Case 580b hydraulic problems

sorry for the long winded post, just want to have as much info out there as possible. I recently bought a 580B backhoe for a good price as it had been sitting for a number of years. The previous owner had said he lost hydraulic pressure and removed the pump to get it repaired, he then lost the pump. I got it running and brought it home , found a used pump that the guy says worked fine before he dismantled the machine. Put it on and seems like not enough pressure, can’t move most circuits, I put a gauge in the main loader valve as seen in the picture. Getting 400-500psi at most. I also removed what I think is the relief valve, disassembled it and cleaned it and reinstalled. No change Anyone have any ideas?