I'm quite skeptical that you have a Wagner tuba. A Wagner tuba does in fact bear a (sonic) resemblance to a French horn, and it would be odd to use it in most ensembles (excepting orchestras playing Wagner operas). You play a Wagner tuba with the left hand on the valve paddles and the bell points off past your right shoulder. More likely (and I certainly hope), you have an oval euphonium where the right hand is used on the valves and the bell points off past your left shoulder. The likelihood of finding a Wagner tuba in a small village in France seems low to me. However, whichever is the case ...
Bob probably has the problem nailed. It's not uncommon to get these horns used (particularly if from Europe) in a condition where the previous owner either hasn't oiled the valves regularly or has abused them in some way. I got mine via Ebay from Bulgaria and the second valve was a little wonky in much the way you describe. Mine is an Amati from the 60s, and so probably virtually identical to yours. Taking the valves apart isn't necessarily the best way to diagnose this -- at least initially.
As a first step, you should detach the linkage at the problematic valve(s) so that you can turn the valve directly by hand. That should tell you if that valve has a problem turning in its bushings (I find the term "bearing" in this case to be a bit inaccurate since you just have a shaft turning in a hole ).
Also, see if by pushing down and pulling up on the top valve lever you can move the valve up and down a bit in the casing. If you can, then that's a problem. If the valve doesn't move up and down in the casing and otherwise moves absolutely freely, then the problem isn't in the valve. If it does wiggle up and down a bit or you feel it being rough or binding when working the valve alone (without the linkage), then the problem is in the valve. Your last paragraph makes me suspect that there is at least vertical play in the valve and bushings.
Another thing you should try is a leak test on the instrument. Put a roll of paper towels in a plastic bag and stuff it firmly into the bell. Then blow through the mouthpipe hard. You should hear where any leaks are. If you have a bad bushing, you will often hear air coming out of it or you may see oil coming out of it (assuming you've oiled the instrument ).
If you want to disassemble and clean the valves, go on Youtube and look for Jeff Funderbunk's series of very good videos on how to do this. And be careful.
If the problem is with one or more of the bushings, you'll probably need to take it to a repair tech to get it fixed. Fixing this problem typically is easy, but you don't have the tools for it. Ferree's sells the tools, but they will likely cost you more than paying a tech to fix it -- and then you need to be able to use them and not screw up your horn beyond repair. The problem can be either with the shaft of the valve (out of round, for example), or with the bushing hole (worn or out of round). These problems CAN be fixed, but you need to have the tools and know what you're doing. Basically, you have to either use a tool to make the hole in the bushing smaller, or use a different tool/technique to make the valve shaft bigger -- but not too much in each case!! Then you have to lap the shaft/hole bushing (which may take disassembling/assembling the valve a couple of times), thoroughly clean off all the lapping compound, oil it all, and reassemble. If this doesn't sound like a lot of fun to you, then don't try it.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)