TW is using coolant

[cooperplace]

None whatever, just the old converter had a big coolant leak.

[flyboy]

If it pressure tested okay, I’d suggest removing and cleaning both pipes to/from the overflow.

I’ve seen a few times where the return has been full of crud. Coolant heats up, gets pushed to overflow, but then after driving as coolant in the block cools down it doesn’t get sucked back in. Air gets in somewhere instead, and over time the overflow fills up more and more until it starts leaking out.

If pressure test showed no leaks, your problem may lay outside the pressure part of the system.

I’d also pull out the overflow bottle and check it carefully for leaks/cracks.