Oldtimer-trucks still have the same system of cables or linkages to the rear axle brakes that we know from the motor car. They however, require a good deal of strength and are certainly not effective enough. Apart from that, the brakes on only one axle have to hold the entire vehicle. This is why, the additional securing of the wheels against rolling is normal nowadays.
Stand-by reserve in the case of total pneumatic collapse
Apart from more comfort, a pneumatically operated handbrake has an important additional safety function, it automatically comes into play if the air-pressure drops. It works together with a spring accumulator. The air-pressure is used for the releasing of the brakes. This occurs by using the above shown hand-lever.
Quicker air-bleeding through the relay-valve
Now, this part of the brakes not only has the task of immobilising- but also the emergency function. In the event of a breakdown of the foot-brake circuits, one could also operate the handbrake lever. However, because it would take too long for all the air from the spring-accumulator to be transported to the drivers cab, and then released into the open air, a relay-valve directly between the the two combi-cylinders turns this
function around. It deaerates if compressed-air is applied from the front.
Releasing of the brakes through counterpressure to the spring chamber
In the drive position, the spring-chambers are supplied, either through the relay-valve or directly, with pressure. When the spring is compressed, the brakes are free. At the same time, in the trailer control valve the pressure to the trailer is cut off through the clutch-head-brake. The brakes on the trailer are also released.
If the pressure drops the trailer automatically causes an emergency braking
When immobilising or emergency braking, the pipes to the spring accumulator and to the trailer control valve, depending on the position of the lever, are deaerated. The spring tension acts on the brake shoes of the rear axle. At the same time the trailer is braked. Sufficient compressed air must exist here, otherwise an emergency braking would have already been carried out.
Testing of the capability of the tractor vehicle as the sole holding force
Should the handbrake lever be pushed slightly forward and to the right, a checking is possible. Thereby, the brake system of the trailer is not operated. It should be checked, whether the entire tractor-trailer unit can be held by the handbrake of the tractor alone, even if no more air pressure were available. 09/07