Well, ya shamed me into digging out the instructions to be sure I had not misled you.
If you look at the diagram (assuming you have the same one I do) the Banjo Bolt 6B-12 is the one with the slot and it goes on the top and 6B-8 goes in the bottom. The top is denoted by being the longer end of the pump body. You have to really look at the drawing, but it does kind of show the slot in part 6B-12 and definitely does NOT show a slot in 6B-8. So I feel better now. I bet that was the problem, you were just supplying so much pressure from the hand pump in the tender that it blew out the (bathtub caulk) seal (the pumps are quite powerful!) It really should be relatively easy to work the tender hand pump... at least until you get 30 or 40 pounds of steam pressure in the boiler.
Another note (though maybe a bit late in your construction... sorry):
The tender pump diagrams show the output and return pipes as being parallel to each other, but some folk say you have to cross them to get the correct pipe to the correct side of the engine, and commented on how difficult it was to cross the rigid pipes under the tender.
Well, yes you do need to cross the connections, but I found it much better to cross the hoses at the tender-to-engine draw-bar, one below the draw-bar and one above, rather than cross the rigid copper pipes under the tender. I found that this eliminates the tendency of the hoses from the tender to the engine to kink and it makes the engine/tender connection much more flexible in curves.
Once you get the axle pump corrected, open the bypass valve and work the tender hand-pump. You should see, by looking down the tender water-fill-hatch, water squirt from the return pipe (that you should have positioned to be visible near the hatch). I always do this every time I start up for the day... this way I know the pipes are not clogged (or the hoses kinked) and all connections from the tender to the engine are correct. Then I close the bypass and work the tender hand-pump to put water in the boiler. It takes several pumps to tell that it is working by seeing a rise in the water level in the sight-glass on the boiler back-head. This also primes the axle pump.
Many folk like to find a theoretical "sweet spot" to have the bypass valve set to. One where the water being pumped in equals the steam being used. I have NEVER found that point, because the amount of steam being used varies a bit regardless of how fast the engine is going. I have R/C control of both the throttle and the valve gear and once up to running speed I back off the valve gear to the "company notch" and use lots less steam that way. But that means that how much steam I am using depends on how often I stop and startup and how well I am controlling the valve gear.
So, I gave up on finding the "Sweet spot" and just monitor the sight glass (a good idea anyway... well, not just a good idea, but a MANDATORY one no matter how you handle water usage and replenishment!) and when the water is low, I close the Bypass valve and when the water is high I open it. All part of the experience of running a real live steam locomotive!