All appears normal except when the portafilla arm is attached

Hi Paul,

The most common cause of these symptoms is a clogged portafilter basket. Take your basket out of the portafilter handle and hold it up to a light; you should be able to see through the majority of the grid of tiny holes in the bottom. If you can't, apply some citrus cleaner or other similar detergent to the bottom of the basket and and work it over with a wire brush. Once you've got most of the holes unplugged, try your shot again.

If your basket isn't plugged, try running a shot through with the portafilter and basket in place, but with no coffee in it. If water still doesn't come out, then your portafilter might be clogged or broken inside, and you'll need to disassemble it to find out. This guide should help you with that: Starbucks Barista portafilter disassembly and cleaning

If water comes through the portafilter when it's empty then there's a slim chance the problem is that your espresso is ground too finely, or you're putting too much in the basket. Try grinding it coarser, fill it only to the top and don't tamp it down. If you still can't get coffee to flow, then as a last resort I would try descaling the machine and see if that helps. If you don't descale regularly it's possible some mineral deposits have built up in the water passages and are inhibiting the flow of water to the coffee.

If none of that helps, the pump you need is an Ulka EP5, and it can be bought online just about anywhere. Make sure you get the version with the right voltage for your country, it comes in 24, 120 and 240.

Good luck!