The valve is not defective. Six months to a year is a pretty normal service interval for the GS/3 vacuum breaker in my experience, and a number of other GS/3 owners have reported similar intervals here. You just have to accept that it's part of the normal maintenance routine required by any machine (such as replacing O-rings on the MP version).

Rather than replacing the entire valve, you may be able to simply clean the internal parts -- an O-ring, a valve stem and a Teflon valve seat -- in diluted white vinegar. Or, you can replace those parts. That's my preference, as it always works, takes less time and is inexpensive. In fact, some have found that replacing just the O-ring works. It depends on the level of contamination. The parts are available from LM USA or your dealer. You can buy a complete rebuild kit consisting of all three parts, or you can buy just O-rings. Or both. I recommend you get a couple of the kits and some extra O-rings. That will set you back less than $25 including shipping.

To clean or replace the parts, you have to remove the valve from the elbow fitting, then remove the male-male fitting at the bottom of the valve to get to the parts. Don't remove the cap on the top -- that won't get you anywhere. If the O-ring is intact, but the parts show signs of mineral contamination, you can try cleaning the parts. If the O-ring is worn out or deformed, replace it. If you can't clean the deposits off the valve stem or Teflon seat, replace them. If the valve seat looks pitted, replace it.

I don't believe the contamination problem is due to scale, per se. I use a cation (salt-based) softening system that completely removes scale-causing hardness. But it doesn't remove all the minerals from the water. Far from it. The TDS is still over 100 ppm and the Alkalinity is about 130 ppm.

I believe what happens is that a combination of water and steam sputters into the valve as it evacuates air from the boiler, and some of it falls on the valve stem, O-ring and seat. When you turn the machine off, and the valve opens, the residual water evaporates and leaves mineral deposits behind. It looks like a very fine powder in my case. Eventually the deposits build up and prevent the vacuum breaker from sealing. I think it's also the case that the O-ring wears out much faster when you cycle the valve every day, as opposed to running the machine 24/7. When the O-ring wears out, the valve leaks.

BTW, a leaking vacuum breaker was really bad back before LM came up with the valve and tube that empties into the drain box. When the valve got stuck open, steam would leak into the machine, condense, and form pools of water on the chassis floor that could leak into the brain box and destroy the logic board. Back then, I ran my machine 24/7 to avoid that potential disaster. But when the upgrade came out I installed it and went back to turning off the machine on a daily basis. I prefer servicing the vacuum breaker ever 6 months to a year versus the unnecessary consumption of electricity.