pressure cooker - help needed

BACKGROUND.. 95 years ago, and prior to domesticated bliss, watso bought a pressure cooker (a namco pressure cooker - and watso must have read somewhere that they were good things to have)- to cut a long story short  - that was a hopeless purchase - and even though it was still in his cupboard at the first experience of domesticated bliss, watso's partner did not show the remotest interest in the appliance.

wives/live in partners, have all been wonderful cooks (probably a trait which appeals to watso) - but nobody was interested in the pressure cooker.  even watso's mother, who was a hopeless cook (sure, her food was healthy, but damned mundane - except that she could cook a mean bit of cheap meat, on low for an hour or two), never tried a pressure cooker.

one thing for sure , is that from the internet, there is a lot more information.  the question that watso is asking, is probably on the internet - but anyway, here is the problem..

as watso seems to recall, he seems to think, that when using the pressure cooker, then when pressure is achieved (using a pressure cooker on a hot plate), then the idea is to turn the heat back to the lowest setting, which will maintain the pressure.. no problems - that is easy to do.

apart from the net, watso's chicky babe GF at a nearby giant retail store, advises watso that when she cooks the lentils in the pressure cooker - she waits for three whistles... lol - watso got all excited, bought a bottle of wine, grabbed one of those buckets from a building site, then sat in the kitchen to check out what happened..

...mmm the pressure built up, the thingy on top of the pressure cooker (not the jenny hawkins one) popped up, watso turned the hotplate down - and about 15 minutes later  , there was an outburst of pressure - watso assumes that was whistle!!!.  having noticed one whistle, watso was expecting another whistle - but that never happened .  to fill in time, watso went to the bottle shop, and bought a cask of wine - but still no extra whistles.

what is going wrong??

for what it is worth, but watso had some brisket bones in the pressure cooker, and after the pressure thingy popped up, watso turned the heat down and cooked for another 3 hours - then took the bones out, and added yellow split peas, garlic and curry powder.

ps - if the young chicky baby gf at the major retail store, played her cards right, and dumped her husband - then she could easily snap up old grumpy watso.  watso could stay home and cook - while she went out to work