I finally fixed the leak in my Hammerli AR20FT and I wanted to post what I did for the sake of future generations (so no one else has to go through what I did).
My issue revolved around three o-rings. The first o-ring is in the air tank female fitting. The o-ring is 7mm(ID) by 2mm. It is a bear to get out of the tank, but the new o-ring pops in fairly easily. I originally thought that this was the source of the leak, but after replacing the o-ring I discovered that it was not.
Now it's time to open the regulator. Not as scary as it sounds, but make sure the gun is degassed first. Take a big crescent wrench and unscrew the regulator from the gun. It will be hard at first, but you shouldn't need to apply heat or anything, just use a little elbow grease. Be careful of how you are holding the gun so you don't break off the pistol grip or something.
Once the regulator is off, there were two o-rings that were dried up and brittle. The AR20 FT has two regulators that are built pretty much the same. To change the o-ring on the high pressure regulator (nearest the air tank), you unscrew the air tank fitting from the first brass section. Again, it will be tight, but two crescent wrenches will get it done. Make sure when you start to unscrew the air fitting that the regulator is pointing down. There is a little brass valve held in place in this area, you don't want it to fall out and roll under a table. Once you have this apart, you can pop out the brass valve and you will see the o-ring. It is 12mm (ID) x 1.5mm. Replace this o-ring and put the high pressure regulator back together. Then do the same thing on the low pressure regulator. It is the same situation, just further up the assembly. So you have the tank fitting, then a 1/2" brass section where you just replaced an o-ring, then a 1 inch brass section where the Bellville washers for the high pressure reg sit, then another 1/2" section for the low pressure reg, then a long brass section where the bellville washers sit for the low pressure reg, then the end cap where the valve sits. You want to separate the second 1/2" brass section from the 1" brass section of the high pressure reg. There is another 12mm x 1.5mm o-ring sitting in there. If you do it right, you won't need to full with spraying bellville washers all over the floor. To get to the bellville washers, you would separate the 1/2" brass sections from their long sections, but I wouldn't do this unless you need to.
Clean up the threads and put everything back together, you should be good to go.
Here is a picture of the high pressure reg with the new o-ring, the little brass valve is sitting next to it. I decided to leave the high pressure reg off my gun for now because I generally don't fill past 200 bar and I don't see a need for the extra weight.
My issue revolved around three o-rings. The first o-ring is in the air tank female fitting. The o-ring is 7mm(ID) by 2mm. It is a bear to get out of the tank, but the new o-ring pops in fairly easily. I originally thought that this was the source of the leak, but after replacing the o-ring I discovered that it was not.
Now it's time to open the regulator. Not as scary as it sounds, but make sure the gun is degassed first. Take a big crescent wrench and unscrew the regulator from the gun. It will be hard at first, but you shouldn't need to apply heat or anything, just use a little elbow grease. Be careful of how you are holding the gun so you don't break off the pistol grip or something.
Once the regulator is off, there were two o-rings that were dried up and brittle. The AR20 FT has two regulators that are built pretty much the same. To change the o-ring on the high pressure regulator (nearest the air tank), you unscrew the air tank fitting from the first brass section. Again, it will be tight, but two crescent wrenches will get it done. Make sure when you start to unscrew the air fitting that the regulator is pointing down. There is a little brass valve held in place in this area, you don't want it to fall out and roll under a table. Once you have this apart, you can pop out the brass valve and you will see the o-ring. It is 12mm (ID) x 1.5mm. Replace this o-ring and put the high pressure regulator back together. Then do the same thing on the low pressure regulator. It is the same situation, just further up the assembly. So you have the tank fitting, then a 1/2" brass section where you just replaced an o-ring, then a 1 inch brass section where the Bellville washers for the high pressure reg sit, then another 1/2" section for the low pressure reg, then a long brass section where the bellville washers sit for the low pressure reg, then the end cap where the valve sits. You want to separate the second 1/2" brass section from the 1" brass section of the high pressure reg. There is another 12mm x 1.5mm o-ring sitting in there. If you do it right, you won't need to full with spraying bellville washers all over the floor. To get to the bellville washers, you would separate the 1/2" brass sections from their long sections, but I wouldn't do this unless you need to.
Clean up the threads and put everything back together, you should be good to go.
Here is a picture of the high pressure reg with the new o-ring, the little brass valve is sitting next to it. I decided to leave the high pressure reg off my gun for now because I generally don't fill past 200 bar and I don't see a need for the extra weight.