Drain valve

A method for draining a pressurised fluid chamber, comprising:

• providing a drain valve with a collection chamber for collecting pressurised fluid from the pressurised fluid chamber, a vent chamber for venting the pressurised fluid from the collection chamber, a master valve between the collection chamber and the vent chamber, and a slave valve between the pressurised fluid chamber and the collection chamber, wherein the master valve directly contacts the slave valve to open the slave valve when the master valve is closed, and closes the slave valve when the master valve is open, wherein the master valve opens the slave valve to allow the pressurised fluid from the pressurised fluid chamber to enter the collection chamber, and closes the slave valve to allow the pressurised fluid from the collection chamber to enter the vent chamber, and wherein the master valve and the slave valve cannot be shut simultaneously;
• and
• switching between a first mode and a second mode in any order, wherein, in the first mode, the master valve is closed and the slave valve is open to collect the pressurised fluid in the collection chamber, and wherein, in the second mode, the slave valve is closed and the master valve is open to vent the pressurised fluid from the collection chamber to the vent chamber.