TM 5-3805-260-24When the brake pedal is released, the air pressureagainst the brake master cylinder is relieved (let out).The master cylinder piston is retracted and hydraulic oilpressure is let off of the caliper pistons. The residualpressure valves (check valves) keep a 3 to 6 psi (20 to40 kPa) back pressure on the pads to keep them againstthe discs. The pistons and pads are not returned bysprings. They just release their grip on the disc. Thepads do not move away from the discs so no adjustmentis needed when the pads begin to wear because thepads keep a little contact with the discs, the pads keepthe discs clean.The air flow comes from three air tanks (3), (6) and(7) filled by air compressor (1) through the air dryer (34)(Models 613BSNS1 and 613BSS1). From the air tanksthe air flow is divided into four separate circuits: servicebrake, emergency brake, parking brake and accessoryair circuits.SERVICE BRAKE CIRCUITWhen the brake pedal is pushed, pressure air is sentfrom brake control valve (28) to the tractor and scraperair chamber and brake cylinders. The tractor andscraper brakes are now engaged.NOTE: Air from the control valve goes through doublecheck valve (27) before it goes to the air chamber andbrake cylinders.EMERGENCY BRAKE CIRCUITWhen there is air pressure in all the air tanks, theservice (wheel) brakes can be used for emergencybrakes. Emergency brake control valve (30) is acti-vatedeither manually or automatically. The valve is controlledmanually by a knob and automatically when air pressurein the system is too low.Air flows from wet tank (7) to emergency brakecontrol valve (30). The air holds the emergency brakecontrol valve in, which keeps the exhaust port closed. Airthen flows from the emergency brake control valve toemergency pilot valve (23).When there is pressure in the line from emergencybrake control valve (30) to emergency pilot valve (23) airfrom reserve air tank (3) can not go through theemergency pilot valve. When air pressure to theemergency pilot valve is released through the emergencybrake control valve, the emergency pilot valve opens. Airpressure from reserve air tank (3) now goes throughemergency pilot valve (23), double check valve (27) andto the tractor and scraper brakes. The tractor andscraper brakes are now engaged.PARKING BRAKE CIRCUITParking brake control valve (3 1) is activated eithermanually or automatically. The valve is controlledmanually by a knob and automatically when air pressurein the system is too low.When there is no pressure in the air system, the knobin parking brake control valve (31) is pulled out (parkingbrake ON position). There is no air pressure in parkingbrake chamber (8). The spring force in the parkingbrake chamber keeps the brake activated.After the engine is started and air pressure is at thecorrect pressure for operation, the knob on the controlvalve must be pushed in and held for a moment (parkingbrake OFF position). Air pressure now goes fromparking brake control valve (31) to parking brakechamber (8). The air pressure puts the springs of theparking brake chamber in compression. The parkingbrake is released.ACCESSORY CIRCUITSPart of the air pressure from wet air tank (7) goes towindow washer valve (18) and air pressure gauge (16).The remainder of the accessory air circuits get airpressure from dry air tank (6) and brake control valve(28).From the dry air tank, the flow of air is to seatsuspension valve (21).From the brake control valve the flow of air is to hornvalve (25) and differential lock valve (26).2-131
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