Brake servo operation
I own a 2005 Jaguar S-TYPE 3.0-litre, automatic. I may or may not have a problem with the vacuum brake assistance. If my mechanical knowledge serves me correctly, one side of the brake servo is at normal atmospheric pressure, while the other side is in a vacuum. I believe that when the engine is turned off, this vacuum is maintained, assuming the system is sealed. However, within a few minutes of the engine being turned off, the brake pedal is solid.
I questioned a mechanic about this, at the garage where the car is regularly serviced, and he said that the vacuum will be lost when the engine is turned off. Having access to a couple of work vehicles, I tried the brake pedals in them and found that the pedals travelled much further. In other words, the vacuum had been maintained.
On the Jaguar, if I start the engine while pushing on the brake pedal, it sinks slightly. During driving the brakes work normally.
So, my question is, do I have a problem with the servo system, or does the braking system on the Jaguar S-TYPE, work differently?
Stephen Hayes
The vacuum in the brake system will be lost over a period of time and will vary across different vehicles. As the vehicle ages the time the vacuum will be maintained for will diminish.
The important factor is that once the engine is started, with pressure applied to the brake pedal, the pedal should drop slightly indicating that the servo is operational. There are different points where the vacuum may be lost, and it is possible that the loss of vacuum is due to one of two common areas.
The first is the grommet where the servo pipe enters the servo. This can perish over time and allow atmospheric pressure to enter the servo. The second is the failure of the small one-way valve in the servo pipe. This would mean replacing the servo pipe as the valve is not a separate unit.
If replacing the vacuum pipe or the grommet care must be taken when removing the pipe to ensure that it is not broken in the process. The easiest way to do this is by gently pulling it to one side and pulling the pipe out one side at a time.