Originally Posted by
dscheidt
Every twin-holed head I've used a lot has eventually refused to pump, because air is going out the wrong hole. Sometimes that's a "it's broken" failure, sometimes it's just whatever magic that's supposed to send air through the path of most resistance has not worked. Never had a single holed head fail that way....
The one two hole head I've owned had that problem. It used a rubber ball as a check valve between the two entry holes and the valve stem in use pushed the ball sideways to block the other hole. At least that was the way it was supposed to work and, for a while, it did. After it stopped being reliable, I spent the money for the Hirame and never had another problem. I believe the :"Hiro" Soil_Sampler posted is the same thing.