Front disc brakes keep locking up
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1st Gear
Joined: Nov 2022
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Front disc brakes keep locking up
Ive got a 79 f150 and have been having problems with the front brakes for a good while. they started off having trouble pushing fluid to the back brakes so i swaped all brake line, master cylinder, brake booster and rear wheel cylinders, that seemed to fix that problem, then my front brakes started locking up, ive put on 2 new sets of front calipers and had the same problem of locking up with both sets. Ive cleand the hell out of the proportioning valve and definitely completely bled the lines with both sets of calipers, not sure if i should go about a new proportioning valve or what
Lead Driver
Joined: Oct 2018
Posts: 7,879
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Are you positive you have the right master ? drum brakes normally have a residual valve that holds fluid back for a bit. shouldn't cause the calipers to stay on long but without return spring pressure I'm not sure what would happen.
Could be a screwed up proportioning valve I suppose, I've had problems with them but not this.
It could be an adjustment issue between the master and booster. or a defective booster that's not returning properly. I'd pull the hose off it when your brakes are locked and see if it changes anything.
Could be a screwed up proportioning valve I suppose, I've had problems with them but not this.
It could be an adjustment issue between the master and booster. or a defective booster that's not returning properly. I'd pull the hose off it when your brakes are locked and see if it changes anything.
Hotshot
Joined: Feb 2004
Posts: 10,808
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From: The Shenandoah Valley
You'll have to pull the master cylinder loose from the booster to do it, but it might pay you to check the actuating rod adjustment between the booster and master cylinder. If the push rod is too long, the brakes will not release once applied.
There are "tools" to measure it or you can make one easily. I'm not sure of the specification, this may be correct for my '77 F-150 ...
... Look down a couple posts to post #5 ...looks like the number is 0.980" min to 0.995" max.
There are "tools" to measure it or you can make one easily. I'm not sure of the specification, this may be correct for my '77 F-150 ...
... Look down a couple posts to post #5 ...looks like the number is 0.980" min to 0.995" max.
Last edited by tbear853; Nov 30, 2022 at 09:22 PM. Reason: corrected what might have mislead.
FTE Legend
Joined: Nov 2015
Posts: 30,915
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From: Angier, NC
If not then you know you have to look else where.
We're the rubber hoses replaced?
They can go bad inside but look good on the outside.
Dave ----
Lead Driver
Joined: Nov 2006
Posts: 5,217
Likes: 506
From: Pa.
You'll have to pull the master cylinder loose from the booster to do it, but it might pay you to check the actuating rod adjustment between the booster and master cylinder. If the push rod is too long, the brakes will not release once applied.
There are "tools" to measure it or you can make one easily. I'm not sure of the specification, this may be correct for my '77 F-150 ...
... but just a picture of the tool in use:
There are "tools" to measure it or you can make one easily. I'm not sure of the specification, this may be correct for my '77 F-150 ...
... but just a picture of the tool in use:
From 1978 Shop service manual APPLIES TO F-100, F-150, F-250, (6200/6800 GVWR), E-100, E-150, F-150 - F-250 (4x4) AND BRONCO
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