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Sunday, 16 June 2013

Diesel engines and brake pedal creep

Diesel engines and brake pedal creep

The MOT test.
image; car mot

If a vacuum servo is fitted, then with the engine off, repeatedly apply the service brake to totally deplete the stored vacuum.

Fully apply the brake and hold at a constant pressure and check whether the pedal can be felt to travel further when the engine is started.

Reason for failure,

  • No dip can be felt when the engine is started, indicating vacuum assistance is not working satisfactorily.
  • sponginess indicating air in the hydraulic system.
  • The pedal tends to creep down while held under pressure

Vehicles with high servo boost

1. Where possible check the brake fluid level in the reservoir then pump the brake pedal several times and check that the fluid level  in the reservoir is unchanged, this will test whether there is fluid loss caused by a leak somewhere in the hydraulic system present.

Urgent attention is required if regular top up of fluid is needed - because this indicates a leak

 2. With the stored vacuum depleted ensure the pedal is not spongy or does not creep.  If there is no loss of fluid, the problem will be caused by fluid being forced past a worn seal or a valve. This could happen in the master cylinder, ABS valve block, or pressure regulator valve.
 3. With servo assistance, fully depress the pedal, if pedal goes down and touches the floor a master cylinder or servo could be the problem. On a single circuit system (one outlet pipe), the fault is likely to be in the master cylinder. Pedal creep (with no loss of fluid) is unusual with tandem (twin circuit) master cylinders; but it may happen.

The answer is to strip down the cylinder and check for damaged or contaminated seals or
corrosion of the cylinder bore strip down and rectify. It may be necessary to replace the cylinder.

Image: Brake Master cylinder

Diesel engines and brake pedal creep


Vehicles with petrol engines uses the inlet manifold to generate the vacuum for servo assistance.
Larger vehicles such as vans and 4x4s with diesel engines usually use a pump to generate the vacuum, Unlike the inlet manifold of a petrol engine, there is no vacuum relief with a pump. If excessive pedal pressure is applied when the vehicle is stationary (and the engine is running) as required for the MOT test the hydraulic pressure required to stop the vehicle will be grossly exceeded and fluid will be forced past seals that are between circuits.

The rare occurrence is known as diesel creep; and it is often incorrectly diagnosed as being caused by a faulty master cylinder.

image: diesel sign

The solution is to stop applying the excessive pressure.

If you can get the brake pedal to creep with the engine switched off and servo exhausted,
or actually under braking there is a serious problem that requires urgent attention!

A road test maybe an option to satisfy yourself, but if in doubt give the benefit of the doubt to the vehicle presenter.

Diesel engines and brake pedal creep

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