PDF associated:
/brake3.pdf
SINGLE DIAPHRAGM VACUUM POWER BRAKES The single diaphragm power head uses engine intake manifold vacuum and atmospheric pressure to provide power assist to the brake system master cylinder The unit is composed of the vacuum power head and the hydraulic master cylinder The vacuum power head consists essentially of a housing separated by a large diaphragm which is connected to the power piston assembly An air valve mechanism controls the application and release of the brakes The unit operates as follows Aline from the engine intake manifold is connected to the vacuum check valve 39 Fig 1 in the front housing 37 of the power brake This check valve is to prevent loss of vacuum when manifold vacuum falls below that stored in the power brake system At the released position Fig 1 the air valve 12 is seated on the floating control valve 13 The air under atmospheric pressure which enters through the filter elements 15 in the tube extension of the power piston is shut off at the air valve 12 The floating control valve is held away from the valve seat 18 in the power piston 10 The vacuum which is present at all times in the space 6 to the left of the power piston is free to evacuate any existing air on the right side 9 of the power piston This air is drawn through two small passages 19 in the power piston over the valve seat 18 in the power piston and then through the power piston into the space at the left of the power piston 6 It is then drawn through the check valve to the vacuum source In this position there is vacuum on both sides of the power piston and the power piston 10 is held against the rear housing by the power piston 14 Chapter Three Power Brake Booster Training Fig 1 Single diaphragm power brake released return spring 4 At rest the hydraulic reaction plate 7 is held against the reaction retainer 5 The reaction levers 8 are held back against the hydraulic reaction plate 7 by the air valve springs 11 some units use only one spring The air valve springs holdthe air valve 12 back so that its retaining ring 20 rests against the power piston 10 The floating control valve assembly 13 is held against the air valve seat by the floating control valve spring 17 As the brake pedal is depressed Fig 2 the pushrod 16 carries the air valve 12 away from the floating control valve 13 The floating control valve will follow until it is in contact with the raised annular seat 18 in the power piston 10 When this occurs the vacuum is shut off in the space 9 to the right of the power piston Air under atmospheric pressure rushes through the air filters 15 and travels past the seat of the air valve 12 and through two passageways 19 into the space 9 to the right of the power piston Since there is still vacuum on the left side of the power piston 6 the force of the air at atmospheric pressure on the right of the piston will force the power piston 10 to travel to the left As the power piston 10 travels to the left the piston rod 35 moves also to actuate the master cylinder and apply the brakes As pressure builds up on the end of the piston rod 35 the hydraulic reaction plate 7 is moved off its seat on the reaction retainer 5 and presses against the reaction levers 8 The levers in turn swing about their pivots and bear against the end of the air valve 12 and push rod 16 assembly In this manner approximately 20 of the load on the hydraulic master cylinder piston 23 is transferred back through the reaction system to the brake pedal This gives the operator a feel which is proportional to the degree of brake application When the desired pedal pressure is reached the power piston 10 moves to the left until the floating control valve 13 which is still seated on seat 18 of the power piston 10 again seats on the air valve 12 Then both air and vacuum are shut off to the space 9 to the right of the diaphragm The power brake will now remain stationary until either more pressure is applied or pressure is released at the brake pedal Reaction force is transmitted through the levers 8 back to the brake pedal 15 Fig 2 Single diaphragm power brake applied As the pressure at the pedal is released the air valve springs 11 force the air valve 12 back until its retaining ring 20 rests against the power piston 10 As it returns the air valve 12 pushes the floating control valve 13 off its seat 18 on the power piston 10 TANDEM VACUUM POWER BRAKES The tandem power brake is similar to the single diaphragm unit except that it has a divided housing with a diaphragm and power piston in each section The power pistons are connected together in tandem or series to provide additional boost without increasing the power head diameter Operation of the tandem unit is as follows In the released position Fig 3 operation of the tandem power brake is the same as the single diaphragm unit except as follows Vacuum in space 52 pulled through the check valve 62 in the front housing 25 is also pulled from space 51 through passage 60 With the air valve 7 seated against the floating control valve 8 atmospheric air pressure is shut off from entering the unit Since the air valve holds the floating control valve off the power piston seat 47 vacuum is also pulled through passage 57 from space 49 and through passage 61 from space 50 When the brake pedal is depressed Fig 4 action in the tandem power brake is the same as in the single diaphragm unit except as follows Movement of the air valve 7 and the seating of the floating control valve 8 on the power piston seat 47 admits air not only to space 49 through passage 57 but also to space 50 through passage 61 With vacuum still present in both spaces 51 and 52 operating force is applied to both diaphragms 14 and 21 and both power pistons 4 and 6 Operating force is transmitted from the power pistons to the master cylinder through the piston retainer 19 and piston rod 27 Areactionary force is applied to the reaction disc 18 which contacts the reaction piston 17 Action of the tandem power brake in the holding position is the same as for the single diaphragm unit except as follows 16 Fig 3 Tandem power brake released 17 With the floating control valve 8 seated on both the air valve 7 and the power piston seat 47 air and vacuum are shut off to the space 49 and 50 to the right of both diaphragms resulting in the stationary or holding action The force applied to the master cylinder is transmitted back through the piston rod 27 and retainer 19 to the rubber reaction disc 18 The reaction disc is compressed and extruded into the cavity where the reaction piston 17 is installed Afraction of the total force is transferred to the reaction piston and through the air valve 7 to the push rod The reaction bumper 58 is a noise control device that will contact the secondary power piston only during maximum assist brake applications Operation of the tandem power brake when releasing is the same as for the single diaphragm unit except as follows Return action of the air valve 7 in seating against the floating control valve 8 and moving the floating control valve off the power piston seat 47 allows vacuum to be pulled from space 49 through passage 57 Simultaneously vacuum is pulled from space 50 through passage 61 Thus vacuum is present on both sides of power pistons and the return spring 1 moves the pistons back against the rear housing VACUUM FAILURE ALLTYPES In case of vacuum source interruption to either the single diaphragm or the tandem power brakes enough vacuum is available in the power brake to make about three power assisted stops If the vacuum check valve should fail or if the vacuum stored in the unit is exhausted it is still possible to operate the power brake by purely mechanical effort However the pedal force required for the manual application of either the single diaphragm or the tandem power brake is considerably greater than with power assist In the single diaphragm power brake push rod force is transferred through the air valve 12 reaction levers 8 reaction plate 7 and piston rod 35 to the master cylinder In the tandem power brake push rod force is transferred through the air valve 7 to the secondary power piston 6 and the reaction piston 17 which in turn conduct the force through the retainer 19 to the piston rod 27 and on to the master cylinder Fig 4 Tandem power brake applied
