Quantcast Troubleshooting Vacuum-Assisted Hydraulic Brakes (Power) Systems

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make any repairs, reset the brake failure warning lamp switch  (fig.  6-7). See table 6-1 for a complete listing of possible brake problems  and  repairs. TROUBLESHOOTING   VACUUM-ASSISTED HYDRAULIC BRAKES (POWER) SYSTEMS Aside from the vacuum booster, the same basic inspection  procedures  given  in  the  hydraulic  brake section  apply  to  the  vacuum-assisted  hydraulic  brake system. When you check this system for a source of trouble, refer to the chart for the standard hydraulic brake  system  (table  6-1).  After  you  isolate  possible causes by consulting this chart, check for causes in the troubleshooting chart of table 6-2. NOTE Conduct the following test BEFORE you check  the  cause  of  a  hard  pedal.  With  the engine   stopped,   depress   the   brake   pedal several times to eliminate all vacuum from the system. Apply the brakes, and while holding the foot pressure on the brake pedal, start the engine. If the unit is operating correctly, the brake  pedal  will  move  forward  when  the engine vacuum power is added to the pedal pressure. If this test shows the power unit is not  operating,  check  the  probable  causes  of vacuum failure in table 6-2. HARD PEDAL A “hard pedal” means the booster is inoperative and you should suspect and check the following as the cause: collapsed  vacuum  hoses,  faulty  vacuum  check  valves, internal  damage  to  the  power  booster,  or  a  broken plunger  stem. GRABBY  BRAKES Uncontrolled stopping is a problem that may be caused  by  grabbing  or  oversensitive  brakes.  This symptom may result from a faulty power booster, a damaged  vacuum  check  valve,  leaky  or  incorrectly connected vacuum lines, or a broken plunger stem. Figure 6-7.—Pressure differential valve with brake tamp warning switch. BRAKES FAIL TO RELEASE When you apply the brakes and they fail to release, the following could be the problem–a broken power booster return spring, a sticking valve plunger in the booster, or a jammed power piston. LOSS OF FLUID Loss of brake fluid may occur through the rear seal of the master cylinder past the piston stop plate and into the power booster. The leak is not visible on the backing plates, the wheels, or the frame because the fluid collects in the power booster. Some of the fluid  may  be  drawn  through  the  vacuum  lines  and burned in the engine. The end result is that you do not see the leak. For a more complete listing of vacuum booster hydraulic brake problems and remedies, see table 6-2. Always consult the specific manufacturer’s manual whenever you replace or repair any vacuum power   booster. HYDROBOOST POWER BRAKE SYSTEMS Diesel  engines  do  not  create  enough  useable vacuum to actuate the vacuum power brake booster. The alternative is a hydraulic-assisted power brake booster or hydrobcmst. This system is currently found in the 1 1/4-ton CUCV and the 3/4-ton CUCV Blazer, both powered  by  the  366  cubic  inch  V8  General  Motors diesel engine. Both units are found throughout the NCF and at some public works stations. The hydroboost uses hydraulic pressure developed by the power steering 6-10



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