Brake booster problem - 79 F350 4x4
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Laughing Gas
Joined: Sep 2006
Posts: 791
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From: Backwoods of western Pa
Brake booster problem - 79 F350 4x4
I'm experiencing a problem with my brake booster on my '79 F350 4x4; I was hoping someone could offer some advice. Here are the symptoms:
When the weather is cold (<40F), and the truck is cold (i.e. first startup of the day) the brake booster doesn't work when you first press the brakes. When the brakes are pressed it makes a loud "whoosh" or hissing sound from the brake pedal area and it has no power assist. Working the brake pedal a few times will help, and eventually the power assist starts working fine and the noise quits. Once the truck has warmed up some and I've worked the pedal a few times, the problem completely disappears until the next cold morning startup.
If the weahter is warm, I never experience this issue. Likewise if the truck is warm the issue is nonexistent. It seems like there is a valve that actuates the power assist when the brake pedal is pressed that is sticking when the booster is cold. Once it warms up, the valve loosens up and it works normal.
I've never had a booster apart, but I was wondering if the pedal-actuated valve is servicable?
When the weather is cold (<40F), and the truck is cold (i.e. first startup of the day) the brake booster doesn't work when you first press the brakes. When the brakes are pressed it makes a loud "whoosh" or hissing sound from the brake pedal area and it has no power assist. Working the brake pedal a few times will help, and eventually the power assist starts working fine and the noise quits. Once the truck has warmed up some and I've worked the pedal a few times, the problem completely disappears until the next cold morning startup.
If the weahter is warm, I never experience this issue. Likewise if the truck is warm the issue is nonexistent. It seems like there is a valve that actuates the power assist when the brake pedal is pressed that is sticking when the booster is cold. Once it warms up, the valve loosens up and it works normal.
I've never had a booster apart, but I was wondering if the pedal-actuated valve is servicable?
Senior User
Joined: Mar 2007
Posts: 362
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usual you only replace it. the valve is the piece that plugs into the booster itself. Your problem may be the booster itself. If memory serves, the diaphram is rubber, on cold mornings it could be contracted to the point it doesn't seal correctly until it warms up.
Senior User
Joined: Apr 2010
Posts: 248
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From: Farmington, New Mexico
That's probably it and it's a cheap repair. Replace the grommet and vacuum hose while your at it.
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Laughing Gas
Joined: Sep 2006
Posts: 791
Likes: 62
From: Backwoods of western Pa
The valve that you are referring to is the check valve to keep the booster from bleeding off vacuum when the engine isn't making any (i.e. when the engine stalls or you are at WOT). That valve is fine, I don't have a problem with it bleeding off vacuum until I press the pedal.
I think the problem is internal to the booster. The diaphragm seems to be fine since it has good power once it warms up. The problem seems to be with the valve that closes off the atmospheric side of the booster when the pedal is pressed. This valve should close, then an atmospheric valve opens the back of the diaphragm to the atmosphere to provide the power assist. I found a good explanation of the inner workings of the booster here: Troubleshoot Power Brakes
I have a 78 F250 parts truck with an unknown booster on it, so I think I'll swap that on to see if it works reliably. If it doesn't work, then I'll have to look into a new booster. Once I get the old one off I'll investigate to see if the internal valves are serviceable.
I think the problem is internal to the booster. The diaphragm seems to be fine since it has good power once it warms up. The problem seems to be with the valve that closes off the atmospheric side of the booster when the pedal is pressed. This valve should close, then an atmospheric valve opens the back of the diaphragm to the atmosphere to provide the power assist. I found a good explanation of the inner workings of the booster here: Troubleshoot Power Brakes
I have a 78 F250 parts truck with an unknown booster on it, so I think I'll swap that on to see if it works reliably. If it doesn't work, then I'll have to look into a new booster. Once I get the old one off I'll investigate to see if the internal valves are serviceable.
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davidc-austin
Brakes, Steering, Suspension, Tires, & Wheels
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Jan 10, 2005 11:13 PM
