Your Air Brakes
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Part 24—Three-line Braking: The Auxiliary System
AS the main air reservoir reaches a pressure of 70 p.s.i., the governor valve is opened and air can flow along both the emergency line and the line to the auxiliary reservoir. If both systems are at a pressure of 95 p.s.i., operating the service line through the foot treadle will draw air from both reservoirs, but if the main reservoir is at only 60 p.s.i, the same brake application will only draw air from the main reservoir.
The same condition applies if the auxiliary hand control is used, as the higher pressure air would be obtained from both reservoirs and, at the lower pressure, from the auxiliary reservoir only. The foot treadle operates on all road wheels, with the hand control applying pressure to the rear tractor and trailer wheels only.
This auxiliary system is operated by the hand control on the right of the steering column, and air from the auxiliary reservoir flows direct to the hand control and then on to the rear wheels of the tractor. In that line there is a T-piece, which takes air from the tractor wheel line to the stand pipe and palm coupling on the tractor, from where a loop line passes the air to a double check valve.
As pressure moves the ball across the
non-return valve, the line to the relay valve is closed and air flows via the service line to the trailer. With the inclusion of the loop line to the double check valve, the auxiliary system becomes a trailer brake control, as with a two line trailer, although with this system there is no separate control of trailer brakes only, the hand control applying equal pressure to .the tractor driving wheels also.
This is a simple system, and although subjected to some misgivings on its first arrival, it is now accepted as a valuable contribution to improved heavy vehicle braking and road safety.