Rowuk I think you are seeing my comments from the wrong direction as though I am recommending that if the valves give trouble just stretch the springs, and as if I am stating that all old instruments have tired springs that dont work properly.
I am not saying that at all.
I am saying that valves have many reasons that cause them to misbehave.
All these must be examined to identify the cause
If everything has been rigorously checked the valve is free to move it is clean well oiled all parts are present and correctly assembled there are no dents and no other possible cause then the spring may be the problem.
It can take weeks to chase down all avenues and when there is nothing else left then when the spring is the only part not cleaned or in some way fixed, then the finger of suspicion points in that direction.
Replacing the spring with a new correct part may then fix the problem and if no correct original spring is available and you end up fitting a non original incorrect spring because that is all that is available you cannot guarantee that it will work correctly.
You cannot expect a wrong part to work correctly it is always a 50/50 chance that it will.
In one recent case everything was adressed and the only thing left that could be causing the problem was the spring so I tried to get a correct replacement spring and there was no correct replacement anywhere in the world.
I found a euphonium spring a new stock item that might fit and work in a trumpet. It was a 50/50 chance it would work. I ordered the set of euphonium springs fitted them and they worked perfectly.
In another instrument everything was checked tested and cleaned repeatedly for weeks the old spring was the only culprit left. No new spring was available so a wrong stock new spring had to be tried, it was a 50/50 chance just like the euphonium spring mentioned above, but this time it did not work.
As a last ditch effort before either retiring the horn never to be played again, or giving it to a tech and saying this horn cannot be fixed, I took the chance and stretched the springs and it worked like magic. Problem fixed forever never to return.
Stretching springs must always be a last ditch effort when everything else has been tried and nothing else has worked.
In 99.999 percent of cases sticking valves is caused by one of the following
2 lack of oiling
3 wrong oil
4 badly assembled parts
5 broken parts
6 hardened residues
7 foreign matter
8 poor operation
9 incorrect parts in the valve
10 felts and corks that are breaking up
11 bent parts
It takes weeks to check and recheck all of these problem area and all of them must be checked for the root cause and fixed before moving on to the springs as the culprit.
If all these areas are fine then the problem must be elsewhere and that means the spring.
By a process of elimination the spring becomes the prime suspect.
That is why in my case every time I decide the springs must be the problem it it is after eliminating all other possible causes, and lo and behold it turns out that the spring was in fact the problem. I had checked or fixed every other possible cause before deciding the spring was the culprit.
And when the spring can be the only problem it makes no sense to declare that the spring cannot ever be a problem, it has to be the problem because everything else other than the spring has been checked and proven to be ok.
There is always a cause our job is to find the cause and resolve it. We do this by a systematic approach and by a process of elimination.
Techs are great they zero in on the problem by the same process of elimination why should we do anything less.
My advice to members is this, do not stretch springs as a first fix, use it as a last resort when every other possibility has been explored or fixed and never stretch springs if a new spring that is correct to the instrument is available.
But if everything else has been addressed and new correct springs cannot be found anywhere and a tech is unavailable or is not an option then stretching a spring can be tried and has a good chance of working.
I am not surprised that you have lots of old equipment and old valve springs that are not giving problems your equipment is very well maintained.
However it is really old equipment older than 1960 where the problems with tired springs surface. I have owned instruments from 1880 1921 1924 1948 1952 1953 1965 and the weak/tired spring issue hit most of these instruments. Instruments of this age have often been played to death.
Many players do not trust instruments of this advanced age with many citing the reason for distrusting them as being that they all have very poor valves. The likes of Sachmo and Bix or anyone else at the time never said that their instruments had poor valves.
I am wondering if weak springs due to advanced age and heavy use over 80 to 100 years is a hidden problem, because all the instruments I have owned and used up to 140 years old have without exception excellent valves.
I do believe in letting a tech work on instruments when problems hit.
I have spent thousands over the years on tech work and they have always done a fantastic job, but there are limits and sometimes it makes sense to stretch a spring when it gives an instant fix at zero cost.
Why wait 3 weeks and spend 20 or 30 dollars if it can be fixed in 30 seconds for nothing. If it doesnt work the tech is still there as an option.
I just have very old equipment that throws up an unusual problem with springs and has led to a solution that does actually work very well and is a useful addition to the tool box.