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automstr, ASE Certified Tech
Category: Car
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Experience:  ASE master tech
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my 1993 acura legend has a hard brake pedal. the booster ...

Customer Question

my 1993 acura legend has a hard brake pedal. the booster checks out OK - master cylinder is relatively new.
Submitted: 8 years ago.
Category: Car
Expert:  Your Auto Inspector replied 8 years ago.

Your issue sounds like a bad brake booster. A leaking brake master cylinder should result in a soft/spongy brake pedal. Your brake booster uses vacuum to amplify your brake pedal pressure to assist you when braking. If the brake booster or check valve fails, the pedal become hard because the vacuum is not assisting you.

How did you check the booster?

Here are the vacuum tests required to verify brake booster and check valve operation:

Functional Test

  1. With the engine stopped, depress the brake pedal several times to deplete the vacuum reservoir, then depress the pedal hard and hold it for 1 5 seconds. If the pedal sinks, either the master cylinder is bypassing internally, or the brake system (master cylinder, lines, modulator, proportioning valve, or caliper) are leaking.
  2. Start the engine with the pedal depressed. If the pedal sinks slightly, the vacuum booster is operating normally. If the pedal height does not vary, the booster or check valve is faulty.
  3. With the engine running, depress the brake pedal lightly. Apply just enough pressure to hold back automatic transmission creep. If the brake pedal sinks more than 25 mm (1.0 in.) in three minutes, the master cylinder is faulty. A slight change in pedal height when the A/C compressor cycles on and off if normal. (The A/C compressor load changes the vacuum available to the booster.)



Leak Test

  1. Depress the brake pedal with the engine running, then stop the engine. If the pedal height does not vary while depressed for 30 seconds, the vacuum booster is OK. If the pedal rises, the booster is faulty.
  2. With the engine stopped, depress the brake pedal several times using normal pressure. When the pedal is first depressed, it should be low. On consecutive applications, the pedal height should gradually rise. If the pedal position does not vary, check the booster check valve.


Booster Check Valve Test

  1. Disconnect the brake booster vacuum hose at the booster or at the booster side of the valve.
  2. Start the engine and let it idle. There should be vacuum. If no vacuum is available, the check valve is not working properly. Replace the check valve and retest.


Please let me know if there is anything else I can do to assist you.

Customer: replied 8 years ago.
Brakes and booster have been checked.
All checked out ok.
Still hard pedal.
Expert:  Your Auto Inspector replied 8 years ago.
It could be a faulty brake proportioning valve. I do not have any diagrams/tech. infomation on replacing the valve .
Customer: replied 8 years ago.
Anyone have a different answer.
The booster and master cylinder have
been checked.
Expert:  automstr replied 8 years ago.
how does the car stop? do you have good vacuum going to the booster? here's the adjustment procedure that most be done whenever the master or booster is replaced . also be sure the seal is on the back of the master click here and here
automstr, ASE Certified Tech
Category: Car
Satisfied Customers: 5314
Experience: ASE master tech
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