air cylinder problems

I am having problems with one of my rear cylinders... when I hit the switch to let air in it tends to not one to go up sometimes. Or i'll hit the switch and it will delay and pop up a second later. It's not the valve because I already tried replacing it with a new one. Also when the cylinder does air up sometimes it drops down to 80 psi and just stops... it sounds like air is coming out of the lower port on the cylinder.. any suggestions or anyone have this kind of problem before?