The valve linkage is in the back, where it is not exposed to damage.
I posted some photos on the TubeNet, but have made some improvements since then. The valve is one that I bought for about $5 more than 40 years ago from the junk pile in a music store in Switzerland; I used large-bore tubing from old sousaphones and a slides from an old YBB201. The large-bore tubing makes the instrument much less stuffy and mellower-sounding in the low register than a standard Conn 5J.
Routing the extra plumbing in front of the pre-existing tubing offers some protection for the pistons and also makes possible a very long 4th tuning slide. This slide can be pushed in to make 12&4 exactly in tune, can be pulled out to make 4 alone in tune, can be pulled some more to make 2&4 in tune, or can be pulled out still more to make 4 alone work instead of 2&4.
I also made an extension on the third valve button; this arrangement is comfortable even for small hands:
The second valve slide now has a pull-ring and a small bungee cord that pulls it back down:
I played around with thumb rings; this is one of them. I made it out of 3/8" soft copper tubing.
As you can see, I did not put any effort into making the instrument look pretty. My goals were comfort, playability, and cost control; it was a success in those areas. I use this tuba almost every day when I give lessons to students and whenever I need a small tuba.