I was advised by the people that service our fire extinguishers here at the apartment complexes not to ever purchase cheap plastic valved extinguishers because they will fail to hold pressure & about the time you need them they won't work. The person that told me this explained that as plastic breaks down it shrinks & he showed me some older plastic valved extinguishers that had leaked down & we examined them along side metal valved extinguishers & you could clearly see why they had leaked down.