Just had this - it's easy to fix: follow the service manual. One thing that wasn't obvious: to disconnect the wires, you need to push in the plastic pin closest (highest) to the wires, and you need something to push it in with (like a screwdriver).
I blew some air over it, but also ordered a...
On the topic of the T-connector, I can confirm it is a pressure relief valve, *but* I think it can sometimes get stuck.
With the pump in dual-action mode, and the T-connector mounted, hold the output nozzle with your finger and pump down. The valve should open at ~7-8 psi. If you pump down...