Flat top piston and valve install - is this right?

[WobblyHand]

From that I gather the piston o-ring seal to the pump tube is intermittent. I've had that problem with dry or damaged o-rings, or a rough surface on the inside of the pump tube. I had one with rust on the inside that did that. Lots of oil and pumping smoothed it out. I wouldn't take 400 grit to the inside of the pump tube though.

Hope I haven't ruined the tube.  I have finer grit sandpaper as well, if I need to polish things further.  I think I have up to 2000 grit.

Are you keeping the oil-wick wet with the supplied (airgunsmith) oil? 

Yes

You are using the right piston o-ring and I assume that you're certain it is not damaged.

At this point, I'm not sure of anything.  I do have a lot of o-rings, since I bought a bag of 100 of 70D and 90D's.  So no harm in replacing them again.  Maybe I'm damaging them when I install them?  Putting on the 90D on the valve was difficult, maybe I rolled it on.  As I understand (now) that isn't a good thing.

But the real test for a good piston / pump tube seal is that it should pull a vacuum on the intake with an audible hiss with the incoming air... as Back_Roads mentioned earlier. You may need a new pump tube.

When I fully open the pump arm, there is a hiss of incoming air.

I would not suspect the valve. If the exhaust valve doesn't seal, you would hear flatulence noises out of the barrel. If the check valve doesn't seal, the pump arm will fly back. If the valve body or tube o-ring doesn't seal you will feel air coming out of the tube (under the breech and above the grip frame).

You completely lost me with this...

got some dieseling for the first 3 shots, and then things calmed down

I had a couple of high velocity (hot) shots right after the rebuild.  615 and 613 fps.  After those 2 good shots the velocities started declining.  The shot string was: (615, 613, 570, 441, 473, 501, 549, 539, 476, 554)fps  Sorry for the poor choice of words.

But I hope you get it sorted...

Wyo

Me too.  Either the tube is damaged (it looks fine, save for the small pit) or I'm damaging the o-rings installing them or something else.  I'm just trying to narrow down the possibilities.  Thanks for your insights.  They are greatly appreciated.

[Tom Tucker]

If the pit is near the beginning of the pump stroke, it wouldn't matter much.  If it's near the middle/end of the stroke, then it's a problem.  I can see it affecting things less when there is more oil present or when the o-ring is warmed up.

One time, I accidentally scored (more like gouged) a tube right at the beginning of the stroke, and it was chewing up o-rings like crazy.  it was a seriously deep gouge.  I cleaned it up with 400- 600 -1000 -1500 grit on a wooden dowel, and that gun was good as new.

Worst case scenario, new pump tubes go for $15 shipped on ebay, sometimes even less. 

[WobblyHand]

Pit seems to be about 1/3 of the way in.  If cold pumped (haven't pumped or used the 1377 in a while) it doesn't seem to seal as well.  As the o-ring warms up it is a little better, at least most of the time.  Adding oil to the back of the o-ring, or to the felt helps out some.  The velocity is rather erratic.  I do have a bunch of fine grit paper in various grades, so I can try that.  Maybe it will knock down a burr that I missed.

Tom, Your estimate for a pump tube was pretty good!  I found a new pump tube for $12.80 shipped on eBay.  So I bought one.  Hope to have that next week.

[WobblyHand]

Well, I found what I was doing wrong...  Basically, taking the supplied instructions at face value . 

I was injuring (cutting) the o-rings on insertion.  I found an interesting pair of videos on youtube by Sean Pero, showing how to prep the tube for the flat top installation.  I followed his video (and Part 1) and after I adjusted the piston length correctly everything seems to behave well.  Pumping gets progressively harder, no burping or slipping any more.  Had to jury rig a few things to lock the piston length, but that's part of the fun.

Now for a 10 shot string (at 10 pumps) I have an average velocity of 649.6 fps and an ES of 9.8 fps, using Excite Econ pellets (7.4gr), much better!  Now I can finally get back to shooting the 1377 again!