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Dodge Repair/'89 dodge diplomat brakes 'soft' when engine running


I have a 1989 dodge diplomat. I have changed both front brake calipers, a brake lin on front driver side and brake master cylinder, and also proportion valve.  I had bench bled the master cylinder before i installed it. I bled the brake system with engine off, rear pass, rear driver, front pass, front driver. i turned the engine and press the brakes and had pressure then pedal slowly went down. i drove the vehicle and had to pump the brakes a few times and pedal goes all the way down for brakes to stop vehicle. I then changed the brake booster and bled brakes again in same sequence. the brakes was still doing the same thing. the pedal has pressure when engine off. when engine is on, i have to pump brakes a few times for brakes to have pressure and pedal goes almost all the way down.  Can u please tell me what i am doing wrong or what i have to do to fix this problem.

Hi Ryan,
Assuming that you are not loosing brake fluid (correct?), and although you changed the brake power booster, what you describe is consistent with there not being sufficient vacuum to the booster for it to operate properly. So I would check the vacuum hose, its connections at the booster and the intake manifold, and the other vacuum lines that it shares with the heater system (a small diameter hose that goes through the firewall to the ac/heater control panel and to the cruise control. A leak in any of those lines will impact the power brake booster function in the manner that you describe.
Let me know what you find.
Thanks so much for the rating and nomination.

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Roland Finston


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