I think the big issue with making repairs will be the integral Bourdon tube. Unlike the Ashton gauges I've worked on that have a bolt in Bourdon tube assembly, this Ashcroft gauge has the Bourdon tube soldered into the case base casting.

I wonder if it works, as-is. Though it wouldn't be fun to get hit in the face with a blast of steam and brass bits while testing.

No hope of it operating as is. The free end of the large Bourdon tube is pinned up against the case. With no possible movement under pressure the black needle will be unable to move.

And note the way the big tube is twisted and captured by the gauge case. It wants to spring out even further.
A comment on the drawing posted, if you look closely at the lead seal represented it seems to have the initials of the Ashcroft company...........mld

There is an imprint on the lead seal. Pretty hard to make out. It looks like A.M. CO. The drawing points these out as drains. The wire and seal prevent them from being opened though…..
Looking at the drawing again the seal is still original.

I've been to the Ashcroft facility. In one of the prior corporate changeovers, they discarded their archives. They don't have much info beyond the 1980s. They certainly won't be able to supply a replacement tube!

Looking at the photos and the drawing that was attached in this thread, I think the gauge works more or less the way I said earlier. The inner tube has two links and affects both movements. The outer (bad) tube has an auxiliary lever with an adjustable fulcrum point, allowing it to counteract the other tube's effect on the lower movement, at an adjustable ratio.

The tube looks like it may have suffered an over-pressure event. Try this: soak everything in penetrant and after a few days, disconnect the lever arm at the tip of the outer tube. Put pressure on it from a deadweight tester or compressed air (don't worry, it isn't going to explode!) and see if the tube leaks. If it is split, you're all done, forget it. But if it holds pressure, and the tip deflects outward with pressure, you may be able to bend the tube back into a position where the gauge still (more or less) works.

The tube does not leak. I hooked up a vacuum pump to it. At 27” hg it did not retract from the case lip though…. Am thinking of trying a large hose clamp around it with a bit of padding so the tube doesn't get marred and see if it will pull back and stay. Don't know if that would be better than putting pressure at the moveable end only.

If the mechanism is jammed up, the tube won't move under pressure (or vacuum). That is why I suggested disconnect the link / lever arm where it attaches to the outer tube tip, thus removing any external mechanical force on the tube. Also, depending on how it is distorted, 27" of vacuum, or -15psig, may not be enough to produce a visible deflection. Once you disconnect the linkage, the tube may have enough play vertically that you can lift it clear of the lip on the case casting.

Nothing is jammed up. I've also discovered under further assessment that the smaller Bourdon tube is also distorted. It is against its outer deflection hard stop and the first few inches of the tube profile is distended. Pretty much destined to be a nice wall hanger. I may try unsoldering both tubes from the casting, reworking their shape and soldering them back. Nothing really to lose in the endeavor…. To bad the drawings posted don't have any dimensions of the Bourdon tubes.

First attempt at pulling the larger Bourdon tube back into shape. You can also see the bulge in the first few inches of the smaller tube at its fixed end.

So I had some pretty good success today making some repairs. I was able to pull the large tube back to pretty close to a correct position. (At least in reference to the drawings) most of the tip in at the free end is also gone.
The small tube had a crack in the solder seam of the free end fitting. The existing solder was soft. I figured it would be hard silver solder but it wasn't. Cleaned up, fluxed and heated with a pin tip butane and flowed some solder into it. No leaks!
The bulging at the first few inches of the fixed end was addressed. Some parallel jaw pliers (with brass shim for no marring) were used to work nearly all the bulge out and dang close to the same profile as the remainder of the tube.
Couldn't fit up a hose clamp to this tube like I did on the larger one. Laced up some stout cording about it and did a Spanish windlass windup and pulled it back so relaxed it is no longer against its overpressure stop. When I had removed this stop I saw two tiny holes, one on each side of the Vee notch. Drawing shows that these held a lead wire for an overpressure indicator. There was no signs of that wire. I fitted a piece of fine electronics solder into them. Not really sure where the free end of the small tube needs to be…
Going to take some experimentation to see if I can make this all work but it's looking like it's possible.
I had thought the large tube was the steampipe pressure but it is the small tube that is.
Need to get the movement reinstalled and figure out some fitting marriages and try some function tests with air and see where things stand.

I believe that the Bourdon tube needs to be hard. Silver solder would have annealed it.

The tubes are certainly hard. They are a pretty tough spring. It was surprising how far they had to be moved to get them to yield a bit. Had to really sneak up on it with repeated pulls to get a new set in at rest position.

According to the drawing the three Milwaukee Road classes that used the gauge were #1-4, Alco 4-4-2; #100-105 Alco 4-6-4; #201-240 Baldwin 4-8-4. All were used on passenger trains and were built in the 1930s, the Alcos had streamlining. The Alcos had 300lb. boilers, the Baldwins 285lb. Unfortunately, all were scrapped, it looks like.