Posting a gee-whiz link just because: The Spudgun Technology Center - Your Source for Spudgun Parts, information, and more!

Yes, it's from a potato-gun site. But it shows the location of the calibrated leak in the valve and how it works. Dirt in there will prevent the valve from opening, if the diaphragm wears it'll stop it since the pressure on the top will be too great. Controlled leaks and venting, not unlike an M-B tranny. That's why you need to keep your system clean and flush it after install.

I had to dig up my system last year to replace the valves since the diaphragm eroded and the manufacturer was bought out. No replacements to be had local and shipping added up to the cost of new valve.