Breaking Valve Springs

['68OrangeSunshine]

This same gripe came up 2-and-a-half years ago:
'68 L6 292 -- Burnt Valve, Brkn Rings?
http://www.67-72chevytrucks.com/vboa...d.php?t=636925

Same truck, same signs and symptoms, different cylinder.
Subject is a 292 L6 in a '68 C/10 Stepside, rebuilt in 2005, under 40,000 SMOH. Engine is original to the truck, but rebuilt after being stored 25 years.

Last time after some diagnosis, we found a broken exhaust spring on Cyl #4. This time the Vacuum gauge is reading about the same [9.5" Hg w/slight bounce] where 17"Hg Steady would be the nominal indication at operating temp. RPMs also dropped. Basically, it's a 5 cylinder.

I ran a Dry Compression Check:
1] 0
2] 136
3] 135
4] 145
5] 128
6] 130

I did not run a Wet check since I found the source in Cylinder #1. Cylinder #4 only looks good since that was the one replaced in July 2014.

One broken spring is a fluke. More broken springs is an alarming trend. This time I will replace all the valve springs and set valves again.

Plug from #1 was smudgy, all others had normal ash.

New springs are on order. Also need to get valvespring compression tool = modified wonderbar. And fresh rubber umbrella seals.

Question is What causes valvesprings to break? This never used to happen in my other Chevy engines, either L6 or V8.