Figure 3-49.-Typical vacuum brake cylinder.results in much greater hydraulic pressure than airpressure admitted to the air cylinder. Valve action varieswith the amount of pressure applied to the brake pedal.When heavy brake pedal pressure is applied by theoperator for hard braking, the hydraulic pressure in themaster cylinder (which operates the valves) causesgreater valve movement. As a result, the valve admitsmore air pressure into the air-over-hydraulic powercylinder and this higher air pressure causes a strongerbraking action.VACUUM BRAKESIn a vacuum brake system, depressing the brakepedal opens a valve between the power cylinder, whichcontains a piston, and the intake manifold to which thepower cylinder is connected (fig. 3-49). When you applythe brakes, air is exhausted from the cylinder head of thepiston. At the same time, atmospheric pressure acts onthe rear side of the piston to exert a powerful pull on therod attached to the piston.When the brake valve is closed, the chamber aheadof the piston is shut off from the intake manifold and isopened to the atmosphere. The pressure is then the sameon both sides of the piston; therefore, no pull is exertedupon the pull rod. The brakes are released and the pistonreturned to its original position in the power cylinder bythe brake shoe return springs.HydrovacTM is a trade name for a one-unit vacuumpower-braking system. It combines a hydraulic controlvalve, a vacuum power cylinder, and a hydraulic slavecylinder into one assembly. This assembly (fig. 3-50) isconnected to both the master cylinder and the wheelbrakes and eliminates the need for mechanicalconnections with the brake pedal.Pressure on the brake pedal forces fluid from themaster cylinder through the check valve to the slavecylinder and to the wheel cylinders. Also, the foot pedalpressure, acting through the master cylinder, acts alsoagainst the slave cylinder piston to help the vacuumpistons and pushrods to press against the brake shoes.Figure 3-50.—HydrovacTM power brake cylinder.3-27
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