Lesson 2/Learning Event 1
The power cylinder consists of a cylinder, piston, piston rod, and
piston return spring. Air pressure admitted at the head end of the
cylinder compresses the piston return spring extending the piston
rod. When the air pressure is released, the spring retracts the rod.
Air in the rod end of the cylinder can pass freely in and out of the
cylinder through a breather air line that is attached to the air
intake system of the vehicle. A liptype piston seal prevents air
pressure from leaking between the piston and cylinder wall.
The slave cylinder is a hydraulic cylinder containing a piston and
piston cup. Some cylinders contain a check valve assembly, at the
hydraulic outlet, for maintaining a slight amount of pressure
(residual) in the hydraulic lines and wheel cylinders. The piston
and piston cup are hollow and contain a check valve that allows brake
fluid to pass through freely when the power cylinder is retracted.
When the power cylinder extends, the check valve blocks the opening
through the center of the slave cylinder piston.