Lesson 1/Learning Event 2
When the pressure drops in the master cylinder, the brake shoe return
springs pull the shoes away from the drum. As the shoes are pulled
away from the drum, they squeeze the wheel cylinder pistons together.
This forces the brake fluid to flow back into the master cylinder.
The returning fluid forces the check valve to close. The entire
check valve is then forced off its seat, and fluid flows into the
master cylinder around the outer edges of the valve. When the piston
in the master cylinder has returned to its released position against
the stop plate, the primary cup uncovers the bypass port and any
excess fluid will flow through the bypass port to the reservoir.
This prevents the brakes from "locking up" when the heat of the
brakes causes the brake fluid to expand.
When the piston return spring pressure is again more than the
pressure of the returning fluid, the check valve seats. The valve
will keep a slight pressure in the brake lines and wheel cylinders.
The brake system is now in position for the next brake application.
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