Brakes grabbing
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Postmaster
Joined: Jul 2001
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From: MD
Brakes grabbing
I just purchased a 1979 F-250 two wheel drive pickup. When stopping, the brakes like to grab too much. I think I need a neck brace now. It is very evident at low to standstill speeds. At first I thought it was the rear self-adjusting brakes. I backed up several times and mashed on the brakes to no avail. What to do now? 
Elder User
Joined: Feb 2004
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From: San Luis Obispo, CA
In pickups there is a design compromise. When loaded there is a lot more weight on the rear wheels, and the vehicle weighs more, so it needs maximum braking capablilty. Most trucks are designed so that they have enough brake for maximum load, so at minimum load there is more rear brake action than needed. The self-energizing action of drum brakes only makes this problem worse. Another thing that makes this problem worse is cold, damp brakes. (You may find that you can improve things by lightly dragging the brakes when first driving it on a cold, damp morning.)
There are changes that can help your problem, but most of them will reduce your effective braking capacity when you are fuly loaded. (A neck brace would be cheaper.) You do not mention whether you have front discs or power brakes, so I will stop for now.
There are changes that can help your problem, but most of them will reduce your effective braking capacity when you are fuly loaded. (A neck brace would be cheaper.) You do not mention whether you have front discs or power brakes, so I will stop for now.
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Postmaster
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From: MD
That would make sense since the truck is totally empty - nothing in bed or cab except from what left the assembly line. Still, it's way too hard - at 5mph, tapping the brake pedal will result in it stopping dead right there.
It's a '79, so yes, it has the front disc brakes, rear drum. Has power brakes. Maybe the booster is working too well?
It's a '79, so yes, it has the front disc brakes, rear drum. Has power brakes. Maybe the booster is working too well?
Elder User
Joined: Feb 2004
Posts: 909
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From: San Luis Obispo, CA
OK - you have a booster, which could be too sensitive. While I have rebuilt a booster, I only replaced all the parts provided by my kit without doing any modifications. I had good results, so I did not go any deeper. Maybe someone will post regarding how easy it is to make a modification that will reduce sensitivity without causing any safety issue. (There may even be a failure mode that causes the sensitivity you notice.) Your could throw $$$ at the problem by replacing the booster, but I recommend you trouble-shoot first.
You have front disks, so your system has some form of valving which includes a proportioning feature. There could be a problem in this valving, but I am not drawing on experience here. I suggest you to an FTE search on "proportioning valve" and see if anyone has already had a brake sensitivity problem.
I'll leave you with a question: If you brake on gravel (have someone watch from outside the vehicle), does the front or rear lock up first? This will help in finding out what the source of your lockup is.
P.S.: I notice you are a "neighbor" - I'm about 20 minutes south of Gettysburg.
You have front disks, so your system has some form of valving which includes a proportioning feature. There could be a problem in this valving, but I am not drawing on experience here. I suggest you to an FTE search on "proportioning valve" and see if anyone has already had a brake sensitivity problem.
I'll leave you with a question: If you brake on gravel (have someone watch from outside the vehicle), does the front or rear lock up first? This will help in finding out what the source of your lockup is.
P.S.: I notice you are a "neighbor" - I'm about 20 minutes south of Gettysburg.
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Postmaster
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From: MD
That's what I was thinking as well. I don't plan to modify brakes.
Okay, will do a search on the proportioning valve and do a gravel road test. Thanks. Finding a patch of gravel is going to be the tough part.
I just discovered my brake light doesn't work at all - not sure if it's the bulb or wiring or what. If it was working, maybe it could light up to tell me the proportioning valve is defective.
Cool! I'm about an hour away from Gettysburg.
Okay, will do a search on the proportioning valve and do a gravel road test. Thanks. Finding a patch of gravel is going to be the tough part.
I just discovered my brake light doesn't work at all - not sure if it's the bulb or wiring or what. If it was working, maybe it could light up to tell me the proportioning valve is defective.
Cool! I'm about an hour away from Gettysburg.
Elder User
Joined: Feb 2004
Posts: 909
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From: San Luis Obispo, CA
I think you will find that your stop light switch is up under the dash and works mechancially off the pedal motion - not likely to be a related problem, but probably easy to fix. (If you mean the brake light in the dash, it tells you that one side of the dual system has failed, which would most likely reduce your braking capacity. All the same you should find out why it is not working. I can help you diagnose this system, but I doubt it is related.)
I agree with greenego and wish I'd mentioned it. If your rears are the problem during the gravel test, then I'd pull the drums. (The gravel test can be done on sand or dirt, or even on hard pavement, but find a place where you will not get into any trouble. Be sure to do it at least for each side.)
You can change out your rear shoes for re-lined ones pretty cheap. NAPA has two levels - the premium are much more expensive and occasionally not the best choice. Maybe somebody will have a tip on what shoes are least agressive.
P.S.: Any more MD folks want to join this thread? (Maybe we can form a mid-MD subchapter of the MD chapter.)
I agree with greenego and wish I'd mentioned it. If your rears are the problem during the gravel test, then I'd pull the drums. (The gravel test can be done on sand or dirt, or even on hard pavement, but find a place where you will not get into any trouble. Be sure to do it at least for each side.)
You can change out your rear shoes for re-lined ones pretty cheap. NAPA has two levels - the premium are much more expensive and occasionally not the best choice. Maybe somebody will have a tip on what shoes are least agressive.
P.S.: Any more MD folks want to join this thread? (Maybe we can form a mid-MD subchapter of the MD chapter.)
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Postmaster
Joined: Jul 2001
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From: MD
So I really need to do the gravel test...sounds like it should pinpoint the problem considerably.
I haven't pulled the rear or front tires. I'll do that and inspect the rear drums. Thanks.
The rear has a Dana 60 axle. Are the studs reverse-thread or normal?
I meant the brake light in the dash. I'm going to fix it while pulling the cluster to R&R and clean (some lights are not illiumiating the panel).
I ought to be able to work on it this weekend. I hope it will get warmer by then. It's quite cold now.
Archie, isn't there the MD Chapter in the Chapter section?
I haven't pulled the rear or front tires. I'll do that and inspect the rear drums. Thanks.
The rear has a Dana 60 axle. Are the studs reverse-thread or normal?
I meant the brake light in the dash. I'm going to fix it while pulling the cluster to R&R and clean (some lights are not illiumiating the panel).
I ought to be able to work on it this weekend. I hope it will get warmer by then. It's quite cold now.
Archie, isn't there the MD Chapter in the Chapter section?
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Joined: Aug 2005
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From: NE Ga
If they're grabbing right off the bat when you just barely tap the pedal then you've got booster probs. Pull and plug the vacuum line that goes to the booster and see if you have the same results. If it still seems sensative but you have to press a little harder then it could be a proportioning valve prob but pull the vacuum line and go from there to eliminate something. Let us know the results.
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Joined: Feb 2004
Posts: 909
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From: San Luis Obispo, CA
I think Ford Dana 60 lug's are all RH threads - my Dana 70 is. (Somebody correct me if I am wrong about 60's.)
You may not be able to pull your rear drums without pulling your axles & taking your hubs apart. Dana's in early Fords were assembled with the brake drum INSIDE the mounting flange. The lug bolts are pressed through from the inside and hold everything together until they are pressed out. (The later Ford Dana's had the studs pressed in from the inside of the hub flange and the drums are removable the way you are describing - just pull the wheels and then the drums, although rust can still make this harder than it should be.) I am not sure when they changed over. Just pull one wheel and take a look. (One more reason to trouble-shoot first.)
P.S.: I am a member of the MD chapter and we just had our first official "meet" in March. All the guys there are from eastern MD, but there is no reason we mid-MD'ers could not get together in one way or another. I am retired, so my schedule is wide open.
You may not be able to pull your rear drums without pulling your axles & taking your hubs apart. Dana's in early Fords were assembled with the brake drum INSIDE the mounting flange. The lug bolts are pressed through from the inside and hold everything together until they are pressed out. (The later Ford Dana's had the studs pressed in from the inside of the hub flange and the drums are removable the way you are describing - just pull the wheels and then the drums, although rust can still make this harder than it should be.) I am not sure when they changed over. Just pull one wheel and take a look. (One more reason to trouble-shoot first.)
P.S.: I am a member of the MD chapter and we just had our first official "meet" in March. All the guys there are from eastern MD, but there is no reason we mid-MD'ers could not get together in one way or another. I am retired, so my schedule is wide open.
Elder User
Joined: Feb 2004
Posts: 909
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From: San Luis Obispo, CA
I agree with 73F100*8Mile's suggestion. Do it before the "gravel" test and be sure to do it in a safe place. Take something to plug the vacuum line during the test so your engine will still idle down properly.
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Postmaster
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From: MD
Will do, 73F100. That's a great tip, thanks.
Archie, it looks like it's an early Dana - I looked at the axle shaft and the its flange - it looks like it must come out if I want to go any further. It also has the cone-looking drum.
Ah, that would be excellent. I saw the MD meet thread a while ago. Are there only a few MDers...?
Archie, it looks like it's an early Dana - I looked at the axle shaft and the its flange - it looks like it must come out if I want to go any further. It also has the cone-looking drum.
Ah, that would be excellent. I saw the MD meet thread a while ago. Are there only a few MDers...?
Elder User
Joined: Feb 2004
Posts: 909
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From: San Luis Obispo, CA
We only got five out to the meet but there are a few dozen on the list. Apparently five is the minimum for a first official meet. I would not mind car-pooling to a future meet over in the Baltimore area or having a "mini-meet" here in central MD if anyone is interested. Anyone interested PM me & I'll start a thread in the proper forum.
Quick, I need to say something about brakes before I am accused of getting off-topic: Grabbing brakes are still better than no brakes.
Quick, I need to say something about brakes before I am accused of getting off-topic: Grabbing brakes are still better than no brakes.
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