1942 - 1947 Chevrolet Shop Manual



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	SEQUENCE OF EVENTS DURING APPLICATION AND RELEASE OF BRAKES As pressure is applied to the brake pedal and is transmitted from the push rod to the piston in the main cylinder, the primary cup closes the compensating port and fluid is forced through the holes in the valve cage, around the lip of the rubber valve cup, into the pipe lines and into the wheel cylinders. This pressure forces the pistons in the wheel cylinders outward, expanding the brake shoes against the drums. As the pedal is farther depressed, higher pressure is built up within the hydraulic system, causing the brake shoes to exert greater force against the brake drums, Fig. 1. As the pedal is released, the hydraulic pressure is relieved and the brake shoe retracting springs draw the shoes together, pressing the wheel cylinder pistons inward and forcing the fluid out of the wheel cylinders back into the lines toward the main cylinder. The piston return spring in the main cylinder returns the piston to the pedal stop faster than the brake fluid is forced back into the lines, creating a partial vacuum in that part of the cylinder ahead of the piston. This vacuum causes a small amount of fluid to flow through the holes in the piston head, past the lip of the primary cup and into the forward part of the cylinder. This action keeps the cylinder filled with fluid at all times, ready for the next brake application. As fluid is drawn from the space behind the piston head it is replenished from the reservoir through the inlet or breather port. When the piston is in a fully released position, the primary cup clears the compensating port, allowing excess fluid to flow from the cylinder into the reservoir as the brake shoe retracting springs force the fluid out of the wheel cylinders. EMERGENCY BRAKE The passenger car emergency brake lever is located under the dash on the left of the steering column and is connected to the idler lever with rod-type linkage. The idler lever is mounted in two brackets riveted to the center of the second frame cross member. Two one-piece cables are pivoted at the center of the idler lever and are attached to the rear brakes. The pull rod from the control lever is yoked at one end of the idler lever and provides proper torque for emergency brake application, Fig. 3. SERVICE OPERATIONS To properly maintain the braking system, servicemen must appreciate that a thorough knowl-	edge of the system, absolute cleanliness, and careful workmanship are very important. Absolute cleanliness is necessary because any foreign matter in the system will tend to clog the lines, ruin the rubber cups of the wheel and main cylinders, and cause inefficient operation or even failure of the

		Fig. 3—Emergency Brake Linkage
		braking system; dirt or grease on a brake lining will cause that lining to take effect first on brake application and fade out on heavy brake application. Careful workmanship will result in a well done job. BLEEDING THE HYDRAULIC BRAKE SYSTEM The hydraulic brake system must be bled whenever a pipe line has been disconnected, when a leak has allowed air to enter the system or at any time the system has been opened. A leak in the system may sometimes be evident through the presence of a "spongy" brake pedal. Air trapped in the system is compressible, and does not permit all pressure applied to the brake pedal to be transmitted through to the brake shoes. The system must be absolutely free from air at all times. The longest pipe line of the brake system should be bled first. The proper sequence for bleeding is: left rear, left front, right rear, and lastly, right front. During bleeding operations the main cylinder must be kept at least half full of hydraulic brake fluid. The main cylinder filler, J-713-C, Fig. 4. automatically maintains the correct fluid level in the main cylinder during bleeding. TO BLEED THE HYDRAULIC BRAKE SYSTEM Carefully clean all dirt from around the main cylinder filler plug. Remove filler plug, install adapter and automatic filler. Open automatic valve in the filler. Remove bleeder valve screw. Attach bleeder drain, Fig. 5, keeping the end of the drain hose below the surface of the fluid in the jar.