For the issue of losing PSI when disconnecting the compressor, I reccomend having a system like the one pictured below using the tire valve attached to a line out the end using the plug from the small red pressure gauge and attach that to the black valve for filling the air tank.
*if at first the valve is letting out air when closed flip the direction indicated by the arrow.
For the issue of the cylinder taking a long time to retract, is there a problem with your air lines? Or is the problem in the solenoid?