I have had the same (and more) problems with my Generac 2300/2.9gpm Pressure Washer. First the unloader valve caused erratic or non-responsive pressure when the trigger was used. A year later, the pump did a "crash n' burn" act, but it is working perfectly now.
1) The original unloader valve seal/washer was a "cup" design. Manufacturer re-designed it to two washers (stacked to seal the split in the washers), and it worked flawlessly, for an out-of-pocket roughly (vaguely remembered) $12.
2) Even though the machine "said" it had ceramic pistons (3 of them). The TH series pistons are chrome or polished stainless, and disintegrated in a short time (hours of use).
I recently replaced the oil and piston seals, lubed with water-proof silicon grease for seals, and replaced the pistons with new ones, which fortunately I still had some warranty time left and got from them through the manufacturer, but I had already outlayed $55 for the seals.
Here's what I would do next time: Search for a General CPH3020/CPH3025 replacement pump (has ceramic pistons). A new pump can run you between $150-$210 mail-order (do a search).
If I rebuild this one again (I won't) I would have to spend about $100 for pistons (part #5704102), and $55 for seals -- That does not cover the cost of replacing the 6 check valves that stage the pressure through the 3 pistons.