Originally Posted by Chopstix
As OP did not specify other symptoms, steam coming out can still mean heat is too high. So just check other indications such as pressure gauge et al. :-) Modern pressure cookers are supposed to be closed systems unless abused.
They aren't entirely closed systems. Then you couldn't control the pressure. There has to be a way to maintain an even pressure, and that's what the petcock and rocker is for. Pressure cookers (and canners) are sealed, but not airtight.
I think the OP gave all the needed information to respond to the question. And the answer is that what is happening is normal and nothing to be alarmed about.
In a closed system, the pressure would just increase and increase until the material (some mechanism in the pot, that is) gave out and it exploded. That's why it's so important to check your valves to make sure they have not been clogged with food and to make sure any gaskets are in good order. You have to have steam release in an amount to maintain the proper pressure. It's normal that as that steam is released, it might condense and drip from the petcock.