Handbrake caliper rebuild

 Brakes, Rolling Chassis  Comments Off on Handbrake caliper rebuild
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

Brake caliper rebuild

 Brakes, Rolling Chassis  Comments Off on Brake caliper rebuild
Jun 242013

The components for the front calipers - including the small circular seals between the two halvesCaliper rebuild kits are readily available from many of the usual suppliers and contain the square sectioned seals which sit in recesses in the caliper piston bores and the outer dust shields.

The dust shields are similar to a expandable bellow with the ends sitting in a recesses in the piston bore and piston. This enables the rubber shield to stop ingress of foreign particles for the full travel of the pistons.

The kits however do not contain the small square sectioned ‘O’ rings that seal the two halves of the caliper.

I think the reason being that the manufacturer provides the following warning:

Girling “split” calipers should not be separated for any reason. They were not designed for separation and reassembly and proper torque specifications are not known, other than the inner and outer bolts are torqued differently.

It seems odd for the manufacturer to give the reason that the torque specifications are not known for their own product! I can understand their wish in avoiding customers trying to split the calipers in case they are not sealed correctly and the likely consequences. Again, being of a cynical nature, I suspect this has as much to do with fear of litigation.

However, when I contacted a caliper reconditioning firm, they suggested that as long as new seals are used and the bolts are sufficiently torqued there shouldn’t be a problem. Leaks were more likely to occur elsewhere. Although I will keep an eye on the calipers once in service … just in case.

Their recommendation was to torque the 7/16″ diameter bolts to 70 lb-ft and the 3/8″ diameter bolts to 40 lb-ft. Also to work from the outside in and alternating sides each time, ie torque the outer bolt on one side then the outer bolt on the other, followed by the inner bolt next to the first outer bolt and finally the remaining inner bolt.

The front calipers have internal passageways to enable fluid to pass between the two halvesThe photo to the left shows the passageways for the brake fluid from the face where the two caliper halves join (the tip of the screwdriver is just about visible at the top of the upper piston bore). A corresponding passageway exists from the face of the other caliper joint and a small channel links the two piston bores. Thus providing the free flow of fluid between all the pistons in both halves of the caliper.

The rebuild is very straight forward. The bores and seal recesses were given a coating of Castrol Red Rubber Brake grease before the bore seal and outer dust rubber shield were fitted.

Grease was also applied to the cylinders before pressing home. The main point is to ensure the pistons were pushed in squarely to avoid damaging the seals. The outer lip of the dust shield pops into place as the piston is inserted.

Piston bore greased and seal inserted Dusty shield inserted before fitting piston

The piston bore and seal recess were covered with Girling Red Rubber grease before the seal was inserted into the recess

The rubber dust shields are insered into their groove in the caliper before pressing in the piston

Two smaller pistons & combined dust shield Greased pistons before pressing home

The front is a 3-pot caliper so one half has two smaller pistons with a combined dust shield

The pistons are also given a generous covering of rubber grease to ease fitting

Finally the two small seals between the caliper halves were greased and inserted into their recesses and the caliper bolts tightened to the torque setting above.

A square section 'O' ring sits in a recess  and provides the necessary seal between the two halves of the front caliper

Completed front and rear main calipers

The main difference between the front and rear calipers is the rears employ an external pipe for the fluid to passed between the two halves and therefore there isn’t the need for the small seals.

Jun 182013

My initial plans were to upgrade the brakes to vented discs with calipers from either Zeus or Coopercraft. By all accounts deficiencies with the original S1 brakes were addressed for the S2 cars so I’ve had a change of heart and will be fitting the original braking system for now. I’ll see how it goes and decide on any changes once I’ve driven it for a while.

All the calipers were sporting many years worth of brake dust, dirt and grease and were to be stripped back, cleaned and rebuilt. The rears were worse but once liberated from the IRS unit they were fairly easy to dismantle.

The rear calipers have the additional handbrake mechanisms attached

Rear caliper with external hydraulic pipe connecting the two halves

The handbrake ratchet mechanism and retraction fork

The handbrake mechanism was simply removed from top of the rear calipers by knocking back the lock tabs and removing the two retaining bolts. The other differences between the front and rear calipers is that the fronts have three pistons compared with two at the rear and the rears have and external hydraulic pipe connecting the two halves.

At the start I was somewhat bemused how one half of the front caliper worked as it didn’t appear to have any hydraulic feed. Obviously there must be an internal passage to transfer the brake fluid but I couldn’t see how the clamping force alone would create the necessary seal between the two halves of the caliper.

All became clear once it was split in half. In the middle of both clamping faces was a channel to the rear of the piston chamber thus providing a path for the brake fluid to flow between the two halves. Machined recesses contained rubber washers which would be compressed to create the required seal.

Unfortunately I’d not documented the rebuild of the calipers as it was done a number of years ago and they had been stored ever since. To my dismay, when I dug them out again, the zinc plating had already started to corrode …. and that was after storing them indoors!

The zinc coating on the refurbished front calipers had already corroded before they were even installed!

The worst corrosion areas were at the joint - I suspect it was a combination of poor plating and a pitted surface

It was probably due to poor plating but they’d certainly look quite shabby very quickly if they were fitted to the car and exposed to the elements.

The calipers would have been cadmium plated which isn’t an option nowadays due to the toxic chemicals involved. I’m relaxed over deviating from originality, especially for practicality reasons, and decided to strip them down again and paint them rather than re-plating. So it was another case of one step forward and two back!

The problem with the plating was limited to the cast iron components so only the main calipers and handbrake caliper arms will be painted. The handbrake ratchet mechanism will be left as passivated zinc.

The pistons were forced out in stages using an air foot pumpThe easiest method of extracting the pistons was to insert a bleed nipple into the hydraulic feed and use a foot pump to force the pistons out.

It’s a good idea to place a piece of wood between the pistons to limit their travel. The pistons can be pushed out in stages be reducing the thickness of wood each time. This enables all of the pistons to be pushed out rather than one popping out and the rest staying put.

It was also necessary to separate the handbrake caliper arm from the ratchet mechanism by removing the split pin and pressing out retaining pivot pin. The internal spring could then be removed more easily.

Handbrake ratchet mechanism and brake pad arms

Removing the handbrake mechanism cover reveals the spring which needed to be detached to remove the handbrake caliper arm.

The calipers were first soaked in a hot degreaser and then the joint faces and piston bores masked ready for painting. The paint kit ordered from Brake Caliper Specialists was a 300ml two pack paint (200ml paint & 100ml hardener) specifically designed for calipers, which was sufficient for all four main calipers and the handbrake caliper arms. Hopefully the 2 pack paint should prove more durable than other VHT paints.

A clothes rack provided an ideal hanging frame for the parts to be painted

All that remains is the caliper rebuilding ….