What if you have a cylinder, with a tight seal, and moving piston.
Like a syringe. When the pressure build up, the piston goes up, and
up, and up juuuust a litle (itsy bitsy) bit more. Until it meets a
small hole where some pressure is let out, and it sinks back a bit.

Started thinking... It might be possible with one of those
"inflatable" water tanks for camping. Pretty sturdy plastic, but
colapsable.

Could be done like this:
The container is fitted with a small walve, from a bikes air-hose(?),
whit a pin/stick that will press down the small needle when the
pressure builds up. (You could also saw away the edges on the walve,
so that the pin will be pressed down when it reaches f.in. a plate).

The container is placed inside a restricting container, so that it
behaves just like you want to at all times.
It could even be made so that there is a lid with tracks on, so that
it's always in the same position when it raises due to pressure.

Some weight is placed on top, so that the walve will only be lifted to
the "releaser" when the pressure is high enough. This should also mean
that you can decide what pressure you want, according to how mutch
weight there is on top.

The only expensive item here, would be the needle-walve that will
restrict the flow of Co2, and thusmaking the pressurization possible.

This may be a bit quirky explanation, and even poor english. Please
feel free to ask if there is something with this desciption you feel
is unclear.

Could also draw a scetch if anyone is interested in trying it out.

Hmm, I feel a bit genious today.:-)

SEE YA !!!

Malawi, The Fisher King - Ruler Of The World
Conquering the world for YOU since 1972.