Well, for any interested, I can't see how I lived without a Chrony - the info provided gave me what i needed to see what was up with my 1377 PFT rig - I'll TRY to keep my findings in a "nutshell"
When I first built this pistol I decided to use the flat-top piston kit - which comes with a pump piston & valve, both of which use black O-rings as seals.
I used a drop of SAE 30 non-detergent oil to lube the pump piston between the first & second PFT matches & experienced a significant power loss. I am now sure it was due to the oil. Everything else logically tested out to NOT be the source of the problem. I stripped & cleaned & re-set & re-tested this weekend & even swapped out(black) O-rings and the absolute BEST numbers I could get were averaging between 440-460 fps but it still varied THAT much over a 10 shot string. ( NOT consistent enough for PFT ) Especially considering I had it at a quite reliable 490+ fps ( according to 2 other peoples reliable Chronys ) for the first match, AND only had to pump 8 times.
So early this morning I decided to get Drastic, and test some things ... first i put a few through to find it the same (450 fps- iSH) then OVER pumped it to find the valve operating normally, easily dumping all air- over 500 fps, and then applied the tiniest titch of oil off a toothpick onto the pump seal and got an immediate drop to around 385 fps.
I figured Unless I could immediately find & acquire some "Oil- Resistant" O-Rings I was "Poo- Outta-Luck" so then decided to see what kind of numbers I could get if I slightly modified the original 1377 valve & pump piston.
I ended up tapping & plugging the bleed-hole, then testing, boring out the transfer port, bit by bit, & testing, porting the valve, & testing, until I got a decent fps read using those original parts.
Here are my last 2 shot strings
1) 10 pumps
475.3
481.7
477.9
482.1
472.1
477.9
477.0
479.1
480.8
478.9
2) 11 Pumps
492.8
489.2
490.1
491.4
492.1
490.7
494.0
491.9
493.9
494.6
491.8
493.7
Don't forget this is a 1377 pumper - my arms ache from the R&D, - Oh well, I'll bring it As -Is for the 7th O' Feb. match, AFTER sighting in at work this week & next, and TESTING at home ...