I've been running an 80 gallon prosumer grade 3.5hp compressor in the shop for a while. Its up to the job, but bigger would be better. I'm not going to tell you the brand because when I went all over town they were the same. Just different paint. Even the same size/power Quincy I saw seemed to only differ by price. That was kind of disappointing.
I've had to work on it a bit, but usually its a same day fix. Usually a same hour fix. The most recurring problem is the check valve. The first time it failed I dropped a buna rubber o-ring in between the plunger and the valve seat. That lasted for six months. I did the same thing six months later. Then I tried a viton o-ring. It lasted for a year. I tried a buna-n o-ring, and it only lasted a couple months. Pretty terrible.
This last time I took it apart I decided to see if I could do something better. I had originally thought some sort of seal/gasket was missing (The o-rings did fix it), but upon closer inspection I could see the dirty plunger was made out of either HDPE or PTFE. Based on the way it cuts I would guess PTFE. The plunger was the seal. It just quit sealing. I don't know why. The plunger looks ok, and the seat looks perfect. Regardless it wasn't working.
I cut a new plunger out of black acetal and it seemed to work, but I could hear some air hissing past and out the pressure relief tube. I'd gone for a simpler design. A recess in one side for the spring and a shallow cone on the other. As indicated. No joy. When I took it apart I looked at it and decided my finish was mediocre, but what got my attention was a shiny ring about 7/8 of the way around. I guess if I left it in there it might have eventually beat it self into seating. I might also have been able to do it by gluing a mandrel onto it and spinning it in place to sort of lap a seal. That all sounded like to much work for a "maybe" solution.
Instead I cut a new plunger out of black acetal, but this time I put a
45 degree cut on it. (90 degree cone) I also used a fresh sharp uncoated TNMG insert instead of the hand ground HSS tool I had used before. The whole surface of the cone looked bright and shiny. It was cut with a single near perfect tool edge. Of course the other side had a recess for the end of the spring like the first. I dropped the new plunger into the check valve and put everything back together. After letting the compressor come up to full pressure I thought I might have heard a hint of an air leak, but I had work to do. I fired up some machines, ran some jobs and didn't worry about it. I close the tank valve and shut off the compressor every night anyway.This morning I checked the tank pressure and it appears I didn't loose any pressure at all over night. I don't know how long the acetal plunger will last, but it seems to be doing the job for now. I imagine it can't be any worse than a buna o-ring. I suppose I should stock some other sizes of acetal rod. It seemed like a shame to turn down a 2 inch piece of stock for a 7/16(apx) inch plunger.