Document kmgBojqMQkg8VmordwaXRde3y

SERVICE BULLETIN MARCH 20. 1978 T i\. 095 Under BRAKES FOR BRANCHES. DISTRIBUTORS AND CUSTOMERS. / // CHASSIS AFFECTED ALLENTOWN HAYWARD OAKVILLE BRAKE IMPROVEMENTS (All With Air Over Hydraulic Disc Brakes) Service Manager aaop Foreman N ignc Foreman Nigat Foreman inspector Leadmar. inspector Leadman Inspector Leadman Unit Over haul Room PURPOSE To advise of improvements to increase disc brake service life and to provide part numbers for disc brake service kits and replacement parts. Shop Office RECOMMENDATION The improvements described in this bulletin will increase service life of disc brakes. Due to a recent change in motor vehicle safety standard 121, older MVSS121 equipped vehicles can incorporate these improvements if the operator desires. These changes are not mandatory however, and are to be made at the operators reques and expense. INFORMATION One of the original stopping requirements for vehicles subject toMVSS121 requirements was 243 feet from 60 mph on dry roads. A revised standard, now in effect, increased the allowable stopping distance to 293 feet under the same conditions. Some Mack vehicles built to comply with the origi nal standard, utilized disc brakes at the front wheels to achieve the high brak ing force required. It has been found that short life of these disc brakes can result from failure to keep the rear brakes in adjustment. Disc brakes are self-adjusting and oper ate at minimum lining to-rotor clearance at all times. As a result, they pro vide the major portion of the braking force under light braking conditions if the rear wheel brakes are out of adjustment. If a particular operation entails considerable light braking, disc brake life can be increased by installing a metering valve in the hydraulic system. The meter ing valve reduces pressure to the disc brakes under low application pressure, causing the rear brakes to assume a larger proportion of the braking effort. At higher application pressure, normal pressure is restored to the disc brakes. Installation of 17 to 1 ratio intensifier in place of a 23 to 1 ratio unit will increase disc brake life. This modification is permissible on highway vehicles equipped with a bogie rated at 55,000 pounds or less. The intensifier uses brake system air pressure to develop hydraulic pressure for the disc brakes. With a 17 to 1 intensifier, 100 psi of air pressure will generate 1700 psi hydraulic pressure. With a 23 to 1 intensifier, 100 psi air pressure will generate 2300 psi hydraulic pressure. Longer lining life can also be achieved by installing current service linings. The less stringent stopping requirements now in effect make it possible to use a dif ferent lining material which has better wear characteristics than the material originally required. Correct adjustment of rear axle manual slack adjusters is essential for achieving correct balance between front and rear brakes. If the operator prefers, auto matic slack adjusters can be installed on the SW56, 57, 571, 573 and 76 series bogies to aid in maintaining correct adjustment. (Continued Over) Do TT <5 A Mark ~ Procedure 1. Installing metering valve 20QE2191R. See Figure 1 (R.U. and DM) and Figure 2 (MB). a. Remove ratio valve 20QE3109R installed in air line between brake application valve and anti-wheel lock controller or quick release valve. This valve is not used when a metering valve is installed. Install a new air line from brake application valve to controller or quick release valve. b. Remove the tee fitting to which the intensifier unit and caliper hy draulic lines are connected. Install the metering valve into these lines in place of the tee. Connect the ,'M" port to intensifier unit and "F" ports (two) to the brake calipers. NOTE The metering valve must be installed vertically with ports facing up and the pin facing down. c. Fabricate a suitable metering valve mounting bracket locally. Utilize any convenient bolts or holes to secure the bracket. Parts required: Qtv. l 3 Part Number 20QE2191R 63AX606 Description Pressure Metering Valve Nut, Inverted Flare 2. INTENSIFIER 3. FRONT BRAKE CONTROLLER 4. AIR LINE - CONTROLLER TO BRAKE APPLICATION VALVE 5. FRONT BRAKE CYLINDER. L.H. 6. RATIO VALVE - REMOVE 7. METERING VALVE 20QE 2191R FIGURE 1 . INTENSIFIER !. FRONT BRAKE CONTROLLER I. RATIO VALVE - REMOVE I. AIR LINE - CONTROLLER TO BRAKE APPLICATION VALVE I. METERING VALVE I. FRONT BRAKE CYLINDER, R.H. '. FRONT BRAKE CYLINDER. L.H. FIGURE 2 (Continued on Sheet 2) MARCH 20, 2. No. Q95 under BRAKES (Sheet 2) Installing low ratio intensifier (a) The 17 to 1 intensifier is directly interchangeable with the 23 to 1 intensifier. Parts required: New Part l 8QE429R( 17 to 1) Description Intensifier ! Replaces j | 8QE430R(23 to 1) 3. Installing automatic slack adjusters on 5W56, 57, 571. 573 and 76 series bogies. a. Remove existing slack adjusters. b. Install new slack adjuster on brake camshaft. Be sure arrow on ad juster points in direction unit rotates for a brake application. See Figure 3. 2. PUSH POO 3. DIRECTION OF BRAKE APPLICATION FIGURE 3 c. Install slack adjuster retainer bracket on brake assembly using existing spider mounting bolts as shown in Figure 4. Do not tighten the bolts. Align clevis pin hole in slack adjuster with push rod clevis by turning worm adjusting nut as shown in Figure 5 and install clevis pin. 1. existing spider mounting bolts 2. BRACKET 72RU 2861-P5, P6 3. SETSCREW 7AX262Z, WASHER 37AX 473, NUT 21 AX 317 4. SLACK ADJUSTER FIGURE 4 (Continued Over) ARM INTO CLEVIS FIGURE S d. Move slack adjuster control arm in direction shown in Figure 6 to be sure it is in contact with the internal stop. Proper operation depends on control arm being correctly positioned against stop. 1. MOVE CONTROL ARM IN THIS DIRECTION 2. CONTROL ARM 3. SETSCREW FIGURE S e. Install control arm setscrew as shown in Figure 4 and tighten retainer bracket screws. Be sure control arm does not move when retainer screws are tightened. f. Check operation of slack adjuster by operating brakes and observing if adjusting nut rotates. After establishing that operation is satis factory, uncage spring brakes, if used. Parts required per bogie: Qty. 4 2 2 4 4 8 Part Number 25QDA335-P6 72RU2861-P5 72RU2861-P6 7AX262Z 21AX317 37AX473 Description Slack Adjuster Slack Adjuster Retainer Bracket Slack Adjuster Retainer Bracket Setscrew Nut Washer MARCH 20, 1978 No. Q95 under BRAKES (Sheet 3) 3. Servicing the disc brake caliper. Various components and service kits are available to expedite overhaul of these units. These items are shown in accompanying parts list. General repair procedures are given in Highway Vehicle Service Manual TS442. Follow these procedures, taking particular care to observe the following points : a. Minor corrosion and stains in the piston bores can be polished with crocus cloth. Do not use emery cloth or other abrasives. Also check seal groove for scoring, nicks or corrosion. If any de fects are noted, replace caliper. b. The piston's outer diameter is plated and sized to a very close toler ance and must not be refinished by any means. If the O.D is scored, knicked, corroded or the plating is worn or damaged, the piston must be replaced. Parts required: Qtv. * ** *** Part Number 4891-^9726602 4891- (79906101 4891-(79829401 4891-(7929501 Description Kit, Boot and Seal Kit, Shoe and Lining Piston - Lining Holdown Kit * Services one caliper ** Service two calipers *** As required