Brake Pedal Goes to Floor When Car is Running

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74DartSwinger360

74DartSwinger360

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But if the car is shut off it has perfect and firm brake pedal.:wack:
The car had the Master Cylinder and booster assembly taken off when we had gotten the Valiant but rest of the system was there.
I replaced the Front Calipers and Master Cylinder with 10 yr old reman units as well as putting brand spanking new Rubber lines and Front pads.
Didn't replace the rear shoes or wheel cylinders as they looked really good when I inspected and cleaned out the insides of the drum assembly with an air gun to remove years of brake dust build up.

I am at a total loss as to what would cause the pedal to do this when you fire the car up. It causes the brakes to engage and car wont move even when you hit the loud pedal some.:banghead:

I researched for a few hours last night but nothing I found said the same thing was the problem or had the solution to the problem.

So if any of you know why this is happening or have experienced this before I would love to get some help here.
Right now it is the only thing keeping this bad boy off the road and being driven a ton.
 

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a bad brake booster leak or bad booster . when the engine is running , its creating a vacuum and causing the booster to do weird stuff would be my guess
 
The booster is just a vacuum powered way to increase the available power to push the pedal, so saying it goes to the floor when running but fine when the car is not running makes a person that knows how it all works go, Huh?
(We understand what you mean, but it's not actually possible for both to be true)

Power brake brakes are stiff as hell when they don't have power to them.

Start with bleeding from the farthest wheel from the master cylinder and work your way to the closest wheel to the master.
If that doesn't do it, step on the pedal with the car running and set your emergency brake.
Test the pedal again.
If it feels alot better with the e brake set, then the rear brakes need adjusting.
(Setting the e brake takes all the free travel out of the rear brakes, basically eliminating them from having to travel when the brake pedal is applied)

The fact that the pedal goes to the floor only when running tells you that the booster is working.

What I suspect is air in the fluid (like already mentioned) or too much travel needed by the rear brakes from way to much play in the rear adjustment.

Just a note:
Most people don't know that these brakes have an automatic adjuster in the rears that when you back up and apply the brakes it moves a plate that is supposed to turn the adjuster.
Sometimes they work, and sometimes things are to dirty or worn out to work.
I any case, when the rear brakes are a long ways out of adjustment it would take 30 or more backing up stops to get them back in adjustment.
(Don't try this with yours if the brakes go to the floor) It was just meant as an FYI.
 
If I'm rading this right, when you start the car the pedal goes to the floor without you even pressing on it? If so, defective brake booster is my diagnoses.
 
/6 Matt said:
If I'm rading this right, when you start the car the pedal goes to the floor without you even pressing on it? If so, defective brake booster is my diagnoses.
Click to expand...

OH!
Thats possible.
If that is the case then it is the booster for sure.
I had a Lincoln that did that.

I just re read his post and I think you are right Matt.

Yup, booster for sure.
 
/6 Matt said:
If I'm rading this right, when you start the car the pedal goes to the floor without you even pressing on it? If so, defective brake booster is my diagnoses.
Click to expand...


Yep This is the problem. But now the car has sat overnight it doesn't do it anymore???:wack:

So Guess will try it each day around neighborhood to see how it acts

Thanks for all the inquiries fellas gonna play this one by day


They had been bleed twice last night and was still goin to floor. So don't know what the hell is goin on. oh well I guess. Just need to get her back down once air shocks are in and drive down the block
 
How about a leak in the vacuum booster rubber line,or a bad booster check valve?. If the booster makes pressure,the check valve should release pressure unplugged. If it doesn't, then I would look at all the cheap small stuff first.
 
If you do drive it, it might lock up at any time and shutting the car off and waiting for the vacuum to bleed off, or unplugging the vac line to the manifold will get it to let go.
 
Abodybomber said:
How about a leak in the vacuum booster rubber line,or a bad booster check valve?. If the booster makes pressure,the check valve should release pressure unplugged. If it doesn't, then I would look at all the cheap small stuff first.
Click to expand...

I have had cars that did that before and you are right about something in the vacuum valve causing it, but I don't remember what.
The last one, everthing would be fine till I touched the pedal and then it would suck the pedal down to the floor and totally lock the brakes.
I had to shut it off, jump out and pull the vac line off to go again, and if I touched the pedal it would do it again.

The problem was that I was not willing to risk a sudden and complete lockup of the brakes at any random speed, with who knows what as far as obstacles.
(My luck it would be at 90 miles an hour in four lanes of traffic.) :banghead:
I just replaced them.
 
We are pretty sure it just needs a new master cylinder and rear brakes adjusted.
When you hit the pedal it pops then brakes go to floor.
So thinkin the piston in the master cylinder sat in one spot for so many years that surface rust happened in rest of cylinder and that when it was bench bleed it messed with the piston seal causing damage to it and allowing fluid to go by.

As it is right now rear brakes don't engage at all when brakes are hit and the fronts immediately lock up once you hear and feel a pop as you are hitting the brakes.

Just need to order a master cylinder
 
74DartSwinger360 said:
We are pretty sure it just needs a new master cylinder and rear brakes adjusted.
When you hit the pedal it pops then brakes go to floor.
So thinkin the piston in the master cylinder sat in one spot for so many years that surface rust happened in rest of cylinder and that when it was bench bleed it messed with the piston seal causing damage to it and allowing fluid to go by.

As it is right now rear brakes don't engage at all when brakes are hit and the fronts immediately lock up once you hear and feel a pop as you are hitting the brakes.

Just need to order a master cylinder
Click to expand...

Myself, I have never experienced a booster being the problem based off doing what you described. Lord knows I haven't seen it all though. Every time I have ran in to that exact combination it has either been air in the lines or a bad component in the brake system. Did you replace the rear rubber brake line too when you did the fronts?
 
I don't think the booster could make the pedal go to the floor unless there was either a bad MC piston seal or air in the lines. You should be able to do manually (engine off) whatever the booster does, maybe by pressing hard with both feet. There is a direct mechanical connection between the pedal and the MC thru the booster, if everything is assembled correctly. The vacuum just assists your movement.
 
I was taught:( initial brake inspection, vacuum brake booster) Run the engine,shut car off.Pump the pedal,count the "softer ,assisted pumps". Should be at least two. After that,should be a flat ,rock hard pedal. Straight from my Bendix handbook,as I remember it.
 
BillGrissom said:
I don't think the booster could make the pedal go to the floor unless there was either a bad MC piston seal or air in the lines. You should be able to do manually (engine off) whatever the booster does, maybe by pressing hard with both feet. There is a direct mechanical connection between the pedal and the MC thru the booster, if everything is assembled correctly. The vacuum just assists your movement.
Click to expand...

The car that I had the problem with would suck the pedal to the floor as soon as you touched it.
Meaning once you touched the pedal you could take your foot off of it and watch the pedal continue on down by itself until the brakes were totally locked up.
Otherwise everything seemed normal.
I got another booster for it and the problem went away.
Same MC and everything, and I have seen it twice in 40 years so apparently it's not very common.
Both were Fords.
 
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