5-14 1955 PONTIAC SHOP MANUAL POWER BRAKES-BENDIX GENERAL DESCRIPTION The Bendix power brake unit can be identified The power brake system provides reduced pedal by the die-cast hydraulic cylinder and pressed steel travel compared to the conventional brake system. filler cap (Fig. 5-17). The Moraine power brake unit The reduced pedal travel lowers the height of the can be identified by the cast iron master cylinder pedal down to approximately that of the accelerator and cast iron filler cap (Fig. 5-39). pedal, permitting the driver to shift his toe from one pedal to the other without lifting his heel from the The Bendix power brake is a combined vacuum floor. Lighter pedal pressures are also obtained for and hydraulic unit for power braking, utilizing en­ normal stops. gine intake manifold vacuum, and atmospheric pres­ sure for its operation (Fig. 5-12). It is a self-con­ Design of the power brake is such that, in case of tained unit requiring no external rods or levers ex­ engine failure and consequent loss of engine vacuum, posed to dirt or moisture. This power brake unit re­ several applications of the brakes can be made utiliz­ places the master cylinder only. Other parts of the brake system are the same as with standard brakes. ing vacuum supply in the vacuum reservoir. In case TV.10 external line connections to the power brake of complete loss of vacuum, the brakes can be applied are necessary. One is a vacuum connection to the manually in the conventional manner. More effort carburetor (and vacuum reservoir). The other is a for manual application is required, however, due to hydraulic eonnection into the hydraulic brake system. the lack of power assist. STOP LIGHT SWITCH .-TO CARBURETOR CHECK VALVE VACUUM RESERVOIR +-FRONT TO WHEEL CYLINDERS Fig. 5-12 Power Brake Schematic www.PontiacSafari.com POWER BRAKES-BENDIX 5-15 LEATHER PACKING------.. HYDRAULlt:: CYLINDER AND RESERvom ASSEMBLY Am CLEANER AND COMPENSATING VALVE ATMOSPHERE PORT------.. RESIDUAL PRESSURE VACUUM POPPET -----__,. CHECK VALVE POPPET COMPENSATING S PUSH ROD AND PLUNGER ATMOSPHERE POPPET'----J VACUUM HOSE------' ROD AND PLUNGER RETURN SPRING PNOTARM-------J COUNTER - REACTION SPRING TO INTAKE MANIFOLD AND VACUUM RESERvom-----'" Fig. 5-13 Cross Section of Power Brake CONSTRUCTION port fitting incorporating a residual pressure check valve. A fluid compensating valve is located between The Bendix power brake assembly (Fig. 5-13) con­ the fluid reservoir and the hydraulic cylinder. Vac­ sists of three operating units, which are: hydraulic uum and hydraulic seals are provided in the bore of cylinder and reservoir assembly, vacuum power cyl­ the hydraulic cylinder to seal around the hydraulic inder, and piston and valve assembly. plunger. The piston and valve assembly is connected to the The piston is connected to the vacuum supply brake pedal by a push rod and plunger. The piston tube by a flexible vacuum hose. The piston contains operates inside the vacuum cylinder moving the hy­ a push rod and plunger, a diaphragm assembly, coun­ draulic plunger into the hydraulic cylinder. A piston ter-reaction spring, a vacuum poppet and poppet return spring is provided in the vacuum cylinder to compensating stem, and an atmosphere poppet. The return the piston and hydraulic plunger to their re­ poppets are actuated by the push rod plunger through leased positions. the pivot arm. The push rod and plunger return The end of the vacuum cylinder is attached rigidly spring is located inside the diaphragm assembly. The by screws to the hydraulic cylinder and fluid reser­ spring returns the push rod and plunger to its re­ voir casting. On the outside of the vacuum cylinder leased position in the piston. A leather packing is is a vacuum supply tube, and an air cleaner and at­ located at the outside diameter of the piston to pro­ mosphere port. Attached to the hydraulic cylinder vide a seal between the piston and the vacuum cyl­ assembly is a fluid reservoir cover and a hydraulic inder. www.PontiacSafari.com 5-16 1955 PONTIAC SHOP MANUAL PRINCIPLES OF OPERATION FLUID RESERVOIR ATMOSPHERE PORT ---_... COMPENSATING VALVE VACUUM POPPET (CLOSED) RESIDUAL PRESSURE CHECK VALVE VACUUM POPPET COMPENSATOR STEM CONNECTION TO WHEEL CYLINDERS COMPENSATOR STEM DIAPHRAGM '---I'IS1rDN RETURN SPRING VALVE PLUNGER RETURN SPRING ATMOSPHERE POPPET (OPEN) PASSAGE TO ATMOSPHERE POPPET SPRING INNER DIAPHRAGM PIVOT ARM----------./ VACUUM TUBE COUNTER- REACTION SPRING Fig. 5-14 Power Brake in Released Position RELEASED POSITION (FIG. 5-14) phere also enters the passage shown at the bottom When the engine is running and the brakes are re­ of the piston and is communicated to the right side leased, vacuum from the engine intake manifold is of the diaphragm assembly. A small hole in the dia­ transmitted through the vacuum check valve to the phragm plate admits atmosphere to the first stage power brake vacuum tube and to a vacuum reser­ (inner) diaphragn •. voir. From the vacuum tube, vacuum is transmitted In the released position, both the power piston and into the unit through a flexible hose which is attached the diaphragm assembly are balanced in atmospheric to the power piston at the left of the vacuum poppet. pressure. The piston is held to the left in its released Atmosphere, after passing through air cleaner and position by the piston return spring; the push rod and atmosphere port, enters the power brake cylinder valve plunger assembly is held in its leftward posi­ chamber at the left side of the power piston. From tion in the piston by the valve plunger return spring. the left side of the piston, atmosphere is communi­ cated through the open atmosphere poppet to the When the piston is in its released position, the vari­ left side of the diaphragm assembly and through the ous parts of the hydraulic portion of the power brake passage in the piston, shown at the top of the dia­ are in their respective released positions. The com­ phragm, to the cylinder chamber at the right of the pensating valve is tilted by the washer at the end of piston. A small passage in the piston admits atmos­ the hydraulic plunger permitting fluid flow from the phere from the left side of the diaphragm assembly hydraulic fluid reservoir to the hydraulic cylinder. to the left side of the vacuum poppet compensator The expansion, contraction, or leakage of fluid in the stem diaphragm. As there is a vacuum on the right hydraulic system is thereby compensated for as in a side of the compensator stem diaphragm, a slight conventional brake system. The residual pressure force to the right is exerted on the vacuum poppet, check valve maintains fluid under slight pressure in partially balancing the force of atmospheric pressure the lines to the wheel cylinders to prevent entrance of against the right side of the vacuum poppet. Atmos- air into the hydraulic system. www.PontiacSafari.com POWER BRAKES-BENDIX 5-17 ATMOSPHERE PORT---~ FLUID RESERVOIR VACUUM POPPET (OPEN) COMPENSATING VALVE VACUUM POPPET RESIDUAL PRESSURE COMPENSA TOR STEM CHECK VALVE CONNECTION TO WHEEL CYLINDERS HYDRAULIC PLUNGER PISTON RETURN SPRING PUSH ROD AND VALVE PLUNGER VALVE PLUNGER RETURN SPRING. ATMOSPHERE POPPET (CI,OSED)--/ ATMOSPHERE POPPET SPRING FIRST STAGE DrAPHRAG~,,"I PIVOT ARM DIAPHRAGM ASSEMBLY VACUUM TUBE-------- COUNTER- REACTION SPRING Fig. 5-15 Power Brake in Applying Position APPLYING (FIG. 5-15) As the brake pedal is applied by the driver, the Initial movement of the plunger allows the com­ push rod and valve plunger move to the right in the pensating valve to seat, trapping fluid in the hy­ power piston allowing the pivot arm and atmosphere draulic cylinder. Fluid under pressure is forced past poppet spring to close the atmosphere poppet. After the residual pressure check valve and through the the atmosphere poppet closes, the pivot arm, which lines to the wheel cylinders. pivots freely on the push rod plunger, opens the vacuum poppet. When the brake pedal is depressed and the plunger moves to the right, a reactionary, or opposing force, The "pre-loading" of the vacuum poppet (in the is provided by the valve plunger return spring (first released position) by the vacuum poppet compen­ stage diaphragm) and differences in pressure on the sator stem reduces the force required to lift the pop­ sides of the diaphragm (vacuum on left side and at­ pet from its seat. Smoothness in the initial applica­ mospheric pressure on the right side). This reaction­ tion of the power brake is thereby obtained. ary force results in 40 % of the load being provided With the vacuum poppet open, vacuum IS com­ by the driver while 60 % is done by the power brake municated to the left side of the diaphragm assembly unit. and to the right side of the power piston. With atmospheric pressure on the left side of the Initial reactionary force is provided by the valve power piston and vacuum on the right side, a pres­ plunger return spring. When the plunger has moved sure differential exists which causes the power piston far enough to the right to open the vacuum poppet, to move to the right. As the power piston moves to vacuum is communicated to the left side of the dia­ the right, the piston return spring is compressed and phragm, the reactionary force is a combination of the hydraulic plunger moves to the right in the hy­ valve plunger return spring force and pressure differ­ draulic cylinder. ential across the diaphragm. www.PontiacSafari.com 5-18 1955 PONTIAC SHOP MANUAL ATMOSPHERE PORT---__..... FLUID RESERVOm VACUUM POPPET \\..L.""""Jj'-___ COMPENSATING VALVE VACUUM POPPET RESIDUAL PRESSURE COMPENSATOR STEM CHECK VALVE HYDRAULIC PLUNGER RETURN SPRING PUSH ROD AND VALVE PLUNGER VALVE PLUNGER RETURN SPRING ATMOSPHERE POPPET (CLOSED) ATMOSPHERE POPPET tll'tllNli-----' PIVOT ARM ------------'" VACUUM TUBE COUNTER- REACTION SPRING Fig.