Can't get over 50psi in bike wheel
The valve isn't the limiting factor for the pressure -- you can go well above 80 psi with a Schrader valve. 

If you're holding 50 PSI in the tire (the tire should feel a bit firm, but you can likely push it in a bit with your fingers), you shouldn't be leaking. I'd try depressing the core (pin in the center of the valve) a bit with a key/small screwdriver/finger nail to make sure its free and try pumping it up again. 

If you're not holding the air in the tire, you either have a bad valve or a leak in the tube (in which case, get a new valve core/tube in the former case, and patch/get a new tube in the latter case and check the tire for damage).

Failing that, I'd try a different pump -- not all pumps are able to reach high enough pressures, but 50 psi is on the low end for the limit of a bicycle pump not specifically intended for low pressure situations like fat bikes (then again, a pump designed to do sports balls only needs to pump about 8-10 psi, so its possible). I'd try a floor pump like this one.