Its called an Accumulator and has a rubber bladder inside and a valve that you connect an air pump to. Sometimes there is a small unit as well behind the electric switch. Turn off the electric water pump, open the tap faucet, connect an electric air pump similar to one you use to top up your car tyres, we pumped ours to about 10-18psi, this will also expel any water in the accumulator/s out the open faucet, when pressure is reached disconnect air pump, turn off the faucet and switch back on your water pump. You may have to experiment with the pressure but that is the normal procedure. All electric switches and accumulators are available from agriculture or irrigation stores.
Best Regards,
Barry and Robyn
Tradewinds III SM 171