Introduction | Fundamentals | Pads | Discs/Rotors | Big Brake Kits
Understanding the bomb (brake pressure accumulator)
This section is intended to dispell some of the mysteries surrounding the brake pressure accumulator as introduced in the fundamentals chapter . If you've got this far, then you will already know that the brake pressure accumulator provides a reserve of pressurised hydraulic assistance to the brake servo in an emergency brake or stall situation.
In addition to containing a chamber filled with highly pressurised Nitrogen gas, the bomb also contains a pressure relief valve and a non-return (check) valve. The unit is sealed at the factory so if it leaks or any internal components fail then in must be properly disposed of and replaced.
Tests - some simple and some not so...
As the bomb ages, it's efficiency can suffer such that the amount of time which it can retain hydraulic pressure is reduced. It is possible for the bomb to leak down over a matter of minutes, hours or days depending on the severity of the internal faults. When hydraulic pressure is too low (below ~30bar), the relevant red alert is displayed by the Autocheck system on the instrument cluster. If this condition occurs with the engine OFF, then the fault is most likely caused by the bomb - assuming of course that the levels of hydraulic fluid and brake fluid are correct. However, if the red hydraulic alert occurs with the engine running, then the problem may be attributable to a faulty bomb or power steering pump.
Testing the bomb -
The easiest test of the bomb, which requires no equipment (other than your foot) is with the engine off, and to count the number of pumps of the brake pedal that are needed to empty the bomb. This results in a 'dead' pedal which will feel rock hard with no servo assistance. A healthy bomb should require more than 30 pumps to achieve this state. A bomb performing less than ideally will require 20 pumps, but if less than 10 pumps are needed then the bomb should be replaced absolutely as soon as possible.
Testing the brake servo for leaks -
Keeping the engine off, disconnect the hydraulic RETURN pipe on the brake servo. A few drops of hydraulic fluid should escape. If a continuous flow is observed, the brake servo is faulty and must be renewed and reconnected to the hydraulic system.
Testing the power steering pump -
If the power steering pump is not generating sufficient hydraulic pressure, then the red hydraulic alert condition will be detected by means of a pressure sensitive switch that resides in the brake servo. In order to do these tests, a special tool kit with a hydraulic pressure guage, in the form of VAG1354/1 is required. The full procedure is covered in Haynes & Bentley for the Coupe so I assume it is also applicable to the S2. Anyway, the PAS pump should deliver in excess of 300ml fluid per minute at idle. If not then it must be repaired or replaced.
As to pressures, assuming that it is physically possible to connect the pressure guage into the hydraulic pressure switch location. With the engine idling it should build up to approx 140bar (yes folks - one hundred and forty bar - Kinda puts the 1.4bar boost of an RS2 into perspective !)
Note that if the pump delivery rate is correct, but the correct system pressure cannot be attained, then the pressure relief valve within the accumulator must be faulty. The bomb must be changed ASAP.
If you've never had the red hydraulic alert -
It would be wise to do some simple checks that your S2's wiring is in tact in this regard. For some stupid reason, the wiring to the brake fluid level switch, the hydraulic fluid level switch and pressure switch is such that a no-fault condition is presented by an open circuit (a break in the wiring). Not good practice in my opinion as you could be driving around in an S2 which has major brake faults that are being masked by a disconnected switch or faulty wiring.
To check that your car's Autocheck system is properly connected to the brake and hydraulic system, the three following tests should be performed. The first two are extremely quick and easy, but the the third is tricky on account of the location of the hydraulic pressure switch.
1. With the engine OFF, unscrew the lid of the hydraulic fluid reservoir. With the wiring connected, temporarily place the lid over an empty jar. Then turn the ignition ON and the red hydraulic alert should be visible. Replace the lid and proceed to the next test.
2. Still with the engine OFF, unclip the 2-pin connector at the brake fluid reservoir and short the two pins together with a suitable connector or wire. This simulates a low brake fluid level which should also be displayed as a red hydraulic/brake alert when the ignition is turned ON. Obviously this does not actually verify that the switch inside the brake fluid reservoir is working, so you might want to invest in a syringe or turkey baster and test again with the wiring loom reconnected to the brake fluid level switch.
3. With the engine OFF (and preferably cold) for this one also - it sounds simple enough to "remove the two wire connection for the hydraulic pressure switch that resides beneath the brake servo", but it requires stout fingers and more than a modicum of patience to do this on the S2. Its tricky enough on the 3B, but its just about impossible on the ABY as access is stupendously restricted by a heat shield in that area. There are a couple of workarounds which will be added here soon...
Last Updated 3rd November 2002