losing pressure on steep hills?

[5.0fox]

I'm having a weird brake issue in my 85 b2 that I can't seem to figure out. While climbing or desending steep hills I am losing all brake pressure. I'm not losing any fluid and the brakes are bled and work great while driving around town but as soon as I go up or down something steep I have to pump the pedal to build all the pressure back up. The bronco has explorer diffs and a. Brand new master cylinder.

 

[Vanner1993]

Maybe the master cylinder is not properly bled.