With many local houses built in the 1920s with Speakman bathroom fixtures and plumbing and brass supply lines, the local word is that when trying to do something like add a shower extension to the bathtub in one bathroom or repair the shower valve in the other bathroom, working on the existing brass supply lines is impossible.
The brass has experienced too much internal corrosion from the water chemistry, and it was more difficult to deal with than copper right from the start.
Am I doomed to replacing all the supply lines, or can I work (carefully) with the brass pipes to add the shower extension and repair the shower valve?
Any ideas on how to repair the gasket, O-ring, or whatever seal in the Speakman shower valve?
I think you should feel very lucky that you got almost a century out of your plumbing system. Stop repairing it, because it would break down repeatedly - money down the drain.
Switch to new pipes, L copper, PEX or something else (CPVC) and new fixtures.
Use Delta/Peerless fixture for durability and customer service (this is not an endorsement, only a suggestion).