Leak I have a leak as well and I've removed the valve and have most of it taken apart. I think I need to replace an O-ring but can not get the middle of the faucet open. After I removed the base, the handle and valve off I can not figure out how to remove the middle. Do I need to remove it? Does the middle come out? It looks like there are parts in the middle that should be replaced.

Are you talking about compression readings? If so, and you only have 30 psi, engine is in serious state of disrepair. The cylinder head will have to be taken off to fix the problem-sounds like a stuck valve, a non sealing valve, or a blown head gasket. There's no way to get to it without removing the head.
Apparently you had a compression test done? If you had done or would do a leak down test of that cylinder, it would pinpoint the source of the problem-whether an intake or exhaust valve, or a head gasket, or piston or ring damage. With a leak down test, wherever the air is escaping from the cylinder points to the source: if for instance, air leaks out the exhaust pipe, you know it is the exhaust valve not sealing in that cylinder. If air leaks out the intake-it's the intake valve.
Air hissing from the oil dipstick tube would mean compression is being lost past the piston rings-going into the crankcase.

Usually if the plug is getting fouled with oil, the leak is rings or valve guides.
Either way the engine would need to be taken apart to fix.

Does anyone know where the check valve is? We’ve taken the machine apart to try and figure why it’s steaming. We’ve checked the hoses and the filter assembly to no avail.

There is a possibility of corrosion between swivel and base. Remove faucet and take to a work table and try and twist swivel spout off. I t may be tight, but the unit is not satisfactory and may need replacement. I have repaired such faucets before and many have been cheaper to replace. There a o rings between spout and body if you get it apart check mating surfaces any erosion discard unit. If OK replace o rings and use silicon grease. No petroleum products as this will cause o rings to swell. Good Luck

Trigger leak is easy to repair. Your nailer uses a valve # tva6 and is sold on ebay or many other web locations for about $20(generic valve works good also). To remove the valve, remove the trigger by removing the pin (3mm). Now remove the pin/s holding the trigger valve. With gloved hand apply air to gun and catch the valve as it pops out. Use caution because the valve could come out with force. If you would like to try to repair the valve, remove two pins holding the valve together. The center spool has three orings. The middle oring is usually the problem. It is a bull nose vinyl ring that splits. You could try replacing the oring with a normal oring, however it must fit very tight to work properly (no need to replace any of the other orings in the valve). I have successfully used regular oring instead of bull nose(purchased from Lowes plumbing department). Good luck

Is it leaking at the joint between the tank and valve, between the two halves of the valve, or from the center of the valve (around the handle)? If It's either of the first two, you may need to replace the o-ring. Take it apart, and check to make sure the o-ring is in good shape, and seated properly. If the o-ring is flat, oblong, or leaving black on your hands, then you should replace it. If it is leaking around the center of the handle, then you need to replace the o-ring and/or teflon seal on the shaft of the stem assembly (the part the handle connects to. To get to this you need remove the top half the valve, then drive out the pin that holds the handle on. (make a not with direction the handle and the stem are. otherwise you can be 180 degrees of when you put it back together. Hope this helps. If you need further assistance post a reply in the comments, and don't hesitate to leave a good thumb rating if you found this helpful. Thanks, and good Luck!

The fitting is called a John Guest fitting. It has a couple of o-ring seals inside. Once the tube is pushed into one end it is locked in. To remove, push in on the tube (opposite of what you would think to remove) then hold down on the ring around the tube, then pull out. There is a good description on this website below

http://www.johnguest.com/

You need to replace the entire cartridge assembly do not waste the time trying to repair

I have never taken apart that model before but they are all relatively similar.

The first step is to close the water supply.
Remove the handle.
Either a set screw or nut around the valve stem (packing nut) will be holding the cartride in place.

Once the valve stem/cartridge has been removed then all of the o-rings are evident....replace all of them and the large flat washer at the end of the stem (if there is one0.

Lease check and make sure that you replace with the right seal, that alone should have taken care of this problem. However if the valve cover gasket is loose or bad it will cause oil to leak around distributor are but NOT into distributor. This seal can be easily replaced by loosing 4 10mm bolts on top of valve cover. Contact if you need more help Good luck and thanks for using FIX YA