Lesson 2/Learning Event 2
2 1/2-TON-SERIES TRUCKS
6x6 trucks can be applied to the airhydraulic brakes of the 2 1/2
tonseries trucks. However, there is enough difference in the design
of the two brake systems so that a short discussion on the 2 1/2ton
truck brakes is needed.
The airhydraulic cylinder of a 2 1/2ton truck looks different from
that of a 5ton truck, but the operation is about the same. Brake
slave cylinder and the fluid passage at one end of the hydraulic
control valve piston. The control valve piston then pushes on the
slave cylinder compensator piston moving it to close the air exhaust
valve and open the air inlet valve. Air pressure from the reservoir
flows through the air inlet opening and out one control line to the
power cylinder and a second control line to the trailer service
Air pressure on the compensator piston and hydraulic pressure on the
control valve piston oppose each other to regulate the controlled air
pressure. When the brake pedal is released, hydraulic pressure is
removed from the control valve piston and the compensator piston
spring returns the compensator piston and control valve to the
released position. This allows the air inlet valve to close and
opens the air exhaust valve. Controlled pressure is exhausted
through the hollow compensator piston and out the air breather port.
The power cylinder and hydraulic slave cylinder work just like the
power and slave cylinders on the 5ton truck with two exceptions.
The hydraulic piston is returned by inner and outer slave cylinder
piston springs. There is no residual line pressure check valve in
the outlet of the 2 1/2ton truck's slave cylinder like on the 5ton
truck. The 2 1/2ton M35series truck has the residual line pressure
The M35series cargo trucks do not have a hand control valve or
trailer couplings at the front of the truck as the 5ton truck does.
However, they do have service and emergency trailer couplings at the
rear of the truck. The 2 1/2ton tractor trucks have a hand control
valve so the trailer brakes can be operated separately.