[Richard Farrant]

Originally Posted by Allan L

Internally, at the top, is a small container of 'something' which when tipped upside down, spills into the main container which holds 'something else' - the resultant chemical reaction creates a foam which is discharged via the hose thingo. PS That exhausts my technical knowledge of those things. So I guess that violent vehicle movements might see some accidental discharges.
Edit: It's coming back, I think. The small container near the top I think was somewhat bottle shaped (perhaps even made of glass??), with a lead stopper to prevent accidental spillage, so needed to be tipped upside down to displace the stopper. Wikipedia probably has a story on them.

Allan,
You have just brought back a bad memory to me. Over 30 years ago when working in an army workshops, a fire occured in the section I was working in and as an appointed fire team member I was straight on to it. The fire extinguishers were the type you describe and must have been old then. Every one of the extinguishers on the nearby fire point failed to work and I was running around grabbing others. We got the blaze out before the brigade arrived but it was touch and go. After this there was an enquiry and all the extinguishers were updated and regularly checked and recharged. Until you described it I was not aware of the lead stopper, so thinking back it must have stuck.