The two main reasons a spring fails or breaks are over lubrication and under lubrication.
From what I have now read in "T to T" correct lubrication is the key to keeping your rifle healthy. In older rifles, and I have no idea how to determine what is old enough. Possibly how may shots through it and age, your lubes will use up all the oil in the grease and lubes used and start to be under lubed, even adding chamber oil to an older leather seal gun isn't enough, the lubricants behind the seal can dry out as they use the oils to lube and keep the seal soft. I opened up an HW 30 the other day that was made in 1989, it may have never been opened before. I had been using chamber oil to keep the seal soft and it is in good condition but all the lubes behind the seal were hard and not doing much at all. The rifle shot like a dream but I could see it needed to be relubed. With a relube it shot as before but...it was even smoother and had a slight increase in power, velocity went from the low 500's up to the high 500's and low 600's. I didn't relube the seal so it must have been what I did behind the seal......
The other end of the cause is over lubrication, they recommend that is you do a rebuild and change the spring to use the old spring for the first break in, any over lube will be burned up and and any detonation won't hurt the new spring, when to know the lubes have been burned off to the point where you can install the new is spring seems to be the trick...so if you insatll and new spring and seal in your rifle it looks like you should do it in two steps, new seal installed then shoot it in with the old spring the install the new spring without removing the piston...
The lubricants used behind the seal play a big part in keeping your rifle healthy, if you relube a rifle and then after 3 or 4 hundred shots open it up again you will find that the lubes uses have migrated to behind the seal, the small amount that gets around the seal are what give you the power. If these lubricants have lost all the oil in them then you are under lubed...the air rifle needs oil to burn they say,,,your mileage may differ.....
Just what the book says filtered through my meager brain....

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Mike:

the proper lube makes the spring work at optimal conditions in any air rifle

under lube and the spring has to compensate for the added friction, over lube and the spring just slides to fast to achieve the max output of the piston

warren

PS: now; what makes a spring fail? according to Mike is the next post

and remember "it's 30% the gun and 70% the shooter"

I have several "old" springers (pre-WWII) which have each seen a good bit of service. All have intact original springs. And all could well use new leather piston seals, so they probably are missing some of that piston dampening as air leaks around them. As to any lubes, those likely evaporated well before I was born. Still, they soldier on. So, is their spring longevity a result of relatively low power (inefficient design), heavier built (over-built?) springs, or a better spring-to-power ratio? I'm betting there are lot of old 10M guns that have run dry but still have good springs. From what I've read, lubing springs was pretty much non-existant prior to the late 1960's, occasional oiling aside.

that are spring powered run on what Cardew calls the pop gun phase, they are made not to use oil, keeps the velocity steady. In older leather seal rifles when the lube dries up the same thing happens, they will shoot at a very steady velocity but slower than if they had the correct lubricants.

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Were the old guns ever lubed? I would not think so. At least, nothing beyond a splash of oil. Then, the "low power" or less efficient design places less stress on the spring? Put another way, has spring breakage become more pronounced with the advent of "magnumitis"? So many little CZ's, Diana 23's, etc, still kicking around, all bone dry. Guess that extra level in fps demands more critical spring care. (Not that older guns don't benefit from a lube.) Another plus for the older sporters like the Diana 27.

some of the stuff in the "bible" is over my tiny head but that seems to be the case plus they were only testing 12 ft/lb rifles due to the law in UK...

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