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TB 9-2300-426-20

c. Hydraulic and air/hydraulic brakes work because of a simple law of hydraulics; pressure cannot compress

fluid, but fluid will transfer pressure.

d. The master cylinder is the heart of the hydraulic brake system. When you press the brake pedal, the push rod

presses against the piston inside the master cylinder. This creates the pressure that makes the brakes work.

e. That's why a leak in the brake system cuts down on the stopping power. When you lose fluid, the space is

filled up with air. Air compresses under pressure, leaving the truck with a sinking or spongy brake pedal. If you

lose enough fluid, and trap enough air, you won't have any brakes.

f. If the fluid level in the master cylinder is low, it usually means there is a leak in the system. The brake system

does not consume fluid.

g. Some mechanics start troubleshooting a leak at the master cylinder. They replace the master cylinder, but the

truck still has trouble. Nothing has been solved and the cost of a new master cylinder has to be absorbed.

TB 9-2300-426-20