Most ordinary industrial pneumatic parts are designed to burst at over 300 lb/in
2; the working ratings (
here 150 lb/in
2) are lower so that you never approach the burst pressure, even in a dynamic load situation, and are consequently very conservative in an essentially static case.
Were you running any liquid in them, or running them in a liquid?
Did the tanks ever get exposed to any chemical agents whatsoever? Markers/paint/organic solvents/adhesives/etc.? (The only thing
1 you should clean them with is water: not even isopropanol is guaranteed to be safe.)
1 Edit: Unless you've properly researched the cleaning agent's compatibility with polypropylene, and particularly its tendency to either cause crazing or to dissolve the plastic.