[DK on the road]

I am putting in 120+/- and getting 120 out. I've used it a lot and I'm familiar with the ports, but its not clicking or knocking as it used to.

 My question is, Are these serviceable?

 I was worried if I open it up, it may have O rings that may or may not be reusable.  Or should I put a little light oil in the service line?  The same oil I put in my air tools.

[Richard]

I would start with air tool oil. i have had them apart a few times, nothing complicated. Ensure all the small orifices are not occluded

[DK on the road]

Update: yes very simple to breakdown and relubricate internally. The oil inside had gelled and restricted the pistons form working back and forth. Simple