Either a problem with the 3pt. servo valve or the internal linkage that controls it. To stop the relief vale from blowing ( will wreck the pump very quickly from overheating) back out the cap screw with the lock nut on it 5 or 6 turns located just above the floorboard near where your right heel rests. this will allow the oil that goes to the servo valve to pass directly to the reservoir, this may also be in the Operators manual.

The servo valve and the internal linkage can be accessed by removing the top cover under the seat. Do you have a shop manual? Copies of the original (they are the most complete) can be purchased from 
 [url:9rmqit6z]http&#58;//www&#46;floydcountymuseum&#46;org/[/url:9rmqit6z] Good luck.