I bought a nice used TH400 pump on eBay for about $100. It was described as good and used, and it was disassembled in the pics which confirmed the condition of the pump gears and such. This thing would have been easy to hit with a little brake cleaner and install. However I pulled it apart to check all of the ports, bushings, and restrict the converter feed, and put in a new pressure relief spring. As I was tapping the converter feed hole prior to cleaning it up, I noticed that air blown into the port to clean out the tapped metal chips caused the chips to be blown back out of the port toward me. The converter feed slot was plugged completely off with the 'crud' in the pics. A person not knowing what to look for would have installed this clean pump and likely cooked their new trans. As you work on this type of stuff try to make sense of how it works. You may just find a hidden issue.