The test as to whether the problem lies with the valve itself or
elsewhere is quite simple. Disconnect the tube from the tracker bar
and in it's place attach a short piece of knotted tubing. Either
hit the switch, or start pumping. If you can still hear the valve
leaking and the pneumatic still partially collapses, the problem
IS with the valve block. If that 30 year old, piece of junk valve
holds tight, with no discernable leakage of vacuum--which is highly
unlikely in my opinion--then the problem lies between the tracker
bar and the nipple. That, too, is equally simple to test. Take a
piece of masking tape and seal off the tracker bar holes. Now suck
on the end of the tube you disconnected at the valve nipple. That
will instantly tell you whether the tracker bar to nipple run is
air-tight or not. Since there are no junction blocks, controls or
fittings between the valve block and the tracker bar, any problem
will be immediately apparent. Its repair will be equally obvious.

Les Smith