I looked more closely at the pump small inner o-ring (labelled "o-ring" in Breville BES830XL with flow problems), and while it wasn't hardened, it was easy to slip it on/off the piston (which you suggested was a bad sign). I took a trip to the local hardware store and got a replacement (https://www.canadiantire.ca/en/pdp/hill ... 9794p.html, 5/16" x 3/16" x 1/16" - $0.79) and put the pump back together. It's a little tighter than the original, maybe due to imperial vs metric dimensions, but it doesn't seem to affect the operation of the pump. The pump behaved much better! Consistent flow on the "hot water" setting, and consistent steam.
However, the extraction (group head) was still very erratic. Loud buzzing noise, irregular flow. Now that I could consistently pull fluid into the machine, I fed in a solution of 1 part vinegar 1 part water, left overnight, and flushed it out. No noticeable improvement but no worsening either.
I started looking at the group head from the exterior. First, the dispersion screen and head gasket. Oh boy, they were gross!
Cleaned these mechanically, then with dish soap and water. Dispersion screen looks much better, but the gasket is clearly cracked. It seems relatively easy to replace and inexpensive, though.
The group head bulk was also covered in dried and burnt grounds, after a vigorous scraping with a soft wooden tool (chopstick), it looks like this:
I think I can actually see small dimples in the group head bulk, I suppose where entire chunks of it corroded and flaked off.
Ran the machine this way, and the water flow through the group head was still irregular. The overall flow diagram is something like:
Since the hot water/steam setting were behaving somewhat reasonable, I figured the problem must be in the thermoblock/group head assembly. This is the large metal bulk in the center of the machine.
_______________ pump -> | thermoblock | solenoid valve <- |.............| | | | | | \-----> | group head | | |---\ /----| hot water/ steam
There is not too much information about thermoblock internals (they seem to have been a short lived design, quickly replaced by thermocoils), but this video does give some basics: It turned out my thermoblock also looks very similar to this one:
and I encountered the same difficulties in disassembly (of the socket head cap screws holding the assembly together, the short ones were extremely difficult to remove; I stripped a (cheap) allen key in the process).
Anyway, the inside of my thermoblock looks like this:
Massive amounts of scale buildup, wouldn't be surprised if it significantly impeded the flow of water. I'm planning to clean it up mechanically, maybe also try to dip it in a vinegar bath for a little extra. Seals look in pretty good shape though!
For the loud buzzing, I'm starting to suspect the solenoid valve. I haven't looked at it yet. With any luck, it's some scale buildup which I can clean out. Hopefully nothing more serious than that. Any idea whether neoprene rings are safe to use on the hot side of the machine? By a quick web search, "Neoprene is rated for 275 F (135 C)" which seems sufficient.
To be honest, I'm losing a bit of .. steam .. on this. I did not imagine how many problems this machine could have, and even if I do get it working, I'm starting to doubt the quality of coffee that will come out of it. I did manage to learn a ton about espresso machines in the process, so hopefully that will serve well if/when I graduate to a real one.