Lesson 2/Learning Event 2
The air pressure that is permitted to enter the control unit pushes
on the diaphragm assembly in an attempt to close the air inlet poppet
valve. Recall that hydraulic pressure from the master cylinder on
the relay piston opened the air inlet poppet. Therefore, the air
pressure against the diaphragm assembly is opposing the hydraulic
pressure on the relay piston. When the air pressure reaches a point
relay valve move slightly allowing the air inlet poppet to close
shutting off incoming air. But the atmospheric poppet remains closed
so the controlled air pressure is trapped in the power cylinder.
This is known as the "holding" or "lap" position.
The airhydraulic cylinder will remain in the holding position
maintaining an unchanging amount of controlled air pressure as long
as the same amount of foot pressure is applied to the brake pedal.
The amount of brake application is determined by the amount of
controlled air pressure trapped in the power cylinder. If more foot
pressure is applied on the brake pedal, more hydraulic pressure is
applied on the relay piston. This opens the air inlet poppet and the
controlled air pressure increases until it is great enough to
overcome the increased hydraulic pressure and move the relay piston
back allowing the air inlet poppet to close.
When the brake pedal is released, hydraulic pressure on the relay
piston is removed. This allows the diaphragm return spring to return
the diaphragm assembly to the released position opening the
atmospheric poppet. The control pressure is released to the outside
air by passing through the drilled center of the diaphragm assembly
and the exhaust port. The piston return spring returns the power
piston, pushrod, and hydraulic piston to the released position. As
the hydraulic piston nears the released position, the check valve in
the center of the piston opens.
The residual check valve assembly in the outlet of the slave cylinder
maintains a slight pressure in the lines and wheel cylinders, just as
To operate properly, the airhydraulic cylinder must have a supply of
compressed air. But if the air supply should fail, the vehicle
brakes will still be applied when the brake pedal is pressed. Brake
fluid from the master cylinder will flow through the check valve in
the center of the slave cylinder hydraulic piston to the wheel
brakes. There will be no boost from the airpower cylinder, but the
vehicle could be operated under emergency conditions.