Once the loader reaches its maximum hydraulic cylinder extension (the cylinders will "dead-head") then the pressure gauge should go to the relief valve setting which could be as high as 1900 psi (tractor relief valve setting). Sounds like one of (if there is more than one I don't know) the relief valves is set at 500 psi. Remember that on a loader you don't necessarily want full tractor pressure going to the loader cylinders. At 1900 psi a 1.5" diameter cylinder can generate around 3300 lbs. of force. So double that for two cylinders. Depending on the geometry of the loader assembly, that would likely end up being too much force on it, much more than the arms and other components are designed for. You really want a relief valve that will prevent these forces from developing to keep it from breaking something. If the control valve going to the loader hydraulics has its own relief valve, that seems like a reasonable set point for it. Ultimately you'll want it set so it doesn't exceed the rated capacity of the loader (maybe 500 lbs. in this case). Note that 500 psi hydraulic pressure and 500 lbs. of lifting capacity would be purely coincidental (in my opinion, but it wouldn't be impossible that it was engineered this way) and the two values are related by the geometry of the design and will typically be a lot different.