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BRAKES
6.4
HYDRAULIC BRAKE SYSTEM OPERATION
A
B
C
D
E
F
Must be clear to allow
proper diaphragm
movement
Compensating port compensates
for temperature changes by allowing
fluid back to master cylinder
Moveable
Brake Pad
G
Typical Hydraulic Brake System
The Polaris brake system consists of the following components or assemblies: brake lever; master cylinder;
hydraulic hose; brake calipers (slave cylinder); brake pads; and brake discs, which are secured to the drive line.
When the hand activated brake lever (A) is applied it contacts a piston (B) within the master cylinder. As the
master cylinder piston moves inward it closes a small opening (compensating port) (C) within the cylinder and
starts to build pressure within the brake system. As the pressure within the system is increased, the piston (D)
located in the brake caliper moves outward and applies pressure to the moveable brake pad. This pad contacts
the brake disc and moves the caliper in its floating bracket, pulling the stationary side pad into the brake disc.
The resulting friction reduces brake disc and vehicle speed. As the lever pressure is increased, the braking affect
is also increased.
The friction applied to the brake pads will cause the pads to wear. As these pads wear, the piston within the caliper
moves further outward and becomes self adjusting. Fluid from the reservoir fills the additional area created when
the caliper piston moves outward.
Brake fluid level is critical to proper system operation. Too little fluid will allow air to enter the system and cause
the brakes to feel spongy. Too much fluid could cause brakes to drag due to fluid expansion.
Located within the master cylinder is the compensating port (C) which is opened and closed by the master cylinder
piston assembly. The port is open when the lever is released and the master cylinder piston is outward. As the
temperature within the hydraulic system changes, this port compensates for fluid expansion (heated fluid) or
contraction (cooled fluid). During system service, be sure this port is open. Due to the high temperatures created
within the system during heavy braking, it is very important that the master cylinder reservoir haveadequate space
to allow for fluid expansion. Never overfill the reservoir! Fill to 1/4s -5/16s (.64 - .80 cm) from top of the cylinder.
This system also incorporates a diaphragm (E) as part of the cover gasket; and a vent port (F) located between
the gasket and the cover. The combination diaphragm and vent allow for the air above the fluid to equalize
pressure as the fluid expands or contracts. Make sure the vent is open and allowed to function. If the reservoir
is over filled or the diaphragm vent is plugged the expanding fluid may build pressure in the brake system leading
to brake failure.
When servicing Polaris ATV brake systems use only Polaris DOT 3 High Temperature Brake Fluid (PN 2870990).
W ARNING: Once a bottle is opened, use what is necessary and discard the rest in accordance with local laws.
Do not store or use a partial bottle of brake fluid. DOT 3 Brake fluid is hygroscopic, meaning it rapidly absorbs
moisture. This causes the boiling temperature of the brake fluid to drop, which can lead to brake fade and possible
loss of control.
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