All brake hydraulic components should be able to withstand pressures far higher than a human being is able to produce via the brake pedal, so in the MOT test, the tester will apply a pressure far greater than a driver ever will even under an emergency stop.


The favourite components shown up by this are the flexi pipes which can also burst, but can also show impending future failure as they swell at the weak point, metal pipes however don't swell when they are weak, they just split, the test also shows up a failing master cylinder when the pedal descends to the floor, but with no signs of a fluid leak, due to the worn piston seals allowing fluid to leak by back into the reservoir.