Powermaster Diagnostics
Ken Mosher
This was published in the Midlands Chapter GSCA newsletter... it comes from a service bulletin from GM.

The following information was sent to us by our far Western Nebraska member Al Musil (308) 452-3020. He dug up some more information on the infamous Powermaster brakes.

Click on the following for graphical charts to assist with troubleshooting:

Bulletin 87-5-13A
This bulletin provides brake diagnostic information that will help to correctly diagnose and resolve concerns with the PowerMaster brake system on 1986 Estate Wagons and 1985-1987 Regal Turbo vehicles.


    "Harder Pedal Feel" or "Increased Pedal Travel"


    A. Harder Pedal feel
    1. Booster pressure switch leaking fluid externally
    2. Accumulator - low gas charge

    B. Increased Pedal Travel
    1. Likely to be caused by air ingestion into the cast iron combination brake valve, requiring a new brass replacement valve.

NOTE: Replacement of the complete PowerMaster assembly is not required to correct most customer concerns. Part kits are available that will resolve most concerns with this system and eliminate unnecessary replacement of the complete assembly.




Check fluid on booster side after 10 brake applies with ignition off. Check reservoir on the side away from pressure switch

Below min. mark See Step 1
Normal No Action

Warning Light Operation
With normal braking light doesn't come on
No Action
With normal braking light comes on, goes off
With normal braking light comes on, stays on
See booster motor or Increased Pedal Travel diagnostic
With rapid brake applies at one per second light may come on & go off


Accumulator Operation
Test after 10 brake applies with ignition off to depressurize accumulator:
Then turn ignition on and time from motor start to light off.

0-2 seconds Accumulator diaphragm is leaking internally. Replace accumulator
3-6 seconds Normal
7 seconds or more See Step 2


Booster Motor Operation
Turn ignition on and make brake applies.

Motor doesn't run, no power assist, and warning light is on See Step 3
Motor doesn't shut off See Step 4


Inspect for External Leaks

Leaks detected Repair leak


Test for Combination Valve air ingestion
Bleed left front caliper only.

Pedal Travel improves after bleeding left front brake Replace combination valve with brass valve #25509419 and rebleed total system


To properly evaluate pedal travel, apply brakes with vehicle parked, ignition off, and without power assist (depressurize accumulator with 10 brake applies). Pedal travel should be 2 1/4" Max. with normally worn brake linings when checked at 100 # pedal force.


Corrective Action Steps
Step #1 - Low fluid in booster side (external leak)
A. Inspect the pressure switch and adjacent areas for external leak. This may require wiping those areas dry and observing during and after motor run. The pressure switch is the most likely cause of external leak, but can also dampen other areas such as the cowl or rear booster areas.
B. Depressurize the system with 10 brake applies with ignition off before replacing pressure switch or other leaking part. Recheck fluid level on booster side and add brake fluid to adjust level to within gauge marks in reservoir.

Step #2 - After accumulator Operation Test indicates seven seconds of more of motor operation.
A. Depressure system with 10 brake applies with ignition off. Fluid level must be within gauge marks on the reservoir.
B. Turn ignition on and wait until motor shuts off.
1. If motor doesn't self cycle in 2 minutes without brake apply, proceed to (C).
2. If motor does self cycle, an internal booster leak requires replacing the Powermaster assembly.
C. Install J35126 Tool and test to determine if fault is in motor and pump assembly or in the accumulator.
1. Remove pressure switch and install J35216 into switch port.
2. Reinstall pressure switch into J35126 and reconnect electrical connector.
D. Turn ignition on. Wait until motor shuts off. Turn ignition off. Slowly bleed off accumulator pressure with J35126 bleed valve-returning fluid to booster side of reservoir. The gauge pressure will suddenly drop to zero at the accumulator gas pre-charge point.
1. If the drop off occurs below 200 psi, replace accumulator.
2. If drop off occurs above 200 psi, replace the pump and assembly.
E. NOTE: J351226 tool in (2C) is the best method; however, if it is not available, an alternate method will help to determine fault between the pump and motor assembly and the accumulator.
1. Depressurize the system with 10 brake applies with the ignition off. Turn ignition on, and check the time from motor start to motor shut off.
2. If time to motor shut off is more than 15 seconds, replace motor and pump assembly, and then retest accumulator per Accumulator Operation Test. If time is seven seconds or more, also replace the accumulator.
3. If time to motor shut off is less than 15 seconds, replace accumulator, and then retest motor and pump per Accumulator Operation Test. If time is seven seconds or more, also replace motor and pump.

Step #3 - Booster Motor does not run.

Confirm that the 30 Amp fuse is OK or replace. Then conduct electrical circuit checks. Disconnect the pressure switch and motor electrical connector and test to the following:

A. Pressure Switch (Ohmmeter)
1. All switch terminals should have continuity with each other. If not, replace switch.
B. Booster Motor (Ohmmeter)
1. Meter should show resistance between Motor "B" and "D".
2. Meter should show open circuit between Motor "A" and "C".
3. Replace motor and assembly if 1 & 2 are not true.
C. Pressure switch Connector and Harness (Volt/Ohmmeter)
1. With ignition on, switch connector terminal "A" should be continuous with terminal "D".
2. With ignition on, switch connector terminal "B" should be 12 volts to ground. (volt meter).
3. If "A" and "D" are not continuous, but "B" shows 12 volts, release parking brake.
4. Disconnect combination valve electrical connector.
D. Pump Connector and Harness (Volt/Ohmmeter)
1. Connector terminal "A" continuous with chassis.
2. Connector terminal "B" 12 volts to ground with ignition on.
3. Terminal "C" 12 volts to ground. This is the main electrical supply to the motor and is fused. If voltage doesn't appear at this terminal, check fuse.
4. Terminal "D" previously checked in step 3C1.

Step #4 - Motor does not shut off with ignition on

A. Check fluid level per Step 2A.
B. Install J35126 tool per Step 2C and test to determine if fault is in motor and pump assembly, pressure switch, or internal booster leakage.
C. Monitor booster pressure on J35126 gauge - After 20 seconds with ignition on and without braking.
1. If pressure exceeds 770 psi, replace pressure switch.
2. If pressure does not reach 540-770 psi pump shut-off range, check the front and rear holes in the bottom of the reservoir on boost side only.
a. If fluid is flowing into the reservoir through either hole, the booster is leaking internally and the complete powermaster assembly should be replaced.
b. If fluid is not flowing into the reservoir through either hole, the pump is unable to provide 540-770 psi and the motor/pump assembly should be replaced.



The following repair kits have been released to assist in servicing the Powermaster brake units.

18011279 Pump & Motor Assembly Package
18014977 Pressure Switch Assembly Package
18014978 Accumulator Assembly Package
18013293 Reservoir Cover Assembly Package
18011398 Boot Package
18010875 Bracket Package
18011403 Sump Hose Package
25509419 Brass Combination Valve

Pump Pressure Ranges
Pump Off 540-770 psi
Pump On 490-530 psi
Light On 355-435 psi

Special Bleeding Instructions
A problem may be encountered in bleeding the PowerMaster if for any reason the pump has not been properly filled with brake fluid. If after any servicer new installation the pump does not circulate fluid after the motor has run for 20 secs, it may be necessary to reprime the pump. In order to reprime the system, the following steps should be performed:

1. Turn off ignition
2. Apply the brake pedal with medium pressure 10-15 times with ignition off to discharge pressure from the accumulator.
3. Clean and remove reservoir cover.
4. Disconnect the booster tube and nut assembly from the casting boss at pump outlet. Refer to illustration.
5. Wait for brake fluid to gravity bleed from the disconnected end of the tubing.
6. When brake fluid begins to flow, reconnect the tube and torque to 13 ft lbs.
7. Check the fluid level in the reservoir and add new brake fluid if needed. Reinstall the reservoir cover.
8. Turn on the ignition. The pump should properly circulate the fluid to the accumulator. The fluid level on the inboard side of the reservoir should diminish if the pump is working properly.
9. Continue with service manual bleed and fill procedure.

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