Q What is the purpose of the inverted line valve used
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in the air pressure system of an Austin FJ 17-ton gross automaticcoupling tractive unit?
AThe purpose of this valve is to prevent
the driver applying two input loads simultaneously—i.e. both main and secondary brakes—to the semi-trailer. The air-pressure line to the servo, which is linked to the mechanical brake connection through the automatic-coupling, is piped through the inverted line valve. The control port of this is fed from the service-brake line. The result is that if the service brakes are actuated at the same time as the secondary brakes, the effort delivered to the semitrailer brakes is limited to a figure corresponding to the higher of the two input pressures.