sounds like you have a leak and have gotten air in the system re bleed your brakes in the right order (check the book) that should fix both problems. also check the auto adjusters on the rear breaks for the abs lights they might be sticking not letting the rear brakes work like there supposed to
j4k3, check out what pont said, if still no luck, you may have a wheel cylinder gone bad in the rear, if you loose one or both, you won't have any break pressure @ the pedal....
Alright guys. I have a guy looking over the rear end I just bought and he told me the used one is fine I just need wheel cylinders on that also. So he's putting axle seals and wheel cylinders in and installing it on the truck for real cheap.