UPDATE .25 cal synrod air leak @gage

bigE Did you try wrapping the threads with Teflon tape ?

There is a large O-Ring at each end of the gauge block. There is a small O-Ring at the bottom of the hole the gauge screws into. Check the small one in the hole where the gauge screws into first. Overtightening the gauge will cause the O-Ring to become distorted and prevent a good seal. The large O-Rings at each end of the block can become nicked and leak. The hole in the tube where the gauge goes through frequently has a very sharp burr which can nick the O-Rings as the gauge block is installed or removed. A small amount of silicon oil wiped on the O-Rings really improve the quality of the seal.

If you have the rifle down this far I would suggest you relieve the sharp edges caused by the manufacturing in the main tube. I took a 1 inch ball hone purchased online and cleaned up that area of my syn rod before reassembly. If you can't get a hone you can take a piece of a broom stick and wrap the end with some Emory Cloth. Use some lubricant as you do either process and clean the main tube thoroughly before attempting reassembly. Removing those burrs will reduce the chances of cut Orings. Use a generous amount of silicone grease during the assembly with the new Orings. I had a similar problem so went through a total rebuild of my SynRod 25 cal. I also replaced the gauge just to make certain it wasn't an internal leak of the gauge itself...Good luck and keep us posted...

I absolutely agree Fr8bum... any time you have it that far broke down, clean, deburr and polish.

What are you using on your orings to lube them? It could be a gauge adapter orings, but if it were the valve oring you would be losing air from every point past the valve; not just gauge.

What video are you watching?

I used some special stuff to seal a thread leak....let me find a pic for you.

Here you go....made for tapered threads and pressures up to 10K psi.

I am not a huge fan of sealants. I always assume that some day I will wish to get back into an airgun to update/modify/repair it. I am a firm believer that nothing which is mechanical lasts forever. 

This sealant isn't permanent. I used it because it was specifically for high pressure applications.

I have to add that this shouldn't be a substitute for a proper o-ring replacement. My part didn't have an oring in there so it was proper for me.

Maybe something dirt or a particle of some sort inside of your fill nipple. Try cleaning it.