Brake Booster
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Mountain Pass
Joined: Jul 2000
Posts: 189
Likes: 12
From: Minnesota
Brake Booster
Hey guys, anyone ever have their brake booster go bad? I think thats what is happening on my 79 F150 short box 4x4. Here's what its doing. When I come to a complete stop I will eventually lose pressure on the brake pedal. If I let off the pedal and the put the brakes on it will rebuild the pressure and stay firm. I have been under the truck a couple of time and can't find any leaks in the brake system and fluid level in the master cylinder has stayed consistant. One thing that does happen occasionally is I get a whining nose, sounds like under the dash, when I put the brakes on. I'm thinking vaccuum leak on the booster where the brake pedal connects. What do guys think? Thanks in advance for your help.
Elder User
Joined: Jan 2002
Posts: 780
Likes: 0
Brake Booster
I have seen several late model Mustangs with this problem. Brake booster leaks where brake pedal rod goes in. Fix is to replace or rebuild brake booster. You might want to check all vacuum lines first and take a look at the check valve where the vacuum line goes into the brake booster. Sometimes the valve fails.
Elder User
Joined: Jun 2001
Posts: 775
Likes: 0
From: Northern Va
Brake Booster
Sound like you have 2 problems. A booster is just that, an assist device. If the pedal is going to the floor when you are sitting at a light, it's not he booster. That sounds like a master cylinder problem. Master cylinders can bypass internally or out the rear seal.
Try this test, with the engine running press the brake pedal "medium hard", wait for a few seconds, then release about 1/2 (or so) of the pressure you are currently applying, pause and reapply to the same pressure as before. Repeat this process a few times, never fully releasing the brake pedal. If the pedal slowly travels all the way to the floor as you do this, the master cylinder is bad.
I have seen master cyl. leak out the rear and fill the booster with fluid causing the booster to go bad, but since your fluid level is okay I would have to guess an internal bypass if the previously mentioned test fails.
A bad booster typically presents as an extremely hard pedal and/or a hissing sound from under the dash from the vacuum leak. I have even heard of the engine stalling when the brakes are applied from the sudden vacuum leak.
Try this test, with the engine running press the brake pedal "medium hard", wait for a few seconds, then release about 1/2 (or so) of the pressure you are currently applying, pause and reapply to the same pressure as before. Repeat this process a few times, never fully releasing the brake pedal. If the pedal slowly travels all the way to the floor as you do this, the master cylinder is bad.
I have seen master cyl. leak out the rear and fill the booster with fluid causing the booster to go bad, but since your fluid level is okay I would have to guess an internal bypass if the previously mentioned test fails.
A bad booster typically presents as an extremely hard pedal and/or a hissing sound from under the dash from the vacuum leak. I have even heard of the engine stalling when the brakes are applied from the sudden vacuum leak.
New User
Joined: Jan 2008
Posts: 0
Likes: 24
Brake Booster
>where the brake pedal connects. What do guys think? Thanks
>in advance for your help.
E-Mail me dorcet for a copy of Ford's Brake Booster Check procedure.
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