Brake problem

i have a 88 wagoneer xj, but having a brake problem, i have ok pressure with jeep off, but when i start it i have no pressure, i replaced the master cyl. and brake booster from junk yard and still nothing, i have new rear wheel cylinders, pas. front caliper, and extend brake lines. i bled them so much, and even gravity bleed them and still noting.. any ideas?

anyone? i really need to get them fixed

double check the master cylinder u got cuz it may have a bad seal and still leaking into the booster also check to see if caliper piston seals are good pull back the first boot and see if they are leaking cuz when u start the jeep u get a lot more pressure in the system

piston seals are good and they move and it seem that the master isnt leaking, i might have to look at the rear brakes and adjust them or something