Problems with brake chambers can leave mechanics baffled. The components can seem like chambers of secrets. But there are clear solutions to some common challenges.
Many issues have been addressed over time with the rollout of related advances in the components themselves. Slack Adjuster Repair Kit
“There are thousands of people who have contributed to whatever happens in a brake chamber,” said TSE Brakes Canadian sales manager Alain Mineault, during a presentation for the Canadian Transportation Equipment Association (CTEA).
George Westinghouse created the first air brake chambers in 1869 to address stopping challenges for locomotives.
Looking to address issues with logging trucks that were losing control as they traveled down mountain grades, MGM introduced the first spring brake in 1956. This allowed heavy vehicles to stop if there were drops in the air system, and it also introduced a secure parking brake controlled by a dash valve.
But good things were made better. In 1961, MGM introduced a crimped parking side on the tandem chamber, making the parking side of the chamber tamper-proof, and ensuring nobody could disassemble the parking side of the brake and release more than 1,200 lb. of potentially deadly force.
Midland Brake followed in 1995 with long stroke brake chambers to help keep brakes adjusted for a longer periods of time and maintaining some level of braking if the drums expanded because of overheating. Integrated caging bolts came at the same time.
Looking to address the threat of contaminants, Anchorlok introduced a breathable Gore-Tex dust plug. MGM introduced a fully sealed brake chamber that same year, using external tubes to exchange air between the service and parking housings. Haldex took things a step further in 1997 with internal breathing elements to allow filtered air to circulate between the service and parking chambers.
Given that brake adjustment continues to be an ongoing issue during roadside inspections, MGM launched its E-Stroke brake chamber in 2001, converting activity in the chamber to an electronic signal which determined if brake stroke was within normal limits.
For its part, TSE Brakes launched its Variable Clocking Technology in 2016, preventing service side air leaks when orienting the brake chamber’s inlet ports. The Rapid Exchange Pushrod (REX), meanwhile, came in 2018 to ensure that all push rods are the proper length based on the suspension. Cutting was no longer required.
“Cutting a push rod was as significant an issue as the clocking was,” Mineault says.
As far as the brake chambers have evolved, there are still maintenance issues to address. Here are five significant issues – and how to fix them.
Have you even installed a new brake chamber only to find that the push rod moves out when air pressure is applied but doesn’t return all the way in when the air pressure is released? If it is sticking out by ¾ to 1 inch and can be pushed all the way in with a little bit of hand pressure, there’s likely a common cause.
The brake chamber was likely installed on the wrong axle bracket holes, or the slack adjuster is the wrong length.
“Every time the push rod is extended out, because it’s not a perfect 90 degree (angle), it gets hung up in the hole,” Mineault said. Signs of that issue can be found in the warranty bin, with chambers showing signs that the push rod filed a hole in the service housing.
In the case of the bracket holes, the top position would be for a five-inch slack adjuster, the bottom position is for a six-inch slack adjuster, and the holes are used diagonally in the case of a 5.5-inch slack adjuster.
The arm span of the slack adjuster is measured from the center of the camshaft to the clevis pin.
Some suppliers have addressed this issue by introducing larger holes and Teflon or plastic discs, Mineault said, but that still doesn’t ensure the optimal 90-degree angle, so the push rod will continue to rub against the diaphragm and service housing.
After a brake chamber is replaced, fleets might find that brakes are not properly applying and releasing on the same axle. Or maybe the wheel end is running hot.
This issue is caused by an incorrect push rod length, Mineault said.
“Push rod length is something like voodoo,” he joked. “Everybody has got a special little recipe to measure the push rod.”
But of all the push rods that are available, the choices tend to boil down to three lengths.
With the exception of lift axles, the lengths are limited to 2.25 inches from the top of the cover to the clevis pin for a truck application, 5.75 inches for a trailer suspension, and 9.75 inches for an Intraax suspension.
For the lift axles, the only option will be to measuring the length when the push rod is at 90 degrees, and making a cut.
When Mineault finds a bent push rod on a brake chamber, it tends to be 9.75 inches long.
“The slack adjuster is the culprit,” he said.
The problem can be traced to an old slack adjuster that has seized and won’t rotate.
Slack adjusters must be replaced along with the brake chamber, Mineault stressed.
If a wheel end is consistently hot after a brake chamber is replaced, and everything is pulling to one side during brake applications, the problem can be mismatched brake chamber strokes across the same axle. Maybe there’s a long-stroke and standard-stroke sitting side by side.
The lengths can be changed, but should always be installed in matched pairs.
But adding a 90-degree fitting to the service side of the chamber can slow the air system as much as six feet of additional hose, he added.
Some issues can be traced to selecting the wrong parts.
If a Meritor trailer slack adjuster was installed, it’s important to verify that the correct clevis was used, he said. A Meritor trailer automatic slack adjuster (ASA) has a pin spacing of 1.38 inches, but a Meritor truck automatic slack adjuster has a 1.30-inch pin spacing.
Since the lengths are not that different, the incorrect choices can still be forced into place by hand.
The answer involves reinstalling the ASA and tightening the adjustment screw all the way in.
But there is a trick of the trade here as well. Because mechanics tend to be using longer wrenches, TSE Brakes recommends backing off the adjustment screw by ¾ of a turn, rather than half a turn. “By backing off ¾ turn, we are 100% sure we’ve cleared the home position,” Mineault said.
Then apply the brakes 12 times so the ASA can find the correct working limit.
John G. Smith is Newcom Media's vice-president - editorial, and the editorial director of its trucking publications -- including Today's Trucking, trucknews.com, and Transport Routier. The award-winning journalist has covered the trucking industry since 1995.
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If the brake adjustors are not adjusted correctly can the truck roll with the air brakes set?
In theory yes. Realistically, that’s extremely rare. If a slack adjuster is not properly adjusted, then the brake shoes may not make proper contact.
A Meritor trailer automatic slack adjuster (ASA) has a pin spacing of 1.38 inches, but a Meritor truck automatic slack adjuster has a 1.30-inch pin spacing. This statement is currently incorrect. With the addition of welded clevis chambers on trailers in the past several years (1.30″ clevis spacing), Meritor has now accepted this addition to trailer axles; IE: Trailers can have either a 1.38 or a 1.30″ clevis. However all chambers must have the same clevis spacing.
I have brand new brake shoes and drums on my tractor, and my truck still doesn’t stop at its full strength. I have put new air lines and brake valves and the valve under the fith wheel. Can someone please help me out. Thanks.
on break chambers to attach the cleaves to the slacker adjuster do you have to us the pins or can you use screws withe nuts .
“But there is a trick of the trade here as well. Because mechanics tend to be using longer wrenches, TSE Brakes recommends backing off the adjustment screw by ¾ of a turn, rather than half a turn. “By backing off ¾ turn, we are 100% sure we’ve cleared the home position,” Mineault said.”
The 3/4 vs 1/2 turn has nothing to do with the length of the wrench, the number of degrees remains the same (270° vs 180° respectively) no matter what the length of wrench is.. The difference is derived from the fine threaded adjusting mechanism used in ASA versus the coarse thread components in MSA.
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