Air compressor won't stop

Hi Guys
My Dad has an issue with his TD5 air compressor. He has changed his hand break shoes (pig of a job) and now his compressor won't stop running. He used the suspension to help lift the car to get extra clearance under while doing the hand break. Now he's finished the compressor started and won't stop. The suspension is holding its height so not a leaking bag. He's had to disconnect the battery to stop it running. Any ideas would be great. We're suspecting a height sensor but can't be to sure.

replace relay R5(compressor) with R3(headlamp wash) for test to see if it isnt stuck

Thanks he's giving it a go now. He has reported that it moves up and down ok and is currently at normal ride height. Do the discos have an air reservoir like the range rovers or are they simply an on demand system from the compressor?

on demand... the ECU energises the compressor's relay and the air inlet valves on the height sensor's input when the car is below level then stops them when it reaches normal level, and if it's higher opens the valves and the exhaust valve untill it's normal again...that's the simple version of SLS management explanation