Rear brakes locking up before fronts

[skokie]

Truck is a 68 1/2 ton with drums all the way around. What's going on is that when you slam on the brakes (to avoid hitting some dufus who cut me off and is slamming on his brakes now) the rears lock up first and the the truck wants to slide off to one side or the other. Other than getting disc brakes on the front ( in the plan ) what can I do to fix this?

Thanks

[dave2953]

Does it do this all the time? Mine used to do this when there was a lot of moisture in the air. Could it be an adjustment issue?

[skokie]

Yep it does. The last time I adjusted them it did it was well. Could it be a wheel cylinder going out on me or something?

[jakeblues652]

i had a my truck do the same thing(but the front), and it seemed that the majority told me wheel cylinder.

[skokie]

I will lock it up again and see if I can find out which one is doing it and then swap the wheel cylinder out and go from there.

Thanks

[jakeblues652]

my WC hasent come in yet(the right one at least) but the way i look at it, is its 20 bucks. id rather pay 40 bucks for both and know that i'm going to stop, rather then having that thought in my head, because im stepping on a 40+ year old brake system.

[leddzepp]

Make sure your axle seals are not leaking. If the brake shoes are getting wet with diff. fluid, they will lock up all the time. If you find out this is the case, also check your vent tube and make sure it is not plugged. If the rear can't vent, this will also cause a leak.

[skokie]

Leddzepp

Whats the best way to check for the axel seals leaking?

Thanks

[leddzepp]

Pull the drums and see if your shoes are wet. If they are that means either your axle seal is leaking, or the brake cylinder is leaking. A real close look and you should be able to tell where it is coming from.

[skokie]

Will do and thanks.

[Longhorn Man]

does it only do this in a panic stop situation?
It's kinda normal.
If there's no moisture in the drum, then there's no point in getting new cylinders. They are obviously working. If anything, I'd start with adjusting the rear shoes up, and then if no improvement, I'd start looking at the front brakes and see if they are working to potential.
But, like I said before, it's kinda normal.

[MikeM]

proportioning valve maybe?

[skokie]

Originally Posted by Longhorn Man

does it only do this in a panic stop situation?
It's kinda normal.
If there's no moisture in the drum, then there's no point in getting new cylinders. They are obviously working. If anything, I'd start with adjusting the rear shoes up, and then if no improvement, I'd start looking at the front brakes and see if they are working to potential.
But, like I said before, it's kinda normal.

Nope it does it in all hard stop situations. I will pull the drums and see if I have any leaks and adjust all around to see what that gets me.

Thanks

[Jacfourteen]

From all the trucks I've had, it seems that the rear locks first because there is no weight back there. I also noticed that when you have a little weight in the back it seems to stop better. I think this is why the 94 f@#d ranger I used to have had ABS in the rear but not the front.

[skokie]

Originally Posted by Jacfourteen

From all the trucks I've had, it seems that the rear locks first because there is no weight back there. I also noticed that when you have a little weight in the back it seems to stop better. I think this is why the 94 f@#d ranger I used to have had ABS in the rear but not the front.

Good point. Maybe I will add a couple hundred pounds back there to see what happens.

[ChevLoRay]

My '69 did it, too. After doing the drum to disc conversion up front, it didn't do it any more before it went into body-shop hell.

Leaking axle seals are a possibility as mentioned previously. Seems a bit odd that grease could make a drum lock up, but it will. Of course, leaking wheel cylinders are another possibility, but if you aren't noticing that the level in your master cylinder is going down, it may not be the problem. Like you said you were going to do, pull the drums and check it all out. That's the only way to know for sure.

[djmachinist]

I had this problem on my 69 GMC after I rebuilt the entire braking system and swithed to front disc brakes. I slid off into a ditch when a person suddenly slamed on brakes in front of me. I hit the brakes hard and the rears locked up before the fronts, the truck swapped ends instantly. After some trail stops after this incident i confirmed the rear drums were locking up before the front Disc did. I thought the proportional valve that came with new front disc brake kit/master cylinder kit would take care of this. Well, it did not in my case.

The solution I used was installing an adjustable proportional valve in the rear brake line just below the original prop. valve. Then i adjusted the new valve where the front disc lock up just before the rear drums lock up.

Dale

[cheyenne shortstep]

Cracked ,old, and or glazed rear brake shoes will also cause rear wheel lockup. My 1 ton '79 GMC 4x4 van had the same problem [no lack of rear axle weight on a van] and a veteran brake & front end guy told me to check for glazing, sure enough. New shoes and a good clean and sand with 80 grit on the drums solved my problem.

[skokie]

Originally Posted by ChevLoRay

My '69 did it, too. After doing the drum to disc conversion up front, it didn't do it any more before it went into body-shop hell.

Leaking axle seals are a possibility as mentioned previously. Seems a bit odd that grease could make a drum lock up, but it will. Of course, leaking wheel cylinders are another possibility, but if you aren't noticing that the level in your master cylinder is going down, it may not be the problem. Like you said you were going to do, pull the drums and check it all out. That's the only way to know for sure.

I will check the master cylinder as well.

[SWEET7T]

One more truck here that locks the rear before the front. I'm voting this as normal operation. No issues with my brake condition; no glazing, no leaking, no air in lines. Just original design.

Think about it, you have pretty much the same setup on the front and rear. There is more weight on the front end because of the motor, and then you have weight shift when you hit the brakes (more weight on the front axle).

This is exactly why when ABS first came out you had ABS on the rear axle but not the front (owned a '90 and '95 Ford trucks with ABS on rear only, '92 GM truck with rear ABS). This was to 'help' keep the rear from locking during a panic stop.

[capev86]

i had that problem on my 72 burb c20. turned out it was a leaking axle seal. luckily i had bought new seals to install when i did the 4.10 gear swap.

[burkeens]

One more truck here that locks the rear before the front. I'm voting this as normal operation. No issues with my brake condition; no glazing, no leaking, no air in lines. Just original design.

Same here.

[badwrench3]

Check your e-brake cables if they are corroded to the point of binding the shoes will not return and cause the rear brakes to apply prematurely

[rob32472]

change to rear disks and the problem will be solved

[cheyenne shortstep]

Rear brakes locking up is not normal operation.