I have a 2001 honda accord i just bought and the guy i bought it from said it needed a brake proportioning valve and i replaced it and the brakes have great pressure when the car is off but then the pedal goes to the floor and has no pressure and then when i let go of the pedal it has pressure for a second and tje loses it again when i press the pedal all the way

 

sounds like the master cylinder....possible wheel cylinder . brake caliper, make sure you bleed the brakes , all air out , if you have to pump the brake pedal a few times to get any brake but still goes down then my first thing to check would be the master cylinder

 

Welcome in,i do believe its the Master, typical problem is brake fade.

 

I have seen people replace the calipers and do it wrong
In their hurry, they put the wrong caliper on the wrong side.
This causes the brake bleeder to be on the bottom and it can never get all the air out of the system
Would check for this. Then would check for any leaks at the calipers and hoses and wheel cylinders
If there are no leaks then master cylinder is suspect
If it is the master cylinder then you will have to bench bleed the MC before installing

Good luck

 

I bled the brakes really good and there is no leaks anywere and the brake pedal is hard when the car is off when i pump it up and then hold it to the floor and start the car it loses all pressure and then i let go of it and it builds pressure again and then when i press the pedal it starts to apply pressure and then it loses everything i thought maybie it is the booster

 

loss of booster while engine running will result in a manual brake, hard to press, but will not go to the floor, master cylinder bypass will cause a step on brake with engine running to go to the floor, sometimes slowly sometimes fast. (providing there are 0 leak of the fluid, and ample fluid in the master cylinder reservoir)

 

There is plenty of fluid,at this point im pretty sure it is the master cylinder thank you