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RM The Manual Setting of Brake Slack Adjusters

The current brake slack adjusters are known as fully automatic RP units. They have internal rotational differences which class them either as left or right handed.

The front adjuster units being smaller and marked NS & OS respectively sit upright, splined & bolted onto a S camshaft. When in situ these resemble a tear drop behind the rear of the front hub assemblies.

The rear units are similar but longer and are marked NR & OR again splined and bolted on to a S camshaft but hang down, similar to a inverted tear drop.

The positioning of the adjusters brass housing/bodies with the square adjustment drive showing, should and will face forward to the vehicle every time.

The terminology too 'rack on' is to 'wind on' to reduce the gap between the liner and drum and to 'rack off' is to 'wind off' is to increase the gap between the liner and drum at any brake position. This is also to facilitate a manual movement of the cam shaft via the brake slack adjusters to release a possible cam turnover,brake shoes having the wrong size rollers fitted, or a brake slack adjuster that requires some other maintenance requirement ie resetting the brake shoe for correct drum and liner clearance.

Operation of NF & OR (To Rack Off)

It can be noted and seen that these two units are diagonally opposite each other, and are termed left handed. Both have the same operation values. They have a connecting rod attached via a spherical bearing which must be unbolted/removed, so that when the square drive on the adjuster is allowed to rotate the brass body in a ANTI-LOCKWISE direction the liners will rack Off.

Operation of NF & OR ( To Rack On)

This time the procedure is similar but the connecting rod remains attached, allowing the square drive to rotate in a ANTI-CLOCKWISE direction to rack ON.

Operation of OF & NR (To Rack Off)

Again these units are diagonally opposite each other and are termed right handed. Both have the same operational internal features including a externally attached connecting rods.

Remove the connecting rod and rotate the square drive in a CLOCKWISE direction to rack OFF.

Operation of OF & NR (To Rack On)

The connecting rod remains attached whilst rotating the square drive in a CLOCKWISE direction to rack ON.

It should be observed that whilst any unit is being racked up with the connecting rods connected, or it is being hand held, a rapid clicking sound will be heard from within the adjusters brass housing. This is normal for the ratchet pawls are moving over the handed ratchet barrel within.


If in the event that the square shaft drive should refuse to rotate in any direction


Check the position of a groove machined into the splined end of the camshaft. This has been positioned 90 deg or at right angles to the S cam operating the nose rollers on the brake shoe iron.
Don't be tempted to exert that extra bit of torque, to try and move the square drive.Remove the unit and examine it at a bench for further investigation.

My bus number (if any): Ex LT Bus Engineering