Incompatible Coupling arrangements and threads are often the cause of major problems with brakes binding on trailers. Below, two possible causes are explained:

CAUSE 1 - Suzy has NPT ½” fitting and Coupling BSP ½” thread

The NPT thread is tapered but BSP is a parallel thread causing the suzy fitting (NPT) to be screwed too deep into the coupling. This will often restrict the opening of the long stem shut off valve on the male coupling.  With every brake application air goes through the coupling to the trailer but not all the air can escape causing a residual build up of pressure and slight brake application, thereby causing brake bind.  Sometimes it may not even be possible to push the male into the female coupling.

CAUSE 2 - Incorrect spacer washer used on the Couplings

If the spacer washer is too thin, the long stem valve protrudes too far out of the male coupling causing the operating plug (adapter plug) to push the valve too far open. Same problem occurs if the spacer washer on the trailer coupling between operating plug and female coupling is too thin. Correct washer is a copper washer - WABCO Part Number SF240/4

NOTE: The specification standards for couplings, SANS 1477 states that the male long stem valve may protrude from the male coupling by maximum 1mm.