Safe Braking and Understanding the Brake Components
A Vehicle’s Brake Pedal Assembly Requires Regular Checking
A vehicle’s brake pedal assembly, applied many times during any one trip, is one of the most neglected areas of the brake system.
Checking or servicing of the brake pedal assembly is not often done. When wear in this area occurs, it is gradual, so the driver is not aware of the slow degradation of the system and the associated dangers.
Because the average age of the passenger vehicle on our roads is approaching 12 years, it is easy to imagine what the guide pins and clevis pins look like after all this time.
If the clevis pin wears through and snaps, there will be no connection between the brake pedal and booster and this will cause brake failure.
Not only can the clevis pin be worn through, but the clevis itself could have elongated holes which will result in excessive pedal travel, giving the driver a longer reaction time.
The purpose therefore of servicing the brake pedal assembly is to give the driver a shorter pedal travel, reducing the driver reaction time.
A third area to be checked is the brake pedal rubber. This rubber ensures that the driver’s foot does not slip off the brake pedal when the brakes are applied, especially when wet.
Brake Booster – Giving Drivers Better Braking Control
The function of the brake booster is to give the driver control of the braking system under light pedal effort.
Because the booster needs a vacuum to operate, the vacuum is created from the intake manifold on petrol engines. For diesel engines this is supplied by a vacuum pump situated on the back of the alternator. On some modern diesel vehicles there is an electric vacuum pump mounted on the inside of the engine compartment.
To determine if your booster is working, switch the engine off. Pump the brake pedal to eliminate any vacuum. Apply the brake pedal and start the car. The brake pedal will start going down a little as soon as the vacuum builds up.
To check if your booster retains vacuum over night, park your vehicle and leave it over night. In the morning before you start the engine apply the brake pedal. You should hear a pressure release. This will be the vacuum that was stored in the booster escaping.
If the brake pedal is hard, then the vacuum escaped over night and an investigation into the cause must be done. The booster diaphragm may be faulty or the non-return valve situated on the booster pipe may not be functioning properly.
Different size boosters have different boost ratios. Let’s say a booster has a boost ratio of 3,5 to 1. This means that if a driver applies the brake pedal with 10 kg of pressure, the booster will multiply that pressure by 3,5. The result being 35kg of pressure being applied to the master cylinder.
If at any time the booster fails or you lose vacuum to the booster, the brake pedal will go hard and the sensation will be of having no brakes. Do not panic. Just apply the pedal with more force. You will still have brakes, but without booster assistance. Bring the vehicle to a stop and call for assistance.
Brake Master Cylinders
The function of the brake master cylinder is to transfer the applied pressure from the brake pedal and booster to the calipers and wheel cylinders using the brake fluid.
When master cylinder seals are worn, the brake pedal will lose pressure under foot. This loss of pressure is the brake fluid escaping back past the seals as a result of them being worn.
When replacing new and complete master cylinders, measure the depth of the primary piston on the old master cylinder and ensure that the depth of the primary piston of the new master cylinder is the same depth. If the depth varies, this may affect the operation of the braking system.
If too deep, the driver will experience long pedal travel, and the technicians trying to bleed the brakes may experience difficulty in bleeding the hydraulic system. If too shallow, the booster pin will push the piston in the master cylinder and this will cause the seals to block the compensating or recuperation port. Technicians will also experience difficulty in bleeding the hydraulic system.
If this port is blocked, the expanding brake fluid (as a result of heat build up at the calipers and wheel cylinders) will not be able to return to the brake fluid reservoir, resulting in a pressure build-up in the hydraulic system. This in turn will cause the brake pads to drag on the brake disc and will ultimately result in the brakes binding and the vehicle coming to a stop.
Once the brake fluid cools down, the vehicle can be driven again. But this problem will continue until the “toe board” clearance is corrected. On some boosters, this clearance cannot be adjusted, which means the master cylinder will have to be replaced with the correct one.
If reconditioning of the old master cylinder is done, the length of the primary piston must be measured and recorded for the re-assembly. The incorrect length of the primary piston will cause the secondary piston to be in the incorrect position, resulting in either a long pedal, or a binding of the brakes, on the affected circuit.
On a cross-split braking system, this may affect one front wheel and the opposite rear wheel. The tamper-proof screw in the primary piston is what adjusts the length of the primary piston and is secured with loctite.
[Special recognition to content compiled by Fabio Dinino]
About Control Instruments
The Control Instruments group is primarily focused on the automotive after-market in sub-Saharan Africa through its subsidiary, CI-Automotive. The company was originally established in 1948 and today manufactures and distributes premium-branded automotive parts and associated products to the sub-Saharan Africa automotive after-market.
Brand names owned or exclusively represented by CI-Automotive in sub-Saharan Africa include Gabriel, VDO, Echlin, Autocom, Acsa-Mag, Mag Brakes, Warn, Hi-Lift, Textar, Eurocable, Contitech and Truck-Lite.
CI-Automotive comprises its trading division in Johannesburg and Gabriel manufacturing facility in Cape Town.
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