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  Temporary Brake Failure
  Causes of Abnormal Overheating
    Binding Brakes
    Operator Error
    Brake Lining Material
  Investigation - Post Accident
  Glossary
    Brake Fade
    Vapour Lock
    Dry Boiling Point
    Wet Boiling Point
    Dual Brake Systems
  Temporary Brake Failure
 

The phenomena of having a brake "pedal to the floor" experience is not confined to car chases in old B movies.

Although it is not a situation that every motorist has experienced it is one that is certainly happening to more and more drivers.

Any make or model of car or van with hydraulic brakes is capable of this type of failure but it is most commonly found on front wheel drive cars. The phenomena described can occur regardless of whether the vehicle has an Antilock Brake System (A.B.S.) or not, although front disc and rear drum or four wheel disc brake systems utilising diagonal split brake designs are most prone to this fault.

Traditionally a "by-passing" master cylinder is blamed as the cause of "pedal to floor" problems but by far the greatest cause of this type of failure can be attributed to Glycol based (normal) brake fluid that has been contaminated by moisture, the higher the moisture content the lower the fluid boiling point can become.

A feature of brake fluid not commonly recognised is that it deteriorates over time. The main reason for deterioration is that brake fluid is hygroscopic, that is, it absorbs moisture from the air.

Because of the heat produced by friction when brakes are operated, brake fluid is designed to have a high boiling point, typically about 265°C / 509°F. Any moisture absorption could find this temperature reduced to a mere 140°C / 284°F over a period of two years. At this level, any period of sustained braking could produce enough friction to bring the fluid to the boil, where it vapourises, leaving the driver with little or no brakes.

Unlike engine oil which should be changed according to the distance driven, brake fluid deteriorates not with distance but with time. It continues to absorb water even when the vehicle is stationery. Most vehicle manufacturers recommend regular brake fluid changes on a time basis, e.g. every year. Unfortunately, such recommendations are not always followed and independent surveys have shown that many vehicles have dangerous brake fluid in them.

 
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  Causes of Abnormal Overheating
 

Binding Brakes - Whether over adjusted or sticking brake parts including handbrake cables, calipers and wheel cyclinder pistons, also master cylinder valves sticking due to corrosion etc. causing residual pressure within the system.

Operator Error - Examples of this are the parking brake not fully released or the driver resting his/her foot on the brake pedal.

Brake Lining Material - The new type of non asbestos brake lining friction material allows greater dissipation of heat from either the disc or drum into the caliper pistons or brake cylinders.

 
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  Investigation - Post Accident
 

The heat in the wheel cylinder / caliper is not immediately lost to atmosphere but is actually increased due to heat saturation from the greater heat in the disc or drum. The problem should still exist for at least 5 minutes and possibly up to 20 minutes dependant on conditions, immediate testing will prove ro disprove the driver's story. Another factor to be considered is that when a vehicle is stopped, after a period of sustained braking, then the journey continued heat transference can cause the brake fluid to boil.

If the braking system has not been touched but left to cool until the vapourised fluid has condensed once again into liquid form, a very slight vacuum can occur within the system, to normalise the system would require the brake pedal to be depressed at least twice which causes a corresponding drop in the reservoir fluid level. However because of differing brake types this cannot be assumed to be the sole reason for disclaiming the fluid as a cause.

Over-heated brakes have a distinct pungent acidic smell, therefore vehicles with drum brakes should be transferred to a building with restricted ventilation, when the rear drums are removed the odour will not only be obvious but it will also linger.

In cases of moderate over-heating the brake drum, to the untrained eye will appear normal, correct examination of the inside of the drum will highlight the fact that it is polished smoothly and slightly dark blue on colour.

The friction material will become darker and have a polished glazed appearance, any paint ot varnish on the shoe metalwork will be oxidised and appear dull with a blue tint. The utilisation of a small mirror and a source of light reflected onto the shaded friction material will enable a close examination of the lining surface to take place, under these conditions both the glazing and the effects of the bonding agent evaporation are more pronounced.

When extreme over-heating occurs the disc pads will turn white at the edges, more so if the pad is not adjacent to a heat absorbing caliper piston. The varnish / paint on the steel backing of the disc pads will usually bubble or flake, except when they are not adjacent to the caliper piston, in that instance the steel backing will turn a dirty white or grey. The friction material touching the disc will be glazed though it can also develop a rough appearance as the material breaks down and transfers between the two.

Extremely hot disc pad friction will have the appearance of black carbon crystal and the metal backs will appear pock-marked with fresh rust. The disc surface will be extremely rough and gouged with a fresh rust coating. Dust boots will be charred, the inside rubber seals will be degraded and leaking and the burning pads will leave a deposit of carbon soot.

 
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  Glossary
 

Brake Fade - If brakes are used to the point of over-heating, the co-efficient of friction is reduced until virtually no friction or stopping power exists between the friction material (pads and shoes) and the discs or brake drums. The driver experiences a normal / high brake pedal but the vehicle will not decelerate at the expected rate no matter how hard the pedal is depressed.

Vapour Lock - When the brakes are overheated the resultant heat transfers through the metal components of the wheel cylinder or caliper and boils the brake fluid. A gas vapour bubble is produced in the wheel cylinder or caliper displacing fluid back towards the master cylinder and the next depression of the brake pedal will allow the pedal to "go to the floor".

Dry Boiling Point - This is the boiling point of the fluid when it is new and has negligible moisture absorption.

Wet Boiling Point - This is the boiling point of the fluid after it has been exposed to moisture during a specified laboratory test, although most test houses merely add 3.5% water as the test is prolonged and finicky.

Dual Brake Systems - These are usually split diagonally and are designed such that if a fault occurs only part (usually 50%) of the braking force is affected. Vapour lock in one brake will cause only partial brake loss, in these cases only one front brake would be operational and an inexperienced driver could easily cause an accident.

 
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