Uncle Barnaby

I am not sure if this is the proper place to submit this if not maybe
someone can give me a pointer to the right area. In any cas in the mean time I'll give it a whirl.

Here is a strange one someone else may have come across at one time.
In addition to restoring decks I also maintain the rigs for other company's
as I have been in the business for 25 years. This one gentleman brought
in an 11hp belt driven machine with an AR pump. It was not passing water
so the first thing I checked were the valves and I found them all melted.
I replaced them and recommended that since it did not have a thermal relief valve on it to tell his men not to let it run on bypass for to long. Long story
short he brought it back the next day and informed me it only ran about 15
minutes before it quit pumping again. This time I found all the valves melted
as well as the pump packings. I removed the head and ran the pump for
awhile and the pump body became somewhat warm. Is this normal.
I intend to change the oil and add a thermal valve even though it ran 3 years
without one. Has anyone ever seen this before? When in bypass mode it does not seem like the motor is struggling so I'm guess the unloader is working. Any ideas?