Hi out there,

Every evening I refill the water reservoir. Lately, water already loaded into the machine has started to flow back into the reservoir resulting in overflow and water on the table. What is the reason for that? Is there some valve I need to fix?

Best Michael

Sounds like your check valves aren't closing. There are three check valves in a GS/3 AV and two in a GS/3 MP. One is on the pump, one is at the input to the tea water mixing block, and if you have an AV the third is at the input to the flow meter.

All of the valves have to be open for water to flow back into the reservoir. This suggests that either your machine is quite old and the check valves are way past their service life, or the check valves are contaminated with debris from unfiltered water or scale. Scale is somewhat unlikely in the cold water path, so I'd bet on debris, maybe from a charcoal filter that wasn't properly flushed.

First thing I'd do is clean the check valves. If that doesn't fix the problem, then they need to be replaced. The latest check valves are pretty expensive, so you could try to replace the O-rings. JakeG has posted a procedure for doing that.

If the check valves in the coffee boiler cold water path are contaminated with debris, it's a good bet the check valves in the steam boiler path are contaminated or will be soon, and you'd best check them too. Much more serious problems can occur if those check valves fail to close.

I had this issue on my first GS/3.
See:
La Marzocco GS/3 water flows back into reservoir on power off

Thanks guys - I will look into it and come back.
My version is the GS3 without the paddle pre-fuse