So was it you that I was bidding against? Right after I won it, because I wanted that style, I actually got another one coming in really good condition from a off eBay seller (that offline deal wasn't final). The eBay seller for this one said someone wanted post close of the auction but he couldn't come to agreement or something like that. I wasn't trying to back out but for some reason he mentioned some other bidder wanted it and I suggested I could back out. It was weird timing - either get the eBay one and end up with two or pass on the eBay one and end up with nada.
If the build up is inside of the shaft you wouldn't necessarily see bubbles. Even with mine being clogged at the bottom where the check ball I didn't see much bubbling. I'd remove the knob and slip nut too. It is interesting you can rotate the shaft but can't pull it (I assume it is fully down). It doesn't travel much past the center (where the ball is at) - I couldn't image what is keeping it from moving up. I wonder if some kind of other debris has got in there and jammed it up. I do know you can't pull the shaft out from the top, you have to remove the pipe from the center. I never been able to unscrew them, so I assume the shaft drops from the bottom.
I did not bid on it. Two is enough for me. I have the one you have and one that has the manometer on it.