Replaced everything but the master cylinder, brakes are bled perfect, wont hold any pressure in pedal. It will build pressure if pumped but wont hold it. Thoughts?

 

Whats everything? And why havent you replaced the master cylinder, thats a very probable cause.

 

brakes booster, line going from booster to hardline(checkvalve included) line from hardline to motor, new, pads rotors etc.

 

Bleed 'em again, RR-LR-RF-LF. Sounds like either air still in the system or a faulty master cylinder piston/seal.

 

I agree..either Air or a faulty master..If you do it in the way that GO48 says,it will get the Air out./..Bleed the Hell out of it..Say 5 times a piece..

 

there is no air in the system. this isn't the first time i touched brakes. Just never had this problem. Ordered a new MC. thanks

 

If there is a general poor feeling to the pedal, then it is usually either air or something on the master side of the system.

If the booster holds vacuum and the brake response doesn't feel erratic (touchy or non-linear response) then it's probably not the booster.