Hi guys, been a member here for a few years but stopped visiting as much as the USA web sites caught up with 900/920 growth and adoption...
I was an early USA adopter having got my -900 in November 2011. About two years later, it got replaced under warranty with a -900 /B which was the last version before the -920 came out, and uses the -920 brew group and notched portafilters and all. So I'm kind of in between the -900 forum and the -920 forum. So my -900 is basically a -920 save for the ability to drain the boilers, with is moot to me, because I use a commercial espresso machine filtration system that does not produce scale.
So now, many years off warranty, a little over six years into BDB ownership (for Breville Dual Boiler), and being active on the forums, these are the things that seem to commonly arise:
1) o-rings: need replacing every 2-3 years, particularly the steam boiler o-rings under the fittings where there are water tubes. Failure to replace these when they are leaking can cause all manner of strange behavior, from constant heating past the set temp, to popping the circuit at the wall outlet as leaking steam wets the triac circuit board under the warming tray. $10USD from Amazon or ebay gets you 100 size #007 o-rings, which is more than you will need for the rest of your life. I used silicone, because I think that's what Breville used. Some use EPDM, and some use regular nitrile. They all cost about the same and I wouldn't be surprised if they all work about the same in this application, despite having different duty ratings.
2) solenoid: seems good for about 3-4 years. I have not had much luck trying to rebuild one that no longer works. Under $50USD delivered for a new one. I have kept my old one though, in case something comes up where I find good information on how to revive it. But if I had to guess, I'd say the electric bit, the coil, is what is bad, and not leaks or blockages.
3) steam ball valve: these eventually begin to leak either through the ball in the middle or out the side of the on-off handle. It is oriented vertically in the machine. the bits that screw into the top and bottom of it are unique to the BDB. the bit in the middle, the valve unit itself could be an off the shelf item from about anywhere (probably China), as the world is awash in g1/4 British thread ball valves, which is what this uses. Problem: most of the parts houses I'm aware of are out of these and may be discontinued. Solution: keep the top and bottom Breville bits and find an off the shelf ball valve from somewhere that uses female-female g1/4 British threads that is about the same length as the Breville one and use it instead. Might have to bend up a new clamp to clamp it to the the machine if the Breville clamp is too small or too big. USA 1/4 NPT female-female ball valves may work too in this type of application. They are the same thread pitch as the British g1/4, but are tapered thread. Many people in the automotive hobbies exchange USA NPT with British BSPP and BSPT for mild applications.