Why does my Cargo 0811 air breaks hiss when engine turned off?

When I turn the engine off on my Cargo 0811 1987 the air hisses and the brake cylinders empty. The brake light also stays on when the parking brake is released. Didn't notice any problem whilst I was driving it and the air cylinder remained at its correct pressure until I turned the engine off and then the cylinders both empty but this slowed down when I released my parking brake. Started lorry again and one cylinder fills quicker than the other. Why does this happen? Thank you