For our newer style valve DA4VM manifold:  Over time there may be some debris inside one of the UP valves, keeping it from sealing completely closed and allowing air to pass from the tank into the shocks or forks.  Or some debris/buildup in one of the dump valves, letting air leak from the shocks or forks.  This will be pretty easy to disassemble and clean out, and that will return the system to its normal function.  The UP valves are the black cylinder shaped components on the 4-valve manifold, orange wire is the rear and green is the front.  The red is the rear dump and white is the front dump.  See attached photo of valve unit with each valve in a different stage of disassembly.  To disassemble each valve: Remove top nut, this will allow you to slide the electric coil off the center shaft.  Then the shaft can be unscrewed from the aluminum block, there is a slot on top of the shaft so you can use a screwdriver to get it started.  Unscrew and remove the shaft, inside you will find a plunger and spring.  Wipe the plunger's rubber seal clean along with the surface of the valve body that it seals against clean.  Blow everything out with compressed air, and if it's greasy at all, clean it with a q-tip or similar.  Needs to be clean and dry.  You can clean all 4 valves, or just the valve that is leaking.  If o-ring is compromised or lost, it can be replaced with a 3/4" o.d. 5/8" i.d. o-ring.  Once cleaned, you can reassemble.  The system should work perfectly, and should not need to be cleaned again, although it is always possible that some debris can float around in the system until it gets stuck in one of the valves, in which case it just needs to be cleaned out again.  It doesn't happen very often but it is good to know how to clean it out since it is pretty easy to do.