how can you tell if your brake servo is knackered?

any help would be much appreciated

How they do it on the MOT is to pump the brake pedal with the engine off, then keep your foot pressing down on the pedal whilst starting the engine. The pedal should go down then when the engine is running if its ok.

What makes you think your servo is dead?

well ive got the same problem as what most people have with the bendix abs system where they wont bleed, ive changed the master cylinder but still no better and i took it to the garage and they said its most likely to be the brake servo, but i didnt know how to tell if the servo was knackered, but i think its the rear load compenstaor as when i jack the back of the car completly off the ground and bleed the brakes my pedal feels as it should but as soon as the wheel go back down the pedal goes again, but im not a mechanic so i dont know, any help would be much appreciated. thanks for replying.