I have one like this and it is in my opinion a very poor design for something that has to be opened periodically to dry out / replace the filter media.
View attachment 349466 The metal tube at the top leaked on mine no matter what I did after taking it apart to check the filters. I replaced it with two male quick connectors and use a short whip now. I had to clamp each round tube in my large bench vise with some sheet lead wrapped around it for grip and to protect the anodizing so I could unscrew the caps which were super tight.
I used a large monkey wrench on the flats to unscrew one of the caps. The other was positioned in such a way that a wrench would not fit on it until the other one was removed, and a rod in the hole would not budge it. Luckily one of the caps had both flats exposed, otherwise it would have been more of a pain to get it opened up.
I had to hit the wrench handle with a dead blow hammer to break the cap free. I then repeated the same process for the other tube. Mine had two half length fiber filters in the first tube so I had to unscrew the entire tube from the flat sided block to get the 2nd filter out. To do that I had to clamp the round tube in the vise again with lead sheeting and put the monkey wrench on the flat sided block. If I had some really long needle nose pliers on hand I probably could have gotten it out without removing the tube. I use a single full length filter now and can pull it out with needle nose pliers, no need to remove the tube. Another way to easily remove the fiber filters if you have two piece filters, is to hook up your air tank to the the compressor side of the filter assembly with the cap off and give it a quick shot of air, launching the filter out of the tube. B sure to keep clear of the "muzzle" when firing it out!
Once taken completely apart, I ended up swapping the tubes left to right so that when both caps were tightened down, I could use the flats on the caps to unscrew either of the caps first as desired. Luckily that worked for me to line the flats up for both caps. It might not for you. Again, a poor design in my view. The hole in the cap makes it easier to unscrew just one cap once they have been broken loose with the wrench but is worthless for breaking them free or tightening them enough to stop leaking.
The provided O-Rings are too fragile, shedding small threads of black rubber apart when under pressure. You may want to replace them with a higher durometer rating part. If it leaks when you put it back together, it is the O-Ring. Get better ones. Don't bother trying to seal it with teflon tape. It will be too hard to assemble and will eventually probably leak anyway. Replacing the metal U pipe is also recommended as repeated removal and reinstallation of that assembly will eventually make it where you cannot get a seal anymore. You can screw male or female QCs in the openings where the U pipe was. I did two male QCs then the short whip mentioned already to connect the two. With the QC fittings on the caps, you can bypass the tube that doesn't have the pressure release fitting on it for when you want to just use one tube.
The second filter had three components in it, silica gel which is worthless for removing moisture as air moves through it, charcoal for removing odors or whatever, also useless for PCP use and some molecular sieve beads which supposedly work fine. You will want to replace the entire contents of the plastic tube with molecular sieve beads. All the filters and molecular sieve beads can be dried out and re-used but the fiber filter will eventually get too much oil in it to work right so you will need to replace it as needed. I have the black pre-filter on my Yong Heng so there is very little oil on the fiber filter after 12 hours of use. The small filters for the Yong Heng black filter are very cheap, so I chuck them once they get dirty looking.
Before reassembly, put some HPA safe silicone oil or grease on all the threads so that it won’t be so hard to open up the next time. Also put silicone grease on the O-rings to help prevent them from being chewed up. Don't over tighten stuff. Check for leaks then give it just a little more torque, then check etc. I can now remove the caps without having to clamp the tubes in my vise.
If I were to start over from square one, I would not get this model. Instead I would go with the single "orange" water trap that is factory sealed with no need to take it apart, and a single tube moisture absorption filter filled with molecular sieve beads.
