Outer Limits 4x4 Board

I've got a 04 gu leaf/coil ute and recently i've been getting a brake failure but its only when im 4wdriving on a steep hill. Basically when the car is on the hill an i reach for the anchors, the pedal goes spungy and straight to the floor. If i pump them quickly i will get very minimal pressure back but its not until the trucks on the flat again that the brakes fully return.
Ive bled them, there is no air in the lines.
Any ideas? any help would be greatly appreciated.
Thanks
Jake.

Maybe its not generating enough vacuum?

Maybe your vac booster is leaking?

Have you checked the front wheel bearings for excessive play - the discs will push the pads back into the caliper thus making you pump the pedal to get the brake pad back on the disc , if it was the booster the pedal would be rock hard but not going to the floor .. good luck

You could also check the brake proportioning valve above the rear diff