Originally Posted by climbo
Yep, all the way on. Every time it twists to the side it leaks, and when the pressure goes UP it can't hold on. Maybe I need extra long vales to get it way on there but at the moment it sucks. I suppose I could try my valve extenders on there and see how that goes to get it WAY on but it should work as long as there is enough to seal it on the valve stem. It's not like pushing it on an extra inch is any different to the mechanism of it.
Sounds like your stem is too short. If the silca head is touching the rim then you're hosed. Remember the seal is recessed inside the head by 2mm and it needs a further 2mm of purchase to make the seal tight.