Figure 3-46.—Hand brake valve.Figure 3-48.-Air-over-hydraulic power cylinder.NOTE: More information about the air brakesystem is in the chapter covering tractor and traileroperations.AIR-OVER-HYDRAULIC BRAKE SYSTEMAn air-over-hydraulic brake system is shown infigure 3-47. This system combines the use ofcompressed air and hydraulic pressure for brakeoperation. The air-over-hydraulic brake system has antrailer brakes without applying the truck or tractorair-over-hydraulic power cylinder (fig. 3-48) thatbrakes. The hand brake valve or independent trailercontains an air cylinder and a hydraulic cylinder incontrol valve, as shown in figures 3-44 and 3-46,tandem. Each cylinder is fitted with a piston and aprovides the operator control of the trailing load at allcommon rod. The air piston is of greater diameter thantimes.the hydraulic piston. This difference in the two pistonsFigure 3-47.-Typical air-over-hydraulic brake system.3-26
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