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1994 F-250: Mushy Brake Pedal w/ Engine Running

 
  #1  
Old 01-26-2019, 02:34 PM
PoorOldBlue92
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1994 F-250: Mushy Brake Pedal w/ Engine Running

Hey Guys,

Now that we've finally had a break in the weather I finally got around to working on the truck again (1994 F-250, 5.0, RWD). I have replaced the brake shoes, all the drum brake hardware, the wheel cylinders, the calipers, and today I replaced the master cylinder. With the help of my neighbor, we bled the brakes starting from passengers rear, drivers rear, the RABS, passenger front, and ending with the drivers front.

With the truck off, the pedal feels firm. As soon as the truck is started the pedal will go all the way to the floor and you can hear a whooshing noise when you depress the pedal. When you turn the truck off, the pedal goes back to firm. The truck stops but it's disconcerting having the pedal go all the way to floor with almost zero resistance.

Could this be indicative of a failing booster or do we just need to bleed the brakes again?
 
  #2  
Old 01-26-2019, 02:38 PM
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95fiveoh
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The wooshing sound is very typical of a bad booster.
 
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Old 01-26-2019, 02:42 PM
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Are there any tests to perform to verify it's the booster?
 
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Old 01-26-2019, 03:14 PM
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I tested the booster check valve and it is working fine. If I disconnect the vacuum hose the brake pedal remains firm. Not sure if that tells me anything or not.
 
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Old 01-26-2019, 03:14 PM
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Originally Posted by PoorOldBlue92 View Post
Are there any tests to perform to verify it's the booster?
Press on the pedal if you hear a whoosh, it's bad. If the pedal feels soft with the engine on versus engine off....the booster is bad.
 
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Old 01-26-2019, 03:25 PM
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Got it. I guess I was under the impression that a bad booster would cause a hard pedal and not a soft one. Looks like I'll be ordering a new booster. Thanks guys!
 
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Old 01-26-2019, 03:53 PM
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Yep if you disconnect the vacuum line to the booster and the pedal gets firm again, it's definitely the booster.
 
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Old 02-03-2019, 02:43 PM
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I got the new booster installed today. The whooshing noise is gone and the brake pedal is much more firm but I'm still able to push the pedal to the floor. I noticed the rear line is leaking at the booster. The fitting is as tight as I get it so looks like I'll be ordering some new Nicopp line.
 
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Old 02-03-2019, 03:02 PM
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Just went thru this with my 94 f250. I ended up replacing all my metal lines, master cylinder, rubber lines and by passing the rabs. I would fix one line, think everything is great and it would even pump up when off but had such a slight leak some where else that it would eventually leak down while started.

Keep in mind the rear metal line going into the wheel cylinder is a 1/4 fitting on a 3/16 line. Orylies has a lot of break hardware compaired go other places. For the relatively cheap expense of metal line and a double flaring tool its actually worth doing them all. You can cheaply by pass the rabs real quick just to make sure thats not your problem before replacing all the lines.

Once I replaced all my lines and everything else I have solid breaks again. Hopefully you can sort it out. I had a super small leak at my master cylinder fitting too, front one and I had to really crank it to get a good seal.
 
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Old 02-03-2019, 06:27 PM
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Originally Posted by Mo8414 View Post
Just went thru this with my 94 f250. I ended up replacing all my metal lines, master cylinder, rubber lines and by passing the rabs. I would fix one line, think everything is great and it would even pump up when off but had such a slight leak some where else that it would eventually leak down while started.

Keep in mind the rear metal line going into the wheel cylinder is a 1/4 fitting on a 3/16 line. Orylies has a lot of break hardware compaired go other places. For the relatively cheap expense of metal line and a double flaring tool its actually worth doing them all. You can cheaply by pass the rabs real quick just to make sure thats not your problem before replacing all the lines.

Once I replaced all my lines and everything else I have solid breaks again. Hopefully you can sort it out. I had a super small leak at my master cylinder fitting too, front one and I had to really crank it to get a good seal.
That's the route I'm heading. The rubber lines have already been replaced and the front metal lines have already been replaced with Nicopp but all the rears are original. I'm hoping I can knock it all out next weekend.
 
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Old 04-14-2019, 09:34 PM
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Now that the weather turned, I finally had time to work on the brakes again. I bled the brakes again using a Motive Products power bleeder but the end result still was a mushy pedal. The truck will stop and the pedal is firm with the engine off, but it feels spongy with the engine running. I may end up running new hard lines as I am not sure what else could be causing the problem.
 
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Old 04-15-2019, 12:36 AM
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Let us know how it goes. I've been dealing with a similar issue on another vehicle for a while and it's driving me nuts.
 
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Old 04-15-2019, 08:49 AM
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Originally Posted by PoorOldBlue92 View Post
Now that the weather turned, I finally had time to work on the brakes again. I bled the brakes again using a Motive Products power bleeder but the end result still was a mushy pedal. The truck will stop and the pedal is firm with the engine off, but it feels spongy with the engine running. I may end up running new hard lines as I am not sure what else could be causing the problem.
Adjust the rear shoes until the wheel don't spin more than a turn then give that a go.
 
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Old 04-19-2019, 01:16 PM
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Originally Posted by rla2005 View Post
Press on the pedal if you hear a whoosh, it's bad. If the pedal feels soft with the engine on versus engine off....the booster is bad.
Your statement made me change my brake booster today. Wow, what a difference. I got the truck late last year and didn't know what the brakes should feel like. I bled them several times but no change. After getting the new brake booster in, they are much, much better. Thank you!
 
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Old 04-19-2019, 01:19 PM
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Originally Posted by minimalist View Post
Your statement made me change my brake booster today. Wow, what a difference.
I am glad you found this worked out for you.
 

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