Brake booster failure?
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Laughing Gas
Joined: Dec 2010
Posts: 924
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Brake booster failure?
Hey guys, I was wondering if Icould get your guys help on my dads f250 i was wondering if i have maybe brake booster failure or maybe master cylinder failure ever since we changed the rear brake shoes the truck has developed spongy brake pedal we've bled and adjusted the brakes multiple times and no change in pedal firmness. The pedal goes almost all the way to the floor also I've noticed that when I turn the truck off I hear a hissing sound from the booster. I know i shouldnt compare because every truck is different but i turn on and off my 86 f150 and it doesnt do that. Maybe I'm missing something.
FTE Legend
Joined: Nov 2015
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From: Angier, NC
The booster may have a leak but I cant see it a cause of the low / spongy pedal.
after you shut it down and the hissing stops push on the pedal is it hard like after you hit the pedal a few times on your other truck with the motor off?
Now you should have a check valve that could be leaking and the cause of a hard pedal of above. To test the valve run the motor to get vacuum in the booster and before you shut it down use vice grip to pinch off the hose then do the test above still a hard pedal replace the booster.
Spongy pedal how are you bleeding the brakes?
Did you bleed all 4 corners?
Dave ----
after you shut it down and the hissing stops push on the pedal is it hard like after you hit the pedal a few times on your other truck with the motor off?
Now you should have a check valve that could be leaking and the cause of a hard pedal of above. To test the valve run the motor to get vacuum in the booster and before you shut it down use vice grip to pinch off the hose then do the test above still a hard pedal replace the booster.
Spongy pedal how are you bleeding the brakes?
Did you bleed all 4 corners?
Dave ----
Moderator
Joined: Jan 2001
Posts: 56,984
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From: Virginia
The low pedal is caused by the rear brakes not being adjusted correctly. Jack the rear of the truck off the ground, and get a screwdriver or a brake adjusting tool, slip it in the slot in the backing plate, and turn the little star wheel in the correct direction to expand the brake shoes. Sometimes I get it wrong and turn it the wrong way, if I really get messed up I have to take the wheel off and check to make sure which way to turn it.
Once you get it going in the right direction tighten it up till you can't turn the tire at all. Then back it off by turning it the other way till you can turn the tire, but there is still some rubbing going on. Leave it there and then do the other side. After this you should have a very high pedal.
If it's still spongy, you still have air in the lines. If you replaced both wheel cylinders, make sure the bleeder screw is on top above where the brake line goes in. If it is below the brake line, the wheel cylinders need to be swapped side to side on the truck. You will never get the air out if the bleeder is on the bottom.
Once you get it going in the right direction tighten it up till you can't turn the tire at all. Then back it off by turning it the other way till you can turn the tire, but there is still some rubbing going on. Leave it there and then do the other side. After this you should have a very high pedal.
If it's still spongy, you still have air in the lines. If you replaced both wheel cylinders, make sure the bleeder screw is on top above where the brake line goes in. If it is below the brake line, the wheel cylinders need to be swapped side to side on the truck. You will never get the air out if the bleeder is on the bottom.
Thread Starter
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Laughing Gas
Joined: Dec 2010
Posts: 924
Likes: 1
The booster may have a leak but I cant see it a cause of the low / spongy pedal.
after you shut it down and the hissing stops push on the pedal is it hard like after you hit the pedal a few times on your other truck with the motor off?
Now you should have a check valve that could be leaking and the cause of a hard pedal of above. To test the valve run the motor to get vacuum in the booster and before you shut it down use vice grip to pinch off the hose then do the test above still a hard pedal replace the booster.
Spongy pedal how are you bleeding the brakes?
Did you bleed all 4 corners?
Dave ----
after you shut it down and the hissing stops push on the pedal is it hard like after you hit the pedal a few times on your other truck with the motor off?
Now you should have a check valve that could be leaking and the cause of a hard pedal of above. To test the valve run the motor to get vacuum in the booster and before you shut it down use vice grip to pinch off the hose then do the test above still a hard pedal replace the booster.
Spongy pedal how are you bleeding the brakes?
Did you bleed all 4 corners?
Dave ----
I should add that with the truck off I pressed the brake pedal until it got hard and then turned the truck and nothing really happened if it did go it was little to none. I also pumped the brakes to see if engine idle would increase and nothing happened.
Cargo Master
Joined: Mar 2002
Posts: 3,132
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From: W (BY GOD) V
http://www.hotrodhardware.com/pdf/mbm.pdf
Page 14 will show how to test a booster.
Also, if you have spongy brakes after bleeding, the RABS valve may require bleeding.
Page 14 will show how to test a booster.
Also, if you have spongy brakes after bleeding, the RABS valve may require bleeding.
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 219
From: W (BY GOD) V
Goes to show what I don't know ...
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FTE Legend
Joined: Nov 2015
Posts: 30,918
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From: Angier, NC
There was talk about this not long ago and it was on the 250/350 trucks IIRC from the post.
Bill even posted a page out of the parts book on it.
Dave ----
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From: Virginia
https://www.ford-trucks.com/forums/1...tique-abs.html
There was talk about this not long ago and it was on the 250/350 trucks IIRC from the post.
Bill even posted a page out of the parts book on it.
Dave ----
There was talk about this not long ago and it was on the 250/350 trucks IIRC from the post.
Bill even posted a page out of the parts book on it.
Dave ----
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 219
From: W (BY GOD) V
i was wondering if i have maybe brake booster failure or maybe master cylinder failure ever since we changed the rear brake shoes the truck has developed spongy brake pedal we've bled and adjusted the brakes multiple times and no change in pedal firmness. The pedal goes almost all the way to the floor ...
hUKd oN fonIx - wERX 4 mee
I remember my third grade (final year of school) teacher mentioning something about KEY WORDS and the importance of noticing and understanding.
As Franklin has mentioned, your problem is most likely in the rear brake adjustment.
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