Sears 20 hp 46in lawn tractor runs but shuts off when brake pedal is released or blades are engaged. I checked the seat safety switch and it meters as OK
Don't know where to go from here

Have you checked the "reverse" switch? Most newer lawn mowers have a safety that doesn't allow for the blades to be turning when in reverse. It is possible something is messed up in that section.

Hi,
Seat switch is the most common culprit. Check the wiring going yo it.
Peace,
Clarke

charn, there are at least 3 safety switches , which one did you check? Like crigby says the seat switch is frequently the first to go out.