GENERAL INFORMATION

Late model M300 and all RM300 models are equipped with Bendix Master-Vac combination dual diaphragm vacuum booster and tandem master cylinder.
The booster unit mounts on a bracket on the frame rail (Fig. 1) and is externally connected to the brake system by a pedal link to the brake pedal, by a vacuum line to the intake manifold (through a check valve) and to the master cylinder bolted to the front. The push rod for this master cylinder is part of the power brake unit.


Master-Vac

Figure 1 - Booster Mounting M300 (RM300)

SERVICE DIAGNOSIS

Most brake problems are not caused by a power booster malfunction. The following brake problems are usually due to causes other than the brake booster:

  • Grabbing Brakes
  • Low Pedal
  • Spongy Pedal
  • Warning Light On
  • Noise
  • Pull to the Side
  • Loss of Fluid

There are three basic types of booster malfunctions. Do not replace the booster unless performance of the appropriate tests confirms the malfunction.

  1. No boost; Hard pedal or excessive pedal effort required to stop the vehicle. (Test 1).
  2. Pedal Does Not Return: Dragging brakes (Test 2).
  3. Vacuum Leak; No reserve, low assist, rough engine idle (Test 3).

Test 1: No boost suspected

  1. With engine running at curb idle (engine at normal operating temperature) check manifold vacuum level and correct as required.
  2. Apply the brakes several times with moderate force and NOTE the general relation between pedal travel and force.
  3. With engine oif, apply the brakes 6 times to remove the booster reserve. Now apply the brakes with moderate force and again NOTE the relationship between pedal travel and force.
  4. If there is a signiticant difference in feel between step "B" and "C" the booster is operating properly.
  5. If there is no significant difference in "Feel" either the booster is failed or no vacuum is getting to the booster.
  6. Check the vacuum hose from the engine manifold to the booster check valve. If the hose is in good condition and not plugged or closed replace the booster.

Test 2: Pedal does not return

  1. With the engine running apply the brakes by hand and slowly release the pedal. Observe whether or not the pedal returns to the fully released position. To determine the fully released position pull up on the pedal gently and release. The pedal should move up and then drop to its normal position.
  2. If the pedal will not fully return check for an improperly adjusted stop light switch and/or a binding pedal linkage or pedal bolt.
  3. lf the stop light switch and pedal linkage are 0.K. and the pedal will not fully return to the unapplied position when released, replace the booster.
  4. If the pedal appears to return correctly but the complaint is dragging brakes, check the master cylinder for compensation (see "Master Cylinders" this Group) and full return and proper function on the bench before suspecting the booster.

Test 3: Vacuum leaks

  1. With engine running at slow idle (normal operating temperature) apply the brakes several times and NOTE the relationship between pedal travel and pedal force.
  2. Stop engine and wait 5 minutes, then apply the brakes ONCE. The relationship between pedal travel and force should be the same as with the engine running.
  3. If the engine is idling rough remove the vacuum hose from the booster check valve and close the open hose end. If this smooths out the idle the booster has a major vacuum leak and should be replaced.
  4. Complaint of low assist can be caused by low engine vacuum. The maximum output of a booster on an engine without 10 inches of manifold vacuum is one half the output of a booster on an engine with 20 inches of manifold vacuum.

Removal

  1. Disconnect brake lines from primary and secondary outlets on tandem master cylinder.
  2. Remove nuts attaching master cylinder to dual diaphragm booster unit. Remove tandem master cylinder.
  3. Disconnect vacuum line from check valve. Remove clamp and disconnect hose to brake booster air cleaner from Master-Vac unit.
  4. Remove nut and bolt from Master-Vac push rod and brake pedal lower lever.
  5. Remove four nuts and washers from studs attaching booster unit to pedestal mount. Withdraw Master-Vac unit from pedestal moimt.

CAUTION:
Do not attempt to disassemble and repair Master-Vac unit. This unit is serviced only by the Manufacturer's Service Station. lf the Master Vac unit is defective, it must be replaced as an assembly.

Installation

  1. Install Master-Vac unit on pedestal mount. Install four nuts and washers on studs and tighten to 150 inch-pounds.
  2. Coat bearing surface of bolt which connects Master-Vac push rod to brake pedal lower lever with lubriplate or equivalent. Install bolt and nut. Tighten to 30 foot-pounds.
  3. Attach vacuum hose to check valve. Reconnect hose from brake booster air cleaner to MasterVac unit. Secure with clamp.
  4. Install master cylinder on Master-Vac unit. Tighten mounting nuts to 200 inch-pounds. Remove cover, replenish fluid level if necessary. Operate brake pedal and verify that the master cylinder compensates (forces a jet of fluid up through front chamber compensation port) with vacuum supplied to Master-Vac. Replace master cylinder cover.
  5. Check adjustment and operation of stop-lite switch.

C-490G