Today that same repair would be a couple of days in the shop, requiring tens of thousands of dollars of specialty tools. Of course those springs don't fail as often as they did back then (a combination of improved materials science and engineering) but when they do the fix is out of reach, even for a trained mechanic without access to a shop with all the required specialty tools.
Have a read:
The good old days of grease and spittle and they didn't even have duct tape yet!
That link is about replacing the entire valve (valves were a wear part, much like tires today, only with much shorter service life).