So I familiarized myself with the internals of the machine and finally worked up the courage to attempt the repair. I siphoned the boiler*, and first attempted to remove the solenoid valve internals (hoping to soak with vinegar). Unfortunately after removing the coil, the fitting at the base of the piston wouldn't budge. Any ideas on how to loosen it up? And can anyone confirm the fitting size? I was using a 17mm deep socket ratchet, getting nowhere and stopped worried that the torque might harm the 6mm copper tube and attached L-fitting into the boiler. Afterwards I descaled the boiler and HX to see if it would help, which it didn't.

*One additional thing I noticed is that I could only siphon 500ml at a time from the boiler (via the water spout fitting), is that normal?

Either a 17 mm deep socket or 11/16" deep should fit nicely. Be sure you are attempting to turn counter-clockwise.