On our fast-up rear system - It is possible to trap or accumulate some debris inside the UP valve, keeping it from sealing completely closed and allowing air to pass from the tank into the shocks.  This will be pretty easy to disassemble and clean out, and that will return the system to its normal function.  The UP valve is the black cylinder shaped component on the lift manifold with one orange and one black wire lead (see photos below to identify the UP valve and see the exploded view of disassembled UP valve).  Before disassembling, turn the valve on the tank off and remove the fuse from the air system.  To disassemble the 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.  Then reassemble, 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 the UP valve, 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.  Let us know if you have any questions and let us know what you find.