I saw a neighbor throw out 3 air nailers last week. A Ridgid R213BNE brad nailer, a Porter Cable 125A brad nailer and a Porter Cable RN175A coil roof nailer. I had no air nailers but have access to a couple of air compressors, so I asked if I could have them and see if they were worth fixing. The neighbor gave them all to me and said none worked and as far as he was concerned it was cheaper just to buy new ones. That's the way it is in America, if it breaks, toss it and buy another.
The Ridgid is supposed to have a lifetime warranty if you buy new and register the tool with Ridgid. Home Depot still sells the exact same nailer, and a clerk there said the warranty meant you took it to a Home Depot and left it with the tool rental department to see if they could repair it. If not they would give you store credit to buy a new one. Time is money, so that kind of warranty isn't very useful unless you have a backup so you can finish the job.

Supplies

Small set of metric hex keys, flat tip screw driver, Dremel or similar cutoff tool with cutoff disks, hammer, bench grinder, locking pliers, spray lube like WD-40, eye protection, lots of spare time and a steady hand, bench vise, permanent fine tip marker, small piece of hardened scrap steel, pin punch, set of nitrile o-rings and a source of clean dry compressed air.

Step 1: Porter Cable Brad Nailer 125A

This only needed to have the top of the cylinder unscrewed (four 3mm hex screws) and the o-ring at the top of the cylinder replaced. The PG nailer o-rings are famous for being made of cheap material that rots. The broken yellowish orange ring is the remains of the top seal. The black ring is the replacement.

I have a set of nitrile o-rings bought about 20 years ago from Harbor Freight for plumbing repair. Nitrile rubber is very stable. The set I have still looks and acts new. The piston with the rod is the rebuilt driver for the Ridgid nailer. The drivers for both were identical except the PG piston is domed and the Ridgid driver shank is slightly longer.

Step 2: Ridgid R213BNE Brad Nailer (18 Gauge)

The driver assembly on this nailer broke when it jammed. The shank broke in two and the shaft was bent 90 degrees. The piston was jammed sideways in the cylinder and had to be hammered out. The driver unit is available for between $30 and $60. The whole nailer retails for $100, so buying a replacement makes no sense.

https://www.partswarehouse.com/Ridgid-R213BNE-18-Gauge-Brad-Nailer-s/10517.htm

I attempted to spot weld the broken pieces together, but the heat of welding destroys the temper of the metal. I was told before hand this would happen, but having nothing else to lose, tried it anyway. If you weld it, it will break just beyond the weld with almost no effort. Bend it and it makes a pathetic little tink sound as it breaks.

Even if I could get the part for free I wondered about the design and how long it would last. The driver shank is grooved and quite thin. I didn't think an identical replacement would work much better.

I had a saw with no handle that was about the same thinkness as the original shank, so I used a Dremel to rough cut a replacement and a bench grinder to rough grind the shank and then back to the Dremel for finish sanding and cutting grooves on one face. Hand saws are made of some very tough steel.

The grooves in the shank fit a bronze guide deep inside the cylinder and also down the back of the shaft guide on the tool. The front of the shaft guide near the tip has a flip lever to open the tip in case of a jam.

I used a permanent marker to mask off areas to be removed and grooved and did the work freehand. The saw steel was so tough I could not drill it for the cylinder pin, so I used a small grinding stone to melt a hole for the pin. I used marine grade JB Weld to fill in the base of the shank after resetting it with the original pin.

Reassembled it works. The piston is aluminum and there is an o-ring under the plastic compression ring. There was some blue-green grease in the cylinder, but Ridgid says not to oil the tool.

Step 3: Porter Cable Roof Nailer

This one misfired. I live in south Florida. We have frequent destructive hurricanes. The building code keeps changing. Anything used on a roof has to meet code. To resist pull-out, nails either have to be ring-shanked or have copper wire connecting them. The wire helps the nail resist pullout. The people that make the nailer have no way of knowing exactly where the wires will be, as there is no universal wind code. Local retailers like Home Depot only buy and sell roof supplies that have been pre-approved. That changes after every major storm.

The coil of wire nails is also too big to fit the gun cartridge anyway. About a foot of nails has to be pulled and cut off just to fit the rest. The coils are also wound left-hand. Point your left thumb up and close your hand. All nail coils are wound this way. That places the gun driver on the left, but the wires could be on either side (top or bottom). In this case they are on the bottom. That means the wires must fit in the slots between the driver fingers or the gun will misfeed. That's what the problem was. Rewinding the nails right hand won't work as there is a 15 to 45 degree step on most coiil nails. That gets the nose of the gun down and away from the base and makes it easier to nail corners and slopes. Reverse the wind and the step now goes up instead of down.

So the fix is the remove some of the nail guide fingers so the copper wire fits down between the slots, and remove about a foot from each new roll and use it later. The picture above without nails shows the shiny areas where material was removed with a Dremel and a cutoff wheel. The flip open section was cut to match.