Hi,

I need your help on the brakes of my Honda FR-V:

Symptoms:
When the motor in not running: the brake pedal feels normal. When the motor runs: the pedal can be pushed completely to the bottom.

I replace the master cilinder and the ABS pump. We have no leakage and we do not loose any oil. But the oil must go somewhere if we brake.... Could it be the brake hoses? Or....?

Anybody any clue or advise?

KR
Danny

 

Did you bench bleed the MBC prior to installation?

 

You likely have an issue with the brake booster. Reason being, if the pedal feels proper when the engine is off, then you are feeling the normal hydraulic pressure working against your foot. The brake booster makes this a lot easier when the engine is running. If that booster failed, then the pedal will drop like you experience.

The brake booster works off of vacuum and atmospheric pressure. There is vacuum on one side of a diaphragm and atmospheric pressure on the other side. When vacuum is applied to one side, it overpowers the atmospheric pressure (~14psi) to provide extra pressure to the hydraulic system. Well if the vacuum booster failed, then no additional brake pressure will be provided, and the pedal will go to the ground like you are experiencing.

The booster leak test is a simple process that ill share with you here:
1. Press the brake pedal with the engine running, then stop the engine. If the pedal height does not vary while pressed for 30 seconds, the vacuum booster is OK. If the pedal rises, the booster is faulty.
2. With the engine stopped, press the brake pedal several times using normal pressure. When the pedal is first pressed, 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.
3. Disconnect the brake booster vacuum hose (check valve built-in) at the booster side.
4. Start the engine, and let it idle. There should be vacuum available. If no vacuum is available, the check valve is not working properly. Replace the brake booster vacuum hose and check valve, and retest.
5. Reconnect the brake booster vacuum hose. Start the engine, and then pinch the brake booster vacuum hose and check valve, and retest.
6. Turn the ignition switch OFF, and wait 30 seconds. Press the brake pedal several times using normal pressure. When the pedal is first pressed, it should below. On consecutive applications, the pedal height should gradually rise.
- If the pedal position does not vary, replace the brake booster.
- If the pedal position varies, replace the brake booster vacuum hose/check valve assembly.


Hope this helps