How do you check the trailer brakes work on a doubles combination?

Test trailer brakes by: (1) applying the trailer brake hand valve (sometimes called the 'trolley valve') and verifying that all trailers brake — you can confirm visually by watching the brake chambers apply or by feel when trying to drive forward; (2) releasing the parking brakes and applying the foot brake (service brakes) to verify that all trailers and the tractor brake together; (3) testing the emergency brake function by disconnecting the air supply line and confirming that the trailers' spring brakes apply automatically. Each component of the combination must brake properly; a non-braking trailer in a doubles combination is a serious safety hazard and disqualifies the vehicle from operation.