Half-inch socket set
Teflon thread sealer
Ball peen hammers
TMR Spring Brakes
Failing air chambers provide little warning of the trouble looming within. Their demise is usually sudden, accompanied by the agonizing sound of escaping air as the parts exhale for the last time.
This has long been the perception among drivers who experience a blown air chamber on the road. In reality, though, it’s not sudden. The problems actually occur long before an expensive service call is required.
“Broken springs account for 85 to 90 percent of the faulty brake chambers we find in our shop,” says Paul Tidrick, a mechanic at Taylor Truck Line in Northfield, Minn. “But leaking diaphragms are responsible for about 100 percent of our brake chamber breakdowns.”
The difference is understandable. Mechanics look for defects, but most drivers do not (beyond their daily inspections, that is). The usual air chamber failure starts when one of the internal springs, fatigued by repeated application and road-chemical-induced corrosion, busts into several pieces. This often goes unnoticed because the remaining brakes continue to work normally, stopping the truck and holding it still when parked. Eventually, though, these jagged parts poke a hole in one of the chamber’s two diaphragms (also called pancakes). That gets people’s attention – and empties their wallets.
Tidrick recommends regularly testing brake power to avoid such needless headaches. This is done by trying to turn your wheels with the parking brakes applied. The test, requiring no more than a few feet of vehicular movement, should be done with an unloaded trailer on dry asphalt or concrete. It’s best to check the tractor and trailer brakes separately to avoid smoking your clutch. Also, remember that not all axles are fitted with parking brakes. Any brake unable to keep a wheel set from turning obviously needs a thorough examination.
The life expectancy of garden-variety air chambers is two to four years, depending on the truck’s operating environment. Haldex offers a long-life chamber, which has no frontal inspection hole (the usual inlet for most road and atmospheric crud) and an integral caging tool that reportedly helps keep the spring-brake, or secondary, chamber sealed.
Brake chambers come in a range of sizes, identified by a number corresponding to their diaphragm size. Most Class 8 equipment uses a Type 30. The products are further divided by stroke lengths: standard and long. You can identify a long-stroke chamber by its square air-hose ports and (when new) a small tag bearing the letters LS.
Safety is key when you’re working on or around brake chambers. The main spring, coiled tightly inside the secondary chamber, has enough pent-up force to remove body parts if it’s accidentally freed. These painful and deadly mishaps occurred regularly until the 1980s, when manufacturers started sealing the most hazardous parts of their products with welded metal bands. Still, caution is always prudent. Large hammers, acetylene torches and welders should never be used when repairing an air chamber.
In fact, these components cannot be repaired at all, just replaced. This work involves three parts, any of which can be changed independently of the others: secondary chamber, mounting housing and diaphragm. Shop time varies according to the task – diaphragm swaps being the quickest – and the difficulty posed by encrusted, petrified connecting hardware.
(These instructions apply to air chambers with removable caging bolts.)
1. CAGE SERVICE SPRING. Park your truck or trailer on a level surface, chock the wheels and apply the parking brake. Position a locking pliers against the mounting housing of the failed air chamber and securely clamp it on the pushrod to prevent movement. Release the parking brakes.
2. CAGE PARKING SPRING. Remove the caging tool, also called a T bolt, from the storage tube on the chamber. Remove the plastic inspection plug from the front of the spring-brake’s dome, or pressure cap. Shine a light inside the dome to locate the spring’s caging-tool hole. Insert the tool into the hole and turn it slightly until the T end hooks into the spring’s corresponding notches. Slip a flat washer over the tool’s bolt end and thread on a nut until it rests against the washer. Do not tighten the nut with a wrench unless you’re unable to use the vehicle’s air pressure for the caging process. When the spring is fully caged, the bolt should protrude about 31/4 inches from the dome. If air pressure is unavailable, you’ll need to crank down the nut with a wrench. In this event, do not exceed 35 pounds-feet of torque once the spring is caged. Reapply the parking brakes.
3. REMOVE AIR HOSES. (This procedure is unnecessary if you’re simply installing a new service diaphragm or mounting housing.) Mark the hoses to ensure proper reassembly, then loosen and remove them. Often the hoses will have a swivel fitting at one end and a crimped-on pipe fitting at the other. Remove both hoses at their swivel fittings. If either of these fittings is permanently bonded with corrosion, however, cut and replace the attached hose.
4. REMOVE SPRING-BRAKE CHAMBER. (This procedure is unnecessary if you’re replacing the mounting hosing and spring brake chamber.) Loosen the clamp holding the spring brake chamber to the mounting housing and tap it free with a hammer. Lift the chamber out of the clamp.
5. REMOVE MOUNTING HOUSING. (This and the following procedure are unnecessary if you’re only replacing the spring-brake chamber.) Pull the pin(s) connecting the pushrod clevis to the slack adjuster. Remove the nuts holding the mounting housing to the axle bracket. Use a steel brush to scrub accumulated corrosion and dirt from the bracket’s mating surface, then apply a coat of rust-inhibiting paint.
6. PREP AND INSTALL NEW MOUNTING HOUSING. While the old service spring is still caged, measure the pushrod length between the clevis and the outer surface of the mounting housing. Cage the new service spring by placing it on the floor – open side down – and standing on the housing while attaching a locking pliers to the pushrod. A jam nut should be installed on the new pushrod. Spin the nut down near the housing’s surface. Cut the pushrod so its length matches that of the old one. Use the jam nut to open any damaged pushrod threads. Install a new clevis and secure it with the jam nut. Mount the housing to the axle bracket and torque the nuts to the manufacturer’s specs. Connect the clevis and slack adjuster, and make sure they form a 90-degree angle when fully applied.
7. PREP AIR CHAMBER MATING SURFACE. (This and the following procedure are unnecessary on sealed chambers or complete kit replacements.) Inspect the mating surface of the chamber being reused. Use a steel brush or Emory cloth to scrub accumulated corrosion and dirt from the mating surface. Always replace clamps, bolts and diaphragms that have been in service more than a month, regardless of their appearance. These items are cheap and not worth the risk of failure.
8. INSTALL SPRING BRAKE CHAMBER. Loosely connect two band-clamp halves with one bolt. Lay a new diaphragm bowl-like inside the spring-brake chamber’s open end. Keeping the diaphragm in place with your fingertips, center the chamber on the mounting housing. Drape the clamp over the top of the chambers’ mating surface and gently tap it down. Repeat the procedure with the clamp’s bottom half and thread in its second bolt. Rotate the chamber for optimum access to the air-hose ports and fittings. Closely inspect the clamp to ensure it’s properly positioned all around the chamber, then tighten the bolts equally – moving from one to the other – until the clamp ends are 1/4 to 1/2 inch apart. Apply no more than 25 pounds-feet of torque to them.
9. APPLY AIR, UNCAGE AND LISTEN. Brush a thin coat of thread sealer on each end of your hose fittings and attach them to the proper ports. Release the parking brake and remove the caging tool from the air chamber’s dome. With an assistant using the brake pedal to apply 80 to 100 pounds of air pressure, remove the locking pliers from the pushrod. Listen for leaks. Lacking a helper, you can squirt soapy water on the mating surface then apply pedal pressure. Any leak will quickly generate a lot of bubbles. If the diaphragm hasn’t sealed, you’ll need to repeat steps 1, 2 and 8, locating the trouble spot in the process. Finish by installing the dome’s dust cap and securing the air hoses with plastic ties so they don’t rub against any hard surfaces.
Affected trucks include model year 2008-2018 Freightliner Cascadia and Western Star 4700, 4900, 5700 and 6900 trucks. DTNA says after hard brake applications, the brake light pressure switch may not activate the brake lights with the light application of the brake pedal.