Overrun capacity - check that it is correct for the gross weight of the trailer.
Coupling mounting bolts - check for tightness.
Coupling head bolts - check for tightness.
Coupling head - using a 50mm test towball (to meetBSAU 113c it must be between 49.61mm and 50.00mm) check that it locates and locks correctly in the coupling head and that any wear/location indicators are functioning. Lubricate the mechanism and lightly grease inside the coupling socket. N.B. If fitted with a 50mm coupling head type stabiliser, DO NOT GREASE.
Eye fittings - If fitted, check for signs of irregular wear.
Draw tube - check, by moving the shaft up and down, that the bearings that support the draw tube are not worn. The amount of end float between the shaft and the brake lever that is acceptable varies between manufacturers and should be checked in their service instructions
Drawtube gaiter - check for splits and tears. Replace if damaged.
Damper - with the handbrake off and wheels chocked, push the coupling in. If it does not move or goes in with no resistance and does not push itself back out, it must be worn and should be replaced. It should move in steadily with uniform resistance. N.B. it is essential that the correct specification of replacement damper is used and that manufacturers' fitting instructions are followed.
Handbrake - check the action for correct movement. If it is a ratchet type, are the condition and action of the teeth and pawl OK? Lubricate the pivots, including any grease nipples underneath.
Grease nipples - apply a grease gun to nipples on top of the housing, ensuring that a liberal amount of grease is applied.
Breakaway cable - check for kinks, fraying, operation of the clip and that the cable is securely attached to the "burst" ring. If replacement is required, ensure that a genuine part is used. It is desirable that the cable passes through a hole or guide in the front of the chassis to ensure straight line pull in the event of a breakaway.
Reverse cut out lever - if the coupling is fitted with a self-correcting reverse cut out lever, check that it returns satisfactorily after being applied. The above are general outline procedures and reference should always be made to the specific coupling manufacturer's maintenance instructions.