Check Valve sticking in hot water recirc

Hi: I have a hot water recirc line with insulation feeding back to the cold input on the h/w tank. Just before the cold connection I have installed a brass flap style check valve in a horizontal position.
The recirc, without pump (convection) works well.
I have had problems with the check valve sticking.
When this happens I get cold showers when the cold short circuits.
Last time, about 6 months ago, I took the valve all apart, the pivot pin, flapper and made sure they were all clean and smooth. Heck of a job to get the pin out!
It has started to stick again as noticed by cold showers.

My water is very pure, rain water that is ozonated, passed through a carbon filter and finally through a whole house R.O.

I know I cant lubricate it, any help appreciated.

Doug Q.

First, is the valve set level? If it is, then I replace the valve. They're so cheap, you can afford to mess with them.

From what I read, RO systems create really agressive water and you may have trouble with many metal components. Water is sometimes referred to as the universal solvent - and by removing all the impurities with RO, it really can attack things. That may be why you have trouble with the check valve. My unprofessional opinion.