and 5-ton M-series 6x6 trucks, which this article addresses. However, the
principles can be applied to many vehicles. The brake systems on the
aforementioned military vehicles include: the master cylinder, the air-
hydraulic cylinder (sometimes called the air pack), the wheel cylinders,
the brake shoes and the brake lines. The trucks are also equipped with a
parking brake — not to be confused with an emergency brake.
The second key component of the brake system is the air-hydraulic
cylinder. Shown here are the three most commonly seen air-
hydraulic cylinders mounted on 6x6s. In the foreground is the unit
from a 5-ton, at center, the unit from a G-749 2-^1 ⁄ 2 ton GMC (such
as a M211) and in the background the unit from a G-742 2-^1 ⁄ 2 ton
(such as a M35A2). Although buffs tend to call all three types “Air-
Pak,” that term is actually a Bendix trade name, and is applicable
only to the 5-ton unit and the G-749 unit.The parking brake is designed to hold a halted truck in position — not
to stop a truck — and it is not effective at stopping a moving truck. It is
also worthwhile to note that the parking brake halts the driveshaft, not
the wheels, so if the rear driveshaft is removed, or a universal joint fails,
the parking brake no longer works. The parking brake is beyond the
scope of this article.
Almost all these trucks have single-circuit power-assisted hydraulic
brakes. Single circuit means that there is no back-up, reserve or safety
brake system. Whereas commercial vehicles and most post-1968 family