.

 

The symptoms don't fit with a bad brake booster. Is the brake fluid low in the reservoir? Look for a fluid leak around the master cylinder and rebleed the system. It's also possible the new master cylinder is bad. Was it new or rebuilt?

 

maybe air in your break lines!

 

check if your fluids ever run low cuz you mite have a leak between your lines. or mite just be air betwween the lines.

 

the reservoir is full and the master cylinder is new never used Ill try bleeding the brakes once more to see what happens I though when the master c. is bad the pedal keeps sinking but mine I wait a little and pump the brake slowly and it get the preassure back like new stiff and brakes good but a period of time passes (arond 2 weeks all the time) and if im going fast and brake the pedal sinks after

 

interesting i had something similar happen ended up being the caliper boot leaking... worth having a look other than that check your brake lines from the master back... but having air in the lines is definately common also