New problem

[solarman216]

Seems like my time for problems, I have two Lister 1970s air cooled diesel gen sets, 3 cyl for main generation and a 2 cyl for standby, started the standby last week it ran up fine but as I started to check battery water it died with only a small whiff, thought air in the fuel, but after hitting the starter realised there was no compression, now two independent cylinders?? so thought cam shaft broke? remove oil filler cap and can see both valves going up and down, cam shaft is driven at front of engine so crank shaft cannot be broke as I can turn it on flywheel end, so question, how can both cyls lose compression at the same time about a minute after start up? have not had time to pull it apart yet to find out, maybe time tomorrow, but interested in theories, Rick