Soft brake pedal only with engine running

themechanic

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This is a boosted front disc/rear drum system properly bled (bench bled MC then 4 wheels). I used the tool to set the booster push rod length. I have the proper power brake pedal linkage. I have the proper disc/drum distribution block. All brake components are new with zero leaks. The car will stop only when the pedal reaches the floor.

What's the solution?
 

Roger63

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I used the tool for pushrod adjustment also, I still had to extend it to get proper pedal.
 

themechanic

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Yes. It can be heard scraping while turning the wheels with them off the ground.
BTW, the one thing I forgot to do is have the drums checked on a machine and turned if necessary. Someone else recently bought new brake drums and recommended this.
 

Don Frelier

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Do you have the calipers with the bleeder at the highest point?
Does the pedal come up if you pump it quickly a couple times?
I've heard about people having troubles with the centering in the proportioning valve. There's either more air in there or
Otherwise I would suspect the master cylinder.
 

MIKESPOLARA

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Master cylinder bore size may be too small. What bore size is your master? The smaller the bore the longer the pedal travel and is softer. Bigger bore reduces travel and firms the pedal feel. You always use a larger bore with power then you do with manual
 

themechanic

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Do you have the calipers with the bleeder at the highest point?
Does the pedal come up if you pump it quickly a couple times?
I've heard about people having troubles with the centering in the proportioning valve. There's either more air in there or
Otherwise I would suspect the master cylinder.
Bleeders are in top. No air in system. Otherwise, would I be able to get a firm pedal after bleeding and with the engine off?

I would think that if the MC internal seals are bad from the manufacturer the pedal would gradually go down with the foot firmly holding pressure on it for a period of time. I have no leak down until I start the car.

The MC stays full.
 

themechanic

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Master cylinder bore size may be too small. What bore size is your master? The smaller the bore the longer the pedal travel and is softer. Bigger bore reduces travel and firms the pedal feel. You always use a larger bore with power then you do with manual
1 1/8" bore disc/drum MC. Dual diaphragm Bendix style booster.
 

68 Sport Satellite

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yep, depends on your front caliper volume too. I also am running front disc/rear drum. When I first ran a 15/16" MC it stopped, but not until the pedal was almost to the floor. I switched to a 1-1/8" MC and now it's fine, although I need to try some different pads for more bite.
 

Geoff 2

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You have air in the system. Engine running, extra force provided by booster is compressing the air, lowering the pedal.
 

6872n73

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How about bleeding the system with the engine running?
 

themechanic

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I'll check booster push rod length furst, then, if necessary, bleed the system again

It doesn't make any difference if the system is bled with the engine running. That's why none of the how tos include starting the car before bleeding brakes.
 

Don Frelier

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We had something similar once on a truck, same type of symptoms.
Turned out one of the rear wheel cylinders was leaking.
We found it when it leaked enough to show on the drum.
Probably not the issue here but worth noting.
 
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