Well, I'm back with an update of what happened. I tried to get the pump out the top and it absolutely refused to come. I ended up splitting the tractor behind the transmission and the pump came out easily. I disassembled the pump and found nothing broken or badly worn. Since the parts to rebuild the pump were over 50% of the cost of a pump, I replaced the pump with a new one. I put three new rings on the lift piston and new o-rings on the standpipe and under the lift cylinder. After putting it all back together it now works better than it has in the last 40 years.
In summary, all that I found was that all of the o-rings were hard and flat. I don't think that any of them could have held any pressure. I did all of the adjustments that the book called for with the cover off. The only adjustment that had to be done through the right-side inspection hole was the nut on the control valve shaft. My only conclusion is that the initial problem must have been associated with the pressure loss via the o-rings. The lift even stays up now and does not drift down. Thanks to all here for the assistance and moral support.
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