Jul 032013

Now the handbrake calipers have had been painted, all that remained was to rebuild them …. for the second time!

The service manual suggests the two rear calipers and handbrake mechanisms should be removed as single units. However this needs the half-shaft inner joints to be broken and the front springs/shock absorbers to be removed, which seemed unnecessary.

The components for the handbrake calipers and self-adjusting mechanism ready to be re-built

I found it much simpler to detach the handbrake mechanism from the rear calipers in situ and then remove them separately. So they won’t be re-united with the rear brake calipers until the refitting of the brakes into the IRS unit.

The handbrake system is self-adjusting using ratchet mechanisms to automatically compensate for the pad wear. The distance between the pad faces is determined by how far the caliper adjusting screw has been screwed into the ratchet gear.

When the handbrake is applied, the operating arm pivots on the pin attaching it to outer caliper arm. This effectively draws the caliper adjusting screw and attached inner caliper arm towards the outer caliper arm, resulting in a clamping force.

The pivoting action also allows the ratchet gear to move relative to the sprung pawl and the extent of movement is determined by the amount of pad wear.

As the wear increases, the movement becomes sufficient such that the pawl slides along the shallow slope of the ratchet gear tooth and its spring forces it to drop onto the next tooth.

The ratchet gear moves back relative to the pawl when the handbrake is released. However the pawl now engages with the steeper slope of the tooth resulting in a turning force on the ratchet gear.

This torque rotates the ratchet gear further onto the adjuster screw and therefore reduces the distance between the two pads. The photos above attempt to show the ratcheting process, although in reality the ratchet gear will only be tightened one tooth at a time.

All the moving parts a given a generous covering of Lucas green brake grease before the cover plates were addedI’m not sure if there is a correct or recommended order for rebuilding the handbrake mechanism. I started with the internal parts of the operating arm; the pawl slots onto protrusions on the arm which guide and limit its travel. A tensioning spring attaches to the pawl and the other end to an anchor pin pushed through the operating arm.

The ratchet gear can then be installed which has a friction clip attached to its base. The clip provides sufficient resistance to stop the gear rotating when the pawl is gliding over the shallow tooth face. Once these were in place all the moving components were covered in high temperature brake grease.

The anchor pin for the return spring needs to be inserted into the outer caliper arm before the pads are fittedThe operating arm could then be attached to the outer caliper arm. I found it easier to fit the operating arm return spring first which is attached at one end to an anchor pin pushed through the caliper arm and the other to a shaft in the operating arm.

Note: the spring passes through a protective cover so this needs to be positioned before the spring is hooked over the operating arm shaft.

The pivot pin between the caliper arm and operating lever was then inserted and fixed with a split pin. A slotted screw and nut then clamp the two protective covers to the operating arm.

Finally the inner caliper arm was attached to the caliper adjuster screw with another split pin and screwed into the ratchet gear to complete the rebuild.

The caliper adjuster screw is fixed to the inner handbrake caliper with a split pin

The completed handbrake mechanisms awaiting fitting

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