How do you know you have a releif valve problem? Did you have a flowrater on the system?
A Late 1650 & 1655 tractors had a pilot operated system relief valve. Earlier had a plain poppet releif valve and apparently that is what the dealer thought you have. The poppet valve would maintain pressure to its initial opening pressure. Inlet pressure is working against the spring + the pressure that is on the back side of the valve. Once the pressure at the inlet hydraulics the valve off it's seat the oil flow holds the valve in a relief position and the result is a drop of approximately 200 pressure below opening pressure.
: The pilot valve will maintain a working pressure the same as the opening pressure. Your best bet is to purchase a new capsule. Here is how it works: When you took it apart there should have been a spring in the cap/plug. The spring + fluid pressure holds the capsule against the inlet pressure. At the same time pressure fluid is allowed to bleed into the area of the cap/plug to aid the spring in holding the capsule aginst the inlet pressure. As you are holding the capsule it has a housing with a hex nut on one end. The nut has a small hole drilled in it. Inside the housing there is a valve that closes the drilling in the nut, valve is held against the drilling with a spring. When pressure gets greater than the spring tension holding the small valve against the drillng in the nut, the valve opens and dumps the fluid pressure that was aiding the spring holding the capsule aginst the inlet pressure. Capsule slides towards the outter nut to releive the excess pressure. Now if you want to take a chance: The hex nut has shims between the head and the capsule body. Removing shims moves the nut closer to body incresing spring tension for the small valve, thus higher pressure is required to unseat it. Look for any foreign material in the valve. To test after installation some kind of adjustable restrictor between couplers with the valve ahead of the restrictor. The factory setting for the valve was 2000 PSI. Good Luck