Help with air compressor!

Over the last couple of months it has been taking longer and longer to go into load phase after turning the unit on. Should only take about 5-10 seconds for the inlet valve to open and begin building pressure but it has been taking up to 10 minutes to finally kick in.
I suspected dirty filters so I gave it a complete maintenance service. Oil and filters changed and belts checked; now it won't go into load at all. Oil level is correct. There is a pneumatic cylinder that operates the air intake valve; possibly sticking so I will attempt to remove and clean it.