Drum Brakes

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Page created by T Dec 4th 2007:

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Torana Rear Brakes. Note the left Pushrod is missing and its Rubber Boot has not been fitted properly. Photo by Wolpolla. Click to Enlargen.

Drum Brakes:

Drum Brakes used on Holdens went through changes from FX on that improved the efficiency and performance. Old Holden Drum Brakes have been of the Duo Servo Brakes type.The Leading Shoe causes the Brake Shoes to rotate in the direction of the Drum to increase their stopping power.

Duo Servo Brakes:

This means that each Brake Shoe has its own Hydraulic Piston activating it. Both Pistons are in the same Housing. 

Energising:

This term describes the idea that the Brakes are designed to apply themselves with greater force from only a light Brake Pedal pressure than would otherwise be possible if the Shoes were simply applied by forcing them against the Drum via the Wheel Cylinder. The process of using the energising force of one Brake Shoe to apply the other Shoe is called Servo Action.The process of using the energising force of both Brake Shoes to apply the other Shoe is called Duo Servo Action,i.e both Brake Shoes work to apply an increased brake force than would be present if the Brakes simply pressed the Shoesagainst the Drum and provided no other function.

The Anchor Point:

The Anchor Point was originally located at the bottom on FX models but migrated to the top from EJ onwards.

Brake Shoe Link:

The Brake Shoe Link was used in Old Holdens from FX to EK but was abandoned with EJ. This was a seperate link that was connected to the Anchor Point to provide leverage between the Shoes. It was attached to the Primary Shoe.

Wheel Cylinder:

The Wheel Cylinder has always been placed at the top on Old Holdens. 

Leading Shoe:

The Leading or Primary Shoe has the least lining material and should be placed towards the front of the car in the image shown.  If the Wheel Cylinder is at the top and it's the Nearside Wheel, the Leading Shoe is placed to the Front of the Car. If the Wheel Cylinder is at the top and it's the Offside Wheel, the Leading Shoe is placed to the Front of the Car.If the Wheel Cylinder is at the bottom and it's the Nearside Wheel, the Leading Shoe is placed to the Rear of the Car.If the Wheel Cylinder is at the bottom and it's the Offside Wheel, the Leading Shoe is placed to the Rear of the Car.

Shoe Placement:

The Brakes used in the image should have the smaller (Leading Shoe) towards the front of the car and the larger (Trailing Shoe) towards the rear of the car. This is true for Old Holdens from EJ onwards. Old Holdens from FX to EK had differing placement of the Brake Shoes depending on the position of the Brake Shoe Link. The Brake Shoe link was attached to the Primary Shoe and the model of Brakes determined whether it was to the front or rear of the Car.Consult a good manual to establish the positioning of the Shoes.

Trailing Shoe:

The Trailing Shoe or Secondary Shoe has the most lining material and should be positioned to the rear of the Car in the image.

Toe and Heel of the Shoe:

The Toe is at the front of the Shoe. The Toe of the Leading Shoe should be found at the top left hand side of the image.The Toe of the Trailing Shoe should be found at the bottom right hand side of the image.The Heel is at the rear of the Shoe.The Heel of the Leading Shoe should be found at the bottom left hand side of the image.The Heel of the Trailing Shoe should be found at the top right hand side of the image.

Operation:

===The Released Position:===Both Shoes are relaxed onto their anchor position at the top of the image.

Application:

On application, both Shoes expand and initially contact the drum equally due to hydraulic pressure in the wheel cylinder.

Energising:

On Energising the Leading Shoe travels with the Drum and takes up any looseness via the Adjustment Link at the bottom of the picture.Both Shoes continue to travel in the direction of rotation. The force applied to the Trailing Shoe travels through the Wheel Cylinder Piston and adds this force to the Leading Shoe via the Leading Shoe Wheel Cylinder Piston. 

Full Braking:

Under full braking the Trailing Shoe anchors itself against the Anchor Point.The Trailing Shoe has now pressed its Wheel Cylinder Piston inwards to a mximum distance which pushes the Leading Shoe piston further out than the Pedal pressure initially caused.The Heel of the Trailing Shoe becomes significant as it is levered against the Drum by the sum of the Footbrake and Leading Shoe forces.The Toe of the Leading Shoe becomes significant as being the primary contact point.

Shoe Reversal:

You've probably heard the saying "A place for everything and everything in its place". It's the same with Brake Shoes on an Old Holden.
 
Note that the Shoes have been reversed on this Car creating a braking problem. The shorter Shoe is the Leading Shoe should be towards the Front of the Car (to the left). The longer Shoe is the Trailing Shoe and should be placed towards the Rear of the Car (to the right). Click to Enlargen. Photo by Hem_Dem.
 
Two heavy return Springs hold the Brake Shoes to the Anchor Point and make sure they release when the Pedal is released. The Handbrake Actuator is above the Axle and below the Anchor Point. The Handbrake Actuator applies the Brake Shoes when the Handbrake is pulled. Note that the Shoes have been reversed creating a braking problem. The shorter Shoe should be towards the Front of the Car (to the left of the image) on this style of Brake design. Click to Enlargen. Photo by Hem_Dem.
The Wheel Cylinder is between the two return Springs, above the Axle and below the Anchor Point.  The Handbrake Actuating Lever is underneath the Wheel Cylinder. Click to Enlargen. Note that the Shoes have been reversed creating a braking problem. The shorter Shoe should be towards the Front of the Car (to the left of the image) on this style of Brake design.  Photo by Hem_Dem.
 
 
Chamfering the leading and trailing edges of the Brake Shoes with  a file prevents squealing and brake snatch on application. Note that the Shoes have been reversed creating a braking problem. The shorter Shoe should be towards the Front of the Car (to the left of the image) on this style of Brake design. Click to Enlargen. Photo by Hem_Dem.
 
When rebuilding brakes the Handbrake must be fully released or the Drum will not fit back on. Sometimes backing off the adjustment in the Handbrake Cable can be required if it's sometime since the last Brake Reline was done.  Note that the Shoes have been reversed creating a braking problem. The shorter Shoe should be towards the Front of the Car (to the left of the image) on this style of Brake design. Click to Enlargen. Photo by Hem_Dem.
 
Rear Drum Brakes. Note that the Shoes have been reversed creating a braking problem. The shorter Shoe should be towards the Front of the Car (to the left of the image) on this style of Brake design. Click to Enlargen. Photo by Hem_Dem.
 
The Anchor Point is at the top of the Image. Note that both the Shoes rest against it. Note that the Shoes have been reversed creating a braking problem. The shorter Shoe should be towards the Front of the Car (to the left of the image) on this style of Brake design. Click to Enlargen. Photo by Hem_Dem.
 
A new Rear Wheel Cylinder fitted to a 6 Cylinder HX Kingswood. Click to Enlargen. Photo by Dusty Dirt Roads.
 
The Adjuster is at the bottom of the Image. It holds the other ends of the Shoes. Rotating its adjustment with a screwdriver causes the Shoes to expand towards or away from the Drum during routine adjustment. Note that the Shoes have been reversed creating a braking problem. The shorter Shoe should be towards the Front of the Car (to the left of the image) on this style of Brake design. Click to Enlargen. Photo by Hem_Dem.
  
Each Shoe has a U-Shaped Spring holding it against the Backing Plate. These springs lose tension with heat over time and no longer hold the Shoe firmly in place. They are called Anti-Rattle Springs. When they are in new condition they keep the Shoes from rattling when the Car hits a bump. When they become loose not only do the Brakes rattle, but the Shoe no longer contacts the Drum squarely. The result is that the Shoe digs into the outside Corner of the Drum under braking and causes snatching when the Brakes are applied. Note that the Shoes have been reversed creating a braking problem. The shorter Shoe should be towards the Front of the Car (to the left of the image) on this style of Brake design. Click to Enlargen. Photo by Hem_Dem.
 
 If the Leading and Trailing Shoes are reversed the Brakes will not Energise properly and the Car will not stop as well.Reversing the Brake Shoes also causes pulsating (motoring) in the Brake pedal under Braking which is caused by the Shoes initiating Energising but not holding Energisation.Motoring will wear the Brake Shoes out very quickly (in around a week of daily driving).

Brake Linings:

Sometimes Brake Shoes are sold identically, that is there is no Leading or training Shoe. It's best to check with the Brake Parts Supplier about their recommendation on this. Often the Brake Shoes themselves are identical and it's only the Linings that make them different.

Removing the Brake Drum:

Often the Brake Drum will bind onto the Axle. To remove it first make sure the Drum is free to rotate. This will make sure the Shoes are a resonable clearance from the Drum. Hit the Drum with a Hammer on its top outer edge at a 45 degree angle from the outside in and down towards the Axle. This is usually enough to make it pop free.

Commodores:

With Commodores the Brake design changed. The anchor point went back to the bottom of the Brake Plate. 

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