The boiler used to generate hot water and steam for tea and coffee making in the RBR and RMB is gas fired and pressurised.  It uses a small feed pump to pressure up the feed water to maintain the water level in the boiler when the gas heating is on.  In the RMB, the feed water comes from a storage tank above the saloon ceiling, but this has insufficient hydrostatic pressure for the boiler, hence the feed pump.

24v positive supply comes through the isolation switch, then through the pressure switch to drive the pump motor.  The 24v supply is usually switched by a second tumbler switch on the main counter control switch box - this tumbler is typically also used to switch the bar extractor fan as well.

The feed pump features a non-return valve, normally the float valve in the boiler, so that when the required pressure has been achieved, and the pressure switch opens, the pressure in the boiler is held until suffient water is drawn off to cause the pressure to drop below the pressure switch cut-in level. Unfortunately, if steam gets past the float valve into the water feed pipe, the pump will cycle continuously.

The pressure switch itself is mechanically operated by the water pressure acting on a bellows to open the contact (thus stopping the pump).  When the contact opens, it is held open by a small permanent magnet which gives about 3psi hysteresis, thus preventing too much switching on and off.  The absolute pressure is set by a thumbwheel at the bottom of the switch which adjusts the tension of a spring against which the bellows acts.

The electrical contacts in the pressure switch are rather flimsy and prone to failure due to the arcing caused by the inductive load of the pump.  There are two contacts at either end of a floating bar on the contact arm.  If one of these contacts fails, a temporary work-around is to connect a (very) flexible wire between the stationary terminal on the failed side and the floating bar.   How to do this is best seen by inspection of the failed switch.  Remember to switch the water off before disconnecting the pressure sense pipe!

The water raiser system used in Mk1 RBR's is similar. Water is stored in a tank below solebar level. This tank is pressured up by a small air compressor so that sufficient hydrostatic pressure can be gained to force water up to the level of the kitchen water taps. The air compressor is controlled by a pressure switch which cuts out the compressor when sufficient head has been built up.