Airforce Texan .45 (full length model) losing power

What might make a .45 Texan lose about 100fps and up its air usage? My brother’s .45 Texan is having this problem. Contrary to what the very nice lady at Airforce told me, it is not the power wheel settings or the way we clean the barrel. We’ve had this gun for a year and a half and are very familiar with how to tune it for different weight projectiles. It is one of 5 Airforce guns we own between the two of us. The gun is totally stock except for the addition of an R&L shroud. Any breech orings or valve/tophat factors I need to be looking at? Are there known issues with hammer spring fatigue? 
 
Bullfrog,

Sorry to hear of your troubles. Recently, I had to replace my breech o'rings to remedy a similar loss of power in my Condor. Fixed it right up. I'm not familiar with the differences between the Condor and the Texan. But here's what Tony at TalonTunes told me about my Condor. If I don't feel a light "click" when I move the breech rearward over the top hat, it's time to change the orings. That "click" that one feels when closing the breech is caused by the oring slipping over and sealing on the top hat. If there's no click, the oring is worn and is no longer a tight fit (seal) over the top hat. 

Hope this helps

BeemanR7
 
When I bought my Condor used I also had to replace the breach orings and that made a big difference. Its not clear to me exactly how the Texan seals. I do know that in any normal airgun, power loss with increased air usage would be a sign of a blown seal somewhere between the valve and the projectile. But on the Airforce guns that work a bit different than a traditional PCP, I'm just not sure. 

When I have the gun in hand I'm going to give a thorough stripdown and check every component. In its current state, its still functional and huntable, its just not up to par. 
 
Hi Travis, just saw this. Took a pic of my Texan showing the breach seal. I wish I could tell you the o-ring size, but as you can see it’s a very THIN diameter. #1 suspect in your missing foot poundage case, haha. 

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LOL Brain.

Here's a pic my brother sent me today.



Also, my chrony is reading low. Very low on some of my guns. I finally saw enough evidence that indicated I shouldn't trust my chrony. I got out my backup Shooting Chrony and it was showing as much as a 80fps gain on some of my guns. Including my .308 Texan SS, which if the backup chrony is correct, is shooting the 99 grain NSAs right at the threshold of 1000fps (999fps). 

But as to my brother's gun, the oring is likely the primary problem.