[mongocanfly]

I wouldn't think it would be the hydraboost.if the hydraboost didnt work the pedal would be hard to push.just like pushing brake pedal with engine not running. If its a soft peddle with no visible leaks it would more likely be the master cylinder. If you only got back brakes your proportioning valve may be tripped and needs reset. Mine was that way when I got it. Po had replaced front brake lines. What happens is it was built in for safety in some trucks in case you blow the brakes on say, the front, the valve trips and sends fluid only to the back brakes.and vice versa.to reset it you would need to loosen back brake bleed screw "to create a leak" and slowly push the brake pedal and the valve will "reset"..that fixed mine..or it could be the front side of master cylinder leaking internally.. I think what he's talking about with rear diff level is he means oil leaking thru axle seal onto brakes but if you only have back brakes I don't think that would be it.