[bgdam]

I think your safety valve is gone. The safety valve should not be releasing pressure constantly once the whistle is on top of the lid.

[tomkinstinch]

That's not necessarily true; there are three types of popular pressure cookers[1]. Fagor-type pressure cookers release steam constantly (this could be the kind GP has), old-school weight-on-top pressure cookers release steam intermittently (this may be what you have in mind), and Kuhn Rikon pressure cookers release no steam at all (this third type is best for flavor since no volatiles are lost to the kitchen with escaping steam).

1. http://www.cookingissues.com/index.html%3Fp=2561.html

[batbomb]

Correction: They release steam if they are over-pressure, but that's not supposed to happen in normal cooking conditions

[rblatz]

Just because you have had a different experience, doesn't mean he is wrong. There are several types of pressure cooker. For example the one my mother used had a weighted thing that sat on top and vented near constantly. Rocking back and forth the whole time.