I aquired a Faema E61 Jubilé A2 last year. Before putting it into service, I completely restored it, including a thorough acid treatment of boiler, all pipes, connections, etc. Everything was flawless when put up for service in april 2020. Last week, it overfilled (and didn't stop until water came out of the safety valve. The water level probe was black and had a rough surface. I cleaned it with steel wool back to shiny and smooth surface, and put it back last week. Yesterday I was told the same problem is back (I haven't been with the machine for the second time yet).
I suspect an acid treatment of the boiler is needed. But why does this happen so “fast”? The machine has been constantly on since april 2020, but not so much in use because of closed restaurant for months at a time, but 1.5 year seems too short for this problem to occur.
Also, what could prevent this for the future?
I also notice line pressure is about 6-8 bars (we have ordered a reduction valve, but it hasn't arrived yet). Not sure if this would be relevant.
If your float is getting black and building corrosion you should test the PH to see if there is still a high level of acidity.
Running acid through a system without neutralizing the system afterwards could lead to corrosion. Obviously I am not there but based off what you are saying that is what I would check to eliminate that as a possibility. Any pool chemical store has PH test strips.
If your probe is getting BLACK, The incoming water probably has either high Iron or Manganese. Have a complete water test done and install the appropriate filtration system. You could also be getting a bit of electrolysis from the heating element if there is a stray current of the wrong polarity to the sensor. I'm not sure on that model of the sensors design, but often they are plated to prevent build up. To abrasive of a cleaning may have removed the plating or surface treatment. All depends on the water analysis result.
I forgot to mention that for electrolysis to happen like you describe, the water would have to be a bit acidic. And the probe have a negative polarity.
Also with the Parker solenoid valves, overpressure of the supply may cause a leak. But line pressure Would have be above 1.5 to 1.8 bar for the relief to lift. (depends on the builders relief setting) So yes, a pressure regulating valve is a must. The solenoid springs do get weak over time. I've stretched a few over the years to stop leaks. Heck, I still haven't put in a new solenoid valve in my own unit that I bought 4 years ago becuase it started leaking.