Courtesy MGM Brakes, 2005 [edited slightly for clarity and consistency – R.L.]
As 3-inch “long stroke” brakes become standard equipment on more and more
new trucks and trailers, it is important for service and maintenance personnel to
recognize what is on the vehicle and always replace “like with like”.
To separate 3-inch long-stroke chambers from the current 2.5-inch “standard”
units, the industry has established the following forms of identification for spring brakes:
1) square air-inlet ports;
2) diaphragm replacement instructions (use only 3-inch long-stroke diaphragm) embossed into flange case;
3) a triangular shaped I. D. tag secured to the service-side clamp band bolt.
On service chambers, the identifiers are:
1) half-inch-high square embossment on top of pressure cap;
2) diaphragm replacement instructions embossed into pressure cap;
3) triangular shaped I. D. tag secured to clamp-band bolt.
Although 2.5-inch stroke and 3-inch long-stroke chambers provide virtually the
same force output at 1¾-inches of stroke, the performance and operational
characteristics beyond that point are dramatically different. Three-inch
long-stroke chambers provide greater braking force at/or beyond the point where
2.5-inch-stroke chambers cease to function. For this reason, standard and 3-inch
long-stroke chambers should never be installed on the same axle.
The heat generated during braking, especially during hard or continuous braking,
causes the drum to “grow”, or expand. The ability of a standard-stroke chamber
to provide adequate pressure of the lining against the ever-expanding drum is
limited by its stroke and performance characteristics. Therefore, as standard
(2.5-inch stroke) chambers exceed 2 inches of travel, the operator will begin to
sense the “brake fade” phenomenon. This is due to the reduced force output of
the chamber as it moves closer to its maximum travel position of 2.5 inches.
When it is at or near that point the brake will cease to provide any braking force
and the vehicle can easily become a “runaway”.
Therefore, if a 2.5-inch-stroke and a 3-inch long-stroke chamber are installed on the same axle, an imbalance may exist that could adversely affect the stopping capability of the vehicle. This is especially true when the 2.5-inch chamber exceeds its recommended (2 inches) readjustment limit. This may result in the vehicle being pulled toward the side with the long-stroke chamber, which, by now,is doing the majority of the braking. This situation may go unnoticed by the driver
during normal brake applications, but could cause vehicle instability during
emergency braking situations. Therefore, when servicing any truck or trailer equipped with either 2.5-inch-stroke or 3-inch long-stroke chambers, it is crucial that the chambers be replaced “like for like”.
And finally, when replacing a 3-inch long-stroke unit, you may come in contact with a yoke that is permanently attached (welded) to the pushrod. The welded yoke
allows manufacturers to significantly shorten the distance between the chamber
and the slack adjuster by eliminating the jam nut and threads needed to attach the
yoke. Because the yoke is permanently attached to the pushrod, removal and re-use of the yoke is virtually impossible. Therefore, unless the non-pressure housing (mounting base), pushrod, and/or yoke have been damaged or are severely worn, replace the single/piggyback unit, diaphragm and clamp band only.
The benefits derived from having a heavy-duty truck, tractor and/or trailer
equipped with 3-inch long-stroke chambers are well documented — i.e. improved
braking efficiency, increased reserve stroke and reduced out-of-adjustment citations — but unless they are properly maintained and serviced, those benefits will never be realized.
For more information regarding 3-inch long-stroke chambers, contact MGM Brakes Customer Service at 1-800-527-1534 or visit them at