SELF SEAL DANGER • I refer to your reply to
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B J Dorling's experience, Commercial Motor, February 1987, with self-sealing air brake couplings which I feel may be misleading in this case. I would point out that the illustration shown, taken from the Freight Transport Association's journal Freight of March 1981, depicts the brake coupling arrangements for the emergency line.
In his letter Mr Darling referred to the service line and, while his problem could have been attributed to a faulty non return valve, it was more likely caused by a component, namely the operating adaptor being missing or incorrectly fitted.
The operating adaptor is fitted to the female "C" coupling on the trailer and its function is to open the self-sealing valve fitted to the male coupling on the tractor air hose. It is axiomatic that without the operating adaptor no air will flow through the coupling. However, even with an adaptor fitted, the air flow can be restricted if the sealing washer between the adaptor and the female coupling is too thick as the self-sealing valve may not be opened fully.
The "C" type coupling and the self-sealing valve arrangements are covered in the British Standard BS AU138a 1980. Before the general acceptance of this Standard in the UK, it was common practice to use manually operated taps in the air lines. Under these arrangements, the operating adaptor was not required and, while all new trailers should have them fitted, and most of the older trailers have since been modified, from time to time rogue trailers are found by ETA inspectors. I would therefore endorse your recommendation for drivers to check the operation of the brakes after coupling their vehicle to a trailer. R B Rider Manager — Engineering Freight Transport Association