Hi there refering to your brake servo mounted to the bulk head under the bonnet.
They,ll test it first with the engine off by pumping the brake pedal then with there foot on the pedal starting the engine there foot should go to the floor this shows the servo is working.
Second they look at it under the bonnet for security and leaks.
Richard
MOT TESTER
mechanic
rbtrans@hotmail.com
Richard, September 2008