Most brake experts agree that even the most complicated balance, timing and compatibility problems can be minimized by a good PM program. We’re talking about steps taken to keep brakes performing as they did when they were new, to prevent problems from occurring and to detect small problems before they become big ones. It’s some of the easiest work your shop can perform, and it has the potential to let you help customers avoid expensive inspection violations, accidents and litigation.
Routine checks: What and when
Because every customer’s operation is a little different, maintenance intervals will vary. Only experience will produce a reliable timetable for preventive brake maintenance.
Most brake manufacturers recommend a visual inspection for wear, broken parts, chafed or badly routed air lines and other obvious damage whenever any brake service is required. Generally speaking, such a check should be made at least every three months.
Whatever the interval, it’s a good idea to accompany the inspection with a thorough operational check. Look at the whole system. The best way to do that is to go from the front bumper to the taillights, making sure everything is doing what it’s supposed to.
Because air is the lifeblood of a brake system, there must be enough of it. And it must be clean. So the compressor’s as good a place as any to start your PM inspection. With the engine running at full governed rpm, air pressure should build from 85 psi to 100 psi in 25 seconds or less, with normal-size reservoirs. If it doesn’t build up in time, and the system isn’t leaking, there’s a good chance a dirty air filter is restricting the compressor intake. That’s not a catastrophe in itself, but it quickly can lead to complications.
When a compressor tries to draw air through a plugged filter, a vacuum is created during the intake stroke. That vacuum is partially relieved by oil sucked past the compressor’s piston rings.
During the compression stroke, the oil is pumped into the discharge line and gums up components downstream- especially relay valves. Periodic checking and replacement of air filters is an easy way to ward off expensive headaches.
Even with a clean air filter, a compressor can pass oil if its oil control rings are excessively worn. Failure to change compressor oil-or engine oil, if the compressor shares oil with the engine-is the most common cause of premature ring damage. Another cause is a damaged or ill-fitting air filter that allows abrasive contaminants to enter the compressor.
Periodically disconnect the discharge line from the compressor. If it’s sludged-up to the point where the inner diameter is substantially reduced, that’s a sign that the compressor is passing excess oil.
Anything leaking? It’s easy to find out without getting your hands dirty. Use the test outlined in the Commercial Driver’s License preparation manual. Run the engine until the system is fully pressurized and shut off the engine. Release the service brakes and time the air pressure drop. The loss rate should be less than 2 psi/minute for straight trucks and less than 3 psi/minute for combination vehicles.
Next, apply the service brakes to at least 90 psi and time the pressure drop (don’t count the initial drop when you hit the pedal). The loss rate should be less than 3 psi/minute for straight trucks and less than 4 psi/minute for combination vehicles.
If leakage is suspected, paint air lines and components with soapy water and watch for bubbles. Or use an ultrasonic leak detector.
Once satisfied that a system is holding its air, make sure all chambers and spring brakes are applying and releasing on cue, and that pushrod travel is within limits, even where automatic brake adjusters are used.
Remember that automatic brake
adjusters aren’t a set-it-and-forget-it item. They need to be selected, installed and maintained in strict accordance with the manufacturer’s instructions.
The maximum allowable pushrod travel is 1-3⁄4 inches for type 16, 20 and 24 chambers, and 2 inches for type 30 chambers. Also remember that, when properly adjusted, the adjuster and pushrod should form about a 90 degree angle, with the brakes fully applied.
Improperly adjusted brakes often are responsible for a host of more complex problems, especially when all the brakes on a vehicle are not adjusted to the same degree. When one or more brakes are out of adjustment, they’re not doing their share of the braking. This results in brake imbalance and increased stopping distance.
Moreover, any properly adjusted brakes on the vehicle are forced to do extra work, resulting in excessive heat and wear. While a heat-damaged drum and prematurely worn lining at one or more wheel end could indicate a dragging brake, the astute technician will be quick to suspect an out-of-adjustment condition elsewhere on the vehicle.
Lubrication is an oft-forgotten PM measure. Whenever a vehicle is brought in for chassis lubrication, brake adjusters, air chamber brackets and anchor pins should be lubed, too. This helps automatic brake adjusters stay that way, keeps manual adjusters easily adjustable, allows camshafts to rotate freely and staves off costly wear. Follow manufacturers’ lubricant recommendations for individual parts, and avoid getting grease or oil on brake linings.
When it’s time to reline
While no one can tell you how long a set of linings will last in your customer’s operation, lining thickness should be measured whenever the vehicle is serviced, or at least every three months. It is generally agreed that linings should be replaced when the thinnest point gets down to about 1⁄4-inch thickness.
When choosing a replacement lining, adhere to manufacturers’ recommendations. Simply matching the edge codings by no means ensures that original frictional and wear characteristics will be duplicated.
And buying lining on price alone is an invitation to incompatibility. Before installation, check that relined shoes haven’t “grown” beyond manufacturer specifications. Check for elongated slots and holes, and look for loose rivets and excessive shoe-to-lining clearances.
Reline time is the time to perform a thorough inspection of axle ends. Check spiders for hairline cracks. Drums should be checked for scoring, cracking, excessive inner diameter or runout. Look carefully at cams and camshafts; cam faces should be free of ridges, cracks and flat spots. Camshaft bearing journals should be smooth. Check for cracked or deformed splines.
Wiggle the S-cam up and down to check for radial play, and in and out to check for end play. Generally, only a few hundredths of an inch in either direction is permitted.
Excessive play can cause uneven application and brake chatter. If it’s a close call, use a dial indicator to be sure.
Check chamber brackets for cracks, bends, looseness and worn or damaged bushings (or bearings, if so equipped) and seals. The chambers themselves should be examined for cracks, clogged vent holes, bent pushrods, and for loose mountings, air fittings and clamp rings. Likewise, brake adjusters should be checked for cracks, damaged splines, worn clevis pin bushings and sticky adjustment nuts.
Examine return springs carefully. Better yet, don’t reuse them. Return springs are highly stressed, and if they are corroded even slightly, there’s a good chance they’ll fail.
All of the above checks can be performed fairly quickly, but they require some attention to detail and a little judgment, which may not be present for rookie brake mechanics.
However, many brake system and component manufacturers offer seminars and clinics for brake mechanics. Taking advantage of these may be one of the smartest ways to spend brake maintenance dollars.
Miscellaneous tips
The following additional, well-accepted recommendations are offered by brake system veterans:
- Occasionally squeeze rubber hoses to check for soft spots that indicate internal damage. Blisters inside hoses can restrict air flow and adversely affect system performance. Never pinch hoses with pliers or vise grips, as this will initiate such damage.
- Have drivers exercise care when making tractor-trailer air connections. The gladhands should be checked for debris and wiped down or tapped out if necessary. There’s no practical way to completely purge an air system, so a lot of what goes in stays in. Accumulated foreign matter eventually will interfere with proper system operation. Also, air lines should be suspended well above deck plates to prevent hose chafing.
- Wherever possible, specify that brakes be mounted so that, as they are applied, S-cams rotate in the same direction as the wheels they serve. When S-cams apply opposite the direction of wheel rotation, hardware and linings wear out more quickly, and brakes are noisier.
- Don’t forget the compressor unloader. Once a year, remove and lubricate it and replace all rubber parts.
- Don’t mix manual and automatic brake adjusters on a vehicle. That’s asking for an adjustment imbalance. If you must do it, never mix them on the same axle.
- Use only recommended air dryer or system antifreeze chemicals. The wrong ones can attack rubber parts and cause serious system leakage.
Maintaining brake systems before trouble occurs will prevent lots of problems and minimize ones that crop up.
While your customers may take the approach of ‘Fix it when it breaks,’ you need to let them know it will cost more in the long run.
