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The 1974 Miraphone, itself, is in great shape- not so much the valve linkage system.
Is it possible to have a repairman replace the 1974 linkage/ valve setup with a new system? Or better to just leave well enough alone. T hanks for your help.

After 40 years, it's possible that it needs to be replaced. However, every time I've taken a horn to a repairman and said "I think the linkage needs to be replaced," they tell me "ok, we'll see" and end up fixing it without replacing it.

If you use and like the tuba as-is and plan to use it for a long time to come, I would say get it replaced if you can afford it. The only downside is money.

Yes, many folks have replaced the linkage with a modern uni-ball type linkage. It is fairly expensive.

I have a '71 186 BBb. It has the original "S" arm linkage. Many times, a good lubrication of the linkage, adjustments of the bumpers and stop arms, popping the valves out and making sure there is no scale on the valves or the rotor casings, and possibly the re-swaging of the first valve linkage bearings, since the first valve takes the most use, and the tuba is good to go. Noise is par for the course. Even that can be minimized. After everything has been checked out for mechanical integrity, put sewing machine oil on the top and bottom rotor bearings (fill the "well" in the bottom valve cap, then pull the valve slides without popping the paddles so the vacuum will draw the oil up into the bearings); then put 3-N-1 oil on the rest of the linkage, including where the "S" arm attaches to the arm of the rotor and everything from there back to the paddles and paddle springs. Put a couple of drops of regular valve oil directly down the outside 1st rotor tuning slide, without letting it drip onto the sides of the tubes (which would do nothing but wash tuning slide gunk down into the rotor, making things worse), to help the rotors clear anything coming down the leadpipe, and enjoy. These are the steps that Roger Lewis recommends, if I have stated everything correctly as I received it from him.

I have done this on a regular bassis (pun intended) ever since I got my tuba, and I don't ever intend on changing the linkage. Here, as I post this, it is almost 6:00 pm on a Thursday evening. Time to sign off and go to community band rehearsal.