Put a stop to poor mail mance
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by Stephen Gray
OUT OF SIGHT — out of mind. That, unfortunately, sums up the attitude towards brakes not only of many drivers but of operators too.
So long as the vehicle they're driving stops when the pedal is depressed, they're ,happy. This is a dangerous attitude. Proper maintenance of brake systems is essential, not only from a safety point of view, but — and in the view of some this is more important — from the cost-saving aspect.
Worn-out linings score and wear drums leading to early replacement. The drums are often five or six times the price of a set of linings.
The extra heat generated by metal contact and relatively poor heat dissipation can cause other problems, such as oil-seal failure and premature bearing destruction.
Graham Montgomerie has .illustrated in an article starting on page 33 the types of brakes available on commercial vehicles. Although the method of application may differ the result, is the same — friction material in the form of pads or shoes, presses up against a disc or drum to stop the.vehicle.
The key to good maintenance of these systems is regu lar, visual checks, it doesn't take that long to take off a brake drum even on the largest truck.
Once the drum is removed linings should be checked for wear, the drum's circumference inspected for cracks, crazing or heat spotting and, in the case of hydraulic systems, the wheel cylinders checked for leaks.
Adjustment
To find out what the operator should look for when checking his brakes, I went to Girling's service department at West Bromwich. A lot of brake problems, says Girling, arise from omissions during servicing operations. For example, where steady posts are fitted, fitters often neglect to re-adjust them after a re-line. Adjustment of these is essential as they keep the surface of the shoe parallel to the drum's rubbing area, preventing tapered wear of the lining from occurring.
Equally important, but also often neglected is the lubrication of areas of contact between the side of the shoe and the back plate. Girling produces an excellent brake grease which can be applied to the back plate where the shoes move againsi it.
When movable cylinders are employed, these should be checked and their rubbing surfaces greased. Failure to de this can result in brake judder and peculiar lining wear.
It's a simple matter to check hydraulic cylinders for leaks. If fluid is not visible externally, the outer rubber dust shield can be lifted up to see if the cylinder is "weeping" internally.
All hydraulic flexible hoses can also be visually checked for leaks, while "bumps," indicating a possible internal fracture, can be shown up by one mechanic pressing the brake while another looks at the hose. Metal hydraulic brake tubing can corrode quickly, despite its protective anodised coating.
Check for leaks
Great care should be exer. cised to ensure that no meta pipes are likely to be chaffed as this will remove the coating and accelerate the wear. Finally, on hydraulic systems, the master cylinder should be examined for leaks. If "pendant''-type pedals are used on the vehicle, leaks from the rear of the cylinder can be seen inside the cab by looking up at the pedal cluster.
Where a servo is used, this too can be easily checked for leaks. If it is of the "through flow" vacuum type, any internal leak will permit fluid to be drawn into the engine and burnt causing excessive smoke from the exhaust.
Few problems are associated with the simple vacuum servo, except for failure of the diaphragm. However, if an air breather is fitted to the servo. it
should be cleaned or renewed according to the manufacturer's instructions, and the vacuum pipe itself must be inspected for cracks or distortion.
Disc brakes are gaining popularity on car derivative light vans and even some larger vehicles. These brakes are very simple and maintenance is also simplified.
An important aspect of disc brake maintenance concerns the caliper. In no circumstances should the pistons be pushed back crookedly. This will cause damage to the cylinder bores and the piston seals.
Girling manufactures a range of brake service tools, and these include a piston clamp which will make sure the pistons go back easily and straight.
If the vehicle's reservoir has been topped up during previous services, some fluid must be removed before the pistons are moved back. Failure to do this will result in fluid spilling out as it rises in the reservoir, causing damage to paintwork.
Single chamber
. All the rules regarding the operation of the shoes in hydraulic systems apply to air brake assemblies. Lubrication is again essental and correct adjustments, especially where bell crank mechanisms are used, must be made. Where external slack adjusters are fitted, these should be lubricated and checked for correct operation.
Single air-brake chambers are easily maintained. If a diaphragm goes, it can be quickly replaced by unclamping the outer retaining ring.
Spring brakes are more complex and it's vital that any repairs to these are undertaken by experts, preferably the manufacturer. Instances have occurred where unskilled people have been injured dismantling these chambers.
A thriving back-street business exists where spring brakes are repaired without replacing the spring. This is a potentially dangerous situation, as the spring is constantly compressed during normal driving, and is therefore subject to great stress.
All of the air brakes pipe work can be visually and audibly checked for chaffing, wear and leaks. Reservoir tanks must be regularly drained to remove water and oil (the oil comes from the brake system compressor) preventing them from forming a sludge which can affect valves and hoses. The most important valve of all is probably the foot valve. These can give out at a momen't's notice although modern split circuits ensure that the vehicle is not left unbraked.
Good contact between the brake shoes and drums is essential for correct 100 per cent effective brakes. When a vehicle is first relined. there is a disparity between the drum's circumferential radius and that of the shoes.
The most common method of aligning the two is to bed the brakes in on the road. While this works up to a point when there is no drum distortion, it is not the best way. Heat builds up where the shoe meets the drum and can cause "spotting" and distortion.
Trist Draper, the brake lining manufacturer, offers a reline service which can include skimming the drum to the same radius as the shoes. The work is carried out on a special lathe which allows the hub to be fitted, thus ensuring complete concentricity. Meanwhile, the shoes are ground to match on a purpose-built cylinder. The results from this treatment are very good and vehicles can go in straight away for their DoE test with full braking.
With all re-lines, Girling recommends that the brake push-off springs are renewed. They are constantly workii and are subjected to tensii both when the shoes a applied and released.
Provided they are regular checked, brakes should prese no more problems than ti components of any of the oth vehicle systems. Correct toc must always be used, such the Girling return spur remover tool, to prevent inju and speed up the repair.
Lubrication makes for corre and smooth operation.