1979 Brake Issues
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Freshman User
Joined: Apr 2013
Posts: 32
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From: Rolla,MO
1979 Brake Issues
I have a 79 F150 4x4 and been having brake issues. When I start the truck the brake pedal drops down a tad to make the brake lights come on and when I turn it off the pedal comes up just enough to turn them off. While it's running or I'm driving if I put my foot under the pedal and lift up on it the brake lights will go out but as soon as I let go they come right back on. I have replaced both rear brake shoes and both rear brake cylinders and the brake light switch and the brake light on the dash is working as it should. Is there anything else it could be? Any advise would be helpful. Thanks in advance.
Senior User
Joined: Oct 2016
Posts: 401
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From: Rockville, IN Temporarily
Have you recently replaced the master cylinder? Sounds like the booster is taking up some slack allowing the pedal to drop. The rod on the booster to master cylinder must be the correct length or the master cylinder has the wrong plunger in it. There also is some adjustment in the brake light switch so it doesn't light up with just a tiny amount of pedal movement. I usually have them set for about 1/2 inch of pedal movement.
Senior User
Joined: Oct 2016
Posts: 401
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From: Rockville, IN Temporarily
The pedal should not go down. Since you have not changed out either the booster or the master cylinder then one or both are suspect. Personally when I change out a booster I do both as a unit. I would look first at the booster being the issue.
Moderator & parts slinger
Joined: Feb 2007
Posts: 50,143
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From: S/W Missouri, Polk county
X2 on the pedal moving on start up/shut down sounds like a vacuum booster issue? X22 on change both as a set. it is fasted. Make to bench bleed the new master cylinder before you install it.
You know that brake light on the dash (should only come on when brielfy you start the truck to do a circuit test) and then any time it comes on after that it is a WARNING that you have a brake system leak problem and it is causing the piston in the frame mounted proportional valve to move and trip the light.
You know that brake light on the dash (should only come on when brielfy you start the truck to do a circuit test) and then any time it comes on after that it is a WARNING that you have a brake system leak problem and it is causing the piston in the frame mounted proportional valve to move and trip the light.
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