# Why wouldn't an increase in steam pressure force water out of a chamber faster?

This morning, as with every morning, I had my coffee. However, today it was burnt, because I slightly overfilled the water. I use an Italian Percolator/Moka Pot on a gas top.

Chamber A is filled with cold water, and B with ground coffee. As you heat the water, steam builds up, increasing the pressure in A. This then forces water up through B, where it is soaked up by the coffee. Once the coffee has expanded, it provides more resistance, and as such the pressure in A increases further. At this point, the water is then forced through B, and up the pipe into C where it then bubbles out to fill the chamber. This makes total sense to me.

However, a problem occurs when you fill A above the safety valve shown in the second picture. The water in chamber A boils much earlier, and is then forced out through the valve, and also from B where the pot screws together. Very little water actually flows up into chamber C.

In my understanding, if you have two chambers connected by a pipe and you increase the pressure of the lower one, then surely water should flow into the upper chamber at a faster rate. It seems like this effect has an optimum pressure, above which an increase in pressure is actually detrimental to the effect. Why should this be the case, and is there a name or term for this process?

• @DanielSank there is less ballistic volume to absorb the pressure spike after a bubble nucleates, which raises the pressure (for a given $\Delta n$, if $PV=nRT$ then $\Delta P=\frac{\Delta n ~RT}{V}$ - smaller $V$, larger $\Delta P$); further, the (volumetric) rate at which liquid can escape the valve will be much lower (viscosity) than for steam; then at the higher pressure water leaks out of the "seal" between A and C and that is where a lot of water is lost- at least that's my observation, and that of OP. – Floris Aug 30 '16 at 8:47