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Brake Pedal is only good when car is off
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no_jive55
Posted 2020-11-10 5:22 PM (#605512)
Subject: Brake Pedal is only good when car is off



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Posts: 14

I recently rebuilt my entire brake system in my 1960 Plymouth, (all new brake lines, new wheel cylinders, front disc brake upgrade). I am using a power brake booster, master cylinder, and proportioning valve off of a 1981 Dodge D150. I completely bled the brakes, now there is no air in the system. There are no fluid leaks. I have a solid pedal when the car is not running, but when I start the car the pedal goes nearly to the floor. Is there an ample amount of vacuum coming off of the port on the intake or could there be a leak in the booster? The system worked fine on the 1981 dodge truck but now I am facing issues. Also, the vacuum port that I am using originally supplied 3 vacuum lines when it was installed on the truck, now it is only supplying the booster.
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56D500boy
Posted 2020-11-10 7:51 PM (#605517 - in reply to #605512)
Subject: RE: Brake Pedal is only good when car is off



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Nobody else chimed in (yet) so I will take a stab at it:

1. If you have firm pedal with the engine off, it should imply that the Master Cylinder is good.

2. If the pedal goes down with the engine off, it might mean that there is a vacuum check valve that was installed backwards and the engine is releasing the vacuum out of the booster, not adding to it. (???)

Next person?
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Old Ray
Posted 2020-11-10 11:05 PM (#605523 - in reply to #605517)
Subject: RE: Brake Pedal is only good when car is off



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I think the test procedure is to push the pedal and than start the engine and the petal should move down. I think the power brake booster is working the way it is supposed to. (kind of) Maybe the pedal is too low but that is a linkage or mounting problem.

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57chizler
Posted 2020-11-11 1:36 PM (#605538 - in reply to #605523)
Subject: RE: Brake Pedal is only good when car is off



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The pedal should drop on engine start but not to the floor..."nearly to the floor" might be OK depending on the size of the master cylinder. Some large-diameter calipers will require a MC with a larger bore; a too-small MC will increase the pedal travel.
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Shep
Posted 2020-11-12 9:55 AM (#605568 - in reply to #605538)
Subject: Re: Brake Pedal is only good when car is off



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Correct 57chizler, the larger bore will require some more pedal pressure to achieve satisfactory results.
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57chizler
Posted 2020-11-13 12:37 PM (#605613 - in reply to #605568)
Subject: Re: Brake Pedal is only good when car is off



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Another possibility; if the car originally had manual brakes and the original pedal is used, the pedal ratio will increase the pedal travel. Many manual brake FL's had a pedal ratio of 8-1 while power brake pedal ratio is 4-1.
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Old Ray
Posted 2020-11-13 4:27 PM (#605627 - in reply to #605613)
Subject: Re: Brake Pedal is only good when car is off



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57chizler - 2020-11-13 10:37 AM Another possibility; if the car originally had manual brakes and the original pedal is used, the pedal ratio will increase the pedal travel. Many manual brake FL's had a pedal ratio of 8-1 while power brake pedal ratio is 4-1.

You are the man !!!!! 

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56D500boy
Posted 2020-11-14 2:04 AM (#605637 - in reply to #605512)
Subject: RE: Brake Pedal is only good when car is off



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Found this on Rock Auto:

"Symptoms of a Failing Power Brake Booster:

Stiff brake pedal that is extremely hard to push
Longer stopping distance
Engine stalls when brakes are applied

Prior to replacing your Power Brake Booster:

Ensure the Vacuum Hose is properly connected and not loose
Check the Vacuum Hose for a blockage or leak
Inspect the Check Valve for cracks, looseness or damage

Testing your Power Brake Booster:

With the engine off, pump the brakes 5-6 times
Turn the engine on while pushing down lightly on the brake pedal
If your brake booster is working, the pedal will sink down about ¼”
If your brake booster is not working, nothing will happen or the brake pedal will push back against your foot"



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Stroller
Posted 2020-11-14 10:57 AM (#605644 - in reply to #605512)
Subject: Re: Brake Pedal is only good when car is off


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Well back to a couple basic questions sir. First did you bench bleed the master cylinder? Did you crack the fittings at the master cylinder and have someone depress the brake pedal after you installed and before bleeding the system out? Did you gravity bleed the system before pressure bleeding? How did you bleed the system? Pressure bleed, vacuum bleed or someone's foot? Is it a new master cylinder or it worked when pulled from the truck? I ask this because if it came off something else, then foot bled it is possible whomever had foot on pedal may have let the pistons in the master cylinder pass beyond their normal rate of travel. The main thing is does the system gravity bleed freely. I always use the 3 times and hold method of bleeding. Tighten the bleeder right after the fluid is shooting out. Should be a good stream strong stream, not a drizzle.
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Stroller
Posted 2020-11-14 10:59 AM (#605645 - in reply to #605512)
Subject: Re: Brake Pedal is only good when car is off


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Sorry 1 other thing. Are the rear shoes adjusted out properly? Just barely touching the drums? Your pedal adjusted comes from the rear shoes. Don't try to raise the pedal with the E-brake. Adjust it after.
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Richbo
Posted 2021-07-11 7:56 PM (#613428 - in reply to #605512)
Subject: Re: Brake Pedal is only good when car is off


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Adjust the shoes to very little clearance . Snug them , then back VERY LITTLE -- just enough to allow to be turned by hand.
With engine off you are pressing against the return springs , but with engine running you are pressing the shoes out to the drums.
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Richbo
Posted 2021-08-19 11:01 PM (#614326 - in reply to #605512)
Subject: Re: Brake Pedal is only good when car is off


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Been a few weeks -- any update ??
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