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If you're having the same problems I had, the pump isn't developing the pressure it should before the unloading valve pops off (also called a pressure relief valve). It works fine without a load but could not lift anything heavy (a common problem with these old tractors). First thing to do is be sure the pump is setup correctly. The pump can be set for all sorts of jobs so be sure that it is set up correctly for the job you need to do. There should be a decal on the battery box just above the pump that shows you how to set the pump up. The pump has four manual adjustment knobs. The most commonly used set up for me is the hold position setting. The lowest knob on the side of the pump is the drawbar adjustment. Its use is for pulling plows and the like and gives traction control. Turn this knob all the way into the body locking the plate into position. The second knob is on top of the pump on the side linkages. This is the hold position adjustment. Turn it so that it locks the two plates together, allowing them to move as one. The third knob is also on top but on the front of the pump. This controls the delay lift. Turn it all the way out until it hits its stop. The last knob is on the front of the pump about half way down. It controls the rate of lift. Turning it all the way out gives you fast lift. All the way in is slow lift. I first bought a pressure gauge (0-5000 psi, cost about $20 plus fittings) and installed it at the small port on the top front of the pump where the 'booster gauge' was normally attached. My pump was only developing about 1000 psi, less than a third of its 3500 psi rating and that wasn't enough to pickup a 4x4 round bale of hay. You'll have to pull the pump off the tractor (three nuts and two bolts and don't forget to drain the pump body when you drain the reservor) to access the front cover plate. Four bolts takes the plate off. The plate covers two cylinders, the first one has a the last manual adjustment knob I mentioned above, the rate of lift control. The cylinder without the adjustment contains the unloading valve. Pull the piston out of the cylinder (you may need to tap it down light to break the o-ring seats). This is the valve seat assembly which has two o-rings outside and contains the valve seat internally. Watch out for the valve ball as it may pop or fall out unexpectedly. Next pull out the upper spring retainer. Watch out for shims falling off the underside of this retainer. Last out is the spring itself. The spring may be broken or may have weakened over the years, or the o-rings around the valve seat assembly may be leaking. Adjustments are made by adding more shims on top of the spring to increase the working pressure. (If you put shims under the spring, be sure they don't block the off-center hole in the lower spring retainer. I've found a M12 washer works nicely.) Its trial and error here. I doubled the number of shims (a little over a sixteenth of an inch). If it isn't enough, you'll have to pull the pump off the tractor again and add more shims. If its over 3500 psi by your gauge, you'll either have to pull some shims out or be very careful not to hit the end of travel on the rams and blow hoses (at best). Of course your problem may be something else entirely, so get that pressure gauge installed as a first step.
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