Time will tell if this repair lasts, but I think I fixed my Harbor Freight air compressor. This is the little 'pancake-style' one I got on sale in 2013 for $40. I really like it because it is small and light, I only use it occasionally for my nail guns (they don't use a lot of volume of air).
The last couple times it took a looong time to get up to pressure. I took it apart yesterday, and looked at some you tube videos. I didn't find anything obvious, one connection seemed a bit loose, so maybe it was leaking as it filled (the tank maintained pressure, but this is before the check valve, so could be a problem still).
I pulled the head, was pretty clean, the reed valves appeared to be OK. I didn't realize at the time that you can pop that reed plate out of the top of the head (it is just sitting in there, with silicone gaskets in between, compressed by the head bolts). The piston has no wrist pin, so it swings in an arc over its stroke. The “piston ring” is apparently made of Teflon, and it is maybe 3/8” high and a 1/4” thick, and is rounded to seal as the piston moves through that arc.
Not sure if it is a good idea or not, but I put some white lithium grease on it (might just catch dirt? I don't see any filter on this thing)? And I put some pneumatic oil on the reeds and crank as best I could. Came right up to pressure in ~ 4 minutes, about like new (and what the label claims). Takes about 50~60 seconds to refill from low trip (~ 85#) to high shut-off (~110#).
They make a new one for ~ 3x the price, quieter and maybe better quality, but it's heavier. I'll just try to keep this running.
I don't see any air filter, and I didn't make a note of where the air comes in, but I think I'll try to rig something up.
A couple videos that show that head/piton/reeds:
https://youtu.be/v-4jWqmBzh4?t=354
https://youtu.be/kDR4t5f117k?t=1172
https://www.harborfreight.com/3-gallon-13-hp-100-psi-oil-free-pancake-air-compressor-61615.html
-ERD50