It depends on where you put the gage, directly in the port, or tee in line, and what is activated. . Some valves were not designed for pressure downstream, and the fluid from the cylinder usage flows out the out /return valve to tank. Other valves are designed for pressure down stream, and are labeled PB. Again, it depends on where you put the gage. 500 psi with no cylinder being used is a bit much. There is a restriction somewhere. It could be from the 3pt lever position. If the 3pt lever is engaged, everything up stream will have pressure on the IN and OUT ports. If you have PB plumbed in, there should be very little pressure in the return/out port. If no hyd unit is being used, there should be no or very little pressure in the system. Fluid expended from the cyl, is supposed to go to tank.
You might be surprised to know how much pressure it takes to raise an empty bucket. It only goes to max pressure or relief pressure when the cyl are fully extended or retracted, and at max load..