Three years ago I tried them and when I pull taps with vac on I noticed some we're stuck open because of the slime in the seat where ball sits, a couple we're stuck shut. I found two completely closed off. So I stopped using them.
This year I had a new 1000 tap bush and my best other Sugarbush which is my oldest is 850 taps of old forest trees most of which are 24"+. I tapped the new one 1-29 and the old one 2-8. The new install on the 1000 taps ran hard until the middle of April averaging well over 1 Gpt per day, or 10 weeks. My older Sugarbush started off great had a couple runs of 2400 gallons of sap in 36 hours on 850 taps, however about six weeks in which would have been the first week of March we had a week and a half freeze up when this was over I was expecting to still see 2-3 gpt per day. However I was only getting a half a gallon Pratap at best, and as I've thought about it for the last couple weeks it has to be because of the old contaminated lines, I will try the check valves again this year now that I have a better Baseline of these sugar bushes. A few years ago I was overlooking the fact that when we are pulling tabs what we are seeing if that current time isn't necessarily indicative of what happened earlier and I guess we need to look at it as what's already been done by that check valve not what it looks like now.
I can clearly see that the bacteria had a lot to do with the holes closing.
One thing that's a fact is that the principal of a check valve is there the idea is completely relevant however there are certain things that in the field throwing different obstacles as I have found and you others as well, but I will try them again even after swearing I never would, mainly because it's going to cost about the same to replace a drop line as it will throw in a check valve and most of my drops are 3 to 4 years old now......
18x30 sugarshack
5100 taps high vac
3x10 inferno with steampan
7'' wes fab filter press
10'' cdl air filter press
D&G 3 post reverse osmosis w/recirculation