Air-Over-Hydraulic Panels
Disc Brake Power Clusters
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GENERAL DESCRIPTION In air-over-hydraulic brake systems power clusters convert moderate applied air pressure to the relatively higher pressure required for hydraulic braking. Compressed air entering the unit forces a large diameter air actuator to stroke a small diameter hydraulic brake master cylinder. Hydraulic pressure is developed in direct proportion to the amount of applied air pressure. DISC BRAKE POWER CLUSTERS The convensional hydraulic brake master cylinder is unsuitable for systems with insufficient initial low pressure displacement. Typical low initial displacement systems are hydraulic actuated disc brake systems, hydraulic operated clutch control systems, and spring set hydraulic released brake systems. A new non-bypass-ported master cylinder which was developed eliminates the need to add a supplementary displacement cylinder in low displacement systems. TYPE HD MASTER CYLINDER OPERATION The Type HD master cylinder is similar to the conventional hydraulic brake master cylinder except a push rod actuated secondary piston is added inside the main piston. During an applying stroke, the actuator push rod moves the secondary piston to close the hole in the center of the primary cup. Further movement of the push rod strokes the main piston in combination with the secondary piston to displace fluid into the sealed system. Upon release, both master cylinder pistons are returned against a piston stop by the master cylinder return spring. The pistons and primary cup are returned to the released position faster than the displaced fluid can return to the cylinder bore. The partial vacuum thus formed draws fluid from the supercharged system forces the secondary piston thru the center hole in the primary cup to relieve residual pressure thru clearance between the secondary and main piston. A back-up washer is used between the primary cup and the main |
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piston face to prevent extrusion of the primary cup into the clearance between the pistons and their bores. This washer increases the expected life of the primary cup in relatively high pressure disc brake systems. PRESSURE RATIO Power clusters with 7 inch diameter air cylinder and 1.75 inch diameter master cylinder develop 1500 psi hydraulic pressure with approximately 100 psi applied air pressure. A power cluster with 5 inch diameter air cylinder develops 800 psi hydraulic pressure with approximately 100 psi air pressure. |
DIMENSIONS
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F-Outlet Ports |
Three 1/2"-20-2B thd. Two ports include F996 Plugs and F17
Gaskets. F4481 Adaptor Fitting and F17 Gasket included with assembly
for 1/4" female pipe thread connection at third outlet port. |
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A75420 |
Mounting Bracket Kit (illustrated on assembly above) is included for right or left hand field mounting. |
| Power Cluster Part Number |
Air/ Hydraulic Pressure Ratio |
Displacement (cu. in.) |
Power Cluster Stroke |
Cylinder I.D. | Dimensions | |||||
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| Porthole | Usable | Hyd. | Air | A | B | C | D | |||
| J98274 J98275 |
1x15 1x8 |
0 0 |
5.0 5.0 |
2.6" 2.6" |
1.75" 1.75" |
7" 5" |
15.70" 15.60" |
7.80" 6.50" |
6.03" 5.83" |
8.40" 7.40" |
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SERVICE INFORMATION After extended service, the hydraulic and/or air seals in the unit may deteriorate, resulting in failure to develop hydraulic pressure proportional to application air pressure. The first symptom of trouble is usually increasing hydraulic pressure when the piston strokes the master cylinder but then gradual loss of pressure after piston reaches full stroke. Full 2.6" movement of the stroke indicator of the unit in a properly bled system is usually and indication of seal failure. These units can be serviced as follows:
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These units replace JF71882 (1x8 ratio) and JF71883 (1x15 ratio) pressure clusters with external displacement cylinder, which are no longer manufactured. The new assemblies function and mount identical to the old assemblies and may be used as replacement. |
| CAUTION: These units are intended for use with SAE J1703 Automotive Brake Fluid ONLY. |