I posted this info on another board, thought I would just paste the info here....

I get many calls on one particular issue relating to both the Jettronic and UP brand valves....

The complaint " either in gear up or gear down position (or both), my valve leaks out the exhaust port"
The problem - This issue typically has NOTHING to do with the valve itself. A leak of this nature, where air is escaping the system out the exhaust port of the valve, is caused by a leaking cylinder. This leak is internal, not external meaning, the air is passing by the piston internally and passing out the opposing airline and back to the valve exhaust port.
The Solution - crimp off each cylinder feed line one at a time with some hemostats. When you have crimped the leaking cylinder line, the leak will cease....Fix or replace that cylinder. It is possible to have multiple cylinders leaking, check them all!