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Rear brake lock-up/proportioning valve

Shran

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This has been an ongoing issue with my stepside truck. I have the standard D35 front discs, but an 8.8 rear from an Explorer with 10" drums. The issue is that the rear brakes will lock up every time under moderate braking. It is almost scary to drive in traffic, the idiots around here like to hammer their brakes while texting and I've put the truck sideways twice now.

I suspect this issue has something to do with the weight of the fiberglass bed but it's kinda hard to tell. For now I am totally ruling out an actual mechanical issue because (a) everything is new-ish and (b) the brakes NEVER did that when the axle was in my Explorer, but the Explorer weighs a good 1000+lbs more than this truck.


That is kinda what I'm looking at to dial back the rear brakes a little. What is the group consensus on that? The reviews suggest that it is not really a "proportioning" valve, it's more of a "flow control" valve but it should more or less do the same thing, right?
 


Uncle Gump

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Does it lock up both sides?

Does it do it under light and heavy braking also?

Have you tried adding some weight to test the theory of being "lighter" with the fiberglass box?

I still hold onto my theory of a tighter rear brake adjustment is better. The less those rear shoes have to move to self energize the less chance of them locking up.
 

Shran

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Yeah, both sides lock up. As far as I can tell anyway.

Light braking - feels a bit more "grabby" than I think it should but it's not bad.
Moderate braking - 50/50 chance of locking up, they ALWAYS do if I give the pedal a quick stab... nice even application of the pedal is usually OK and they don't lock up.
Heavy braking they will lock up every time

Haven't tried weight yet. I am honestly not sure how much weight difference there is. The fiberglass box in my mind feels heavier but I was also very careful moving around and paranoid of breaking it...whereas a steel box it's ZFG, much harder to damage steel. So it being lighter might be my imagination?

I have the rear brakes adjusted pretty tight I think. Usually my process is to tighten the adjuster until the wheel starts to get difficult to turn, then back it off a hair so that it spins but you can hear the shoes rub the drum slightly. Like I said they were fine when the axle was under the Explorer, the parking brake is working perfectly, nothing else really changed except it's under a different vehicle..
 

don4331

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The fiberglass box (and associated tailgate and bumper) is actually 10lbs heavier than the equivalent styleside box, per Ford! Those fiberglass fenders aren't light.
Slightly heavier still if you leave the styleside bumper on as it is wider, but there are aesthetic issues with that.​
A styleside box which has been lined will be heavier as there is considerably more area to coat.​

Did your Ranger originally have 9" or 10" rear brakes? Ford's proportioning valve puts far more pressure to the 9" rear brakes than it does to the 10" ones.

Do you still have RABS working? It should prevent lock up of the Expo axle...

The wife's Explorer Sport is >500lbs heavier on rear axle than my daughter's B4000 SC (both are 4.0 SOHC, Auto, 4x4). And it not just raw difference, it's percentage difference. Explorer is almost 50/50 balance, B4000 is 60/40.
Front axle weight is surprising close between Explorer and Ranger with same engine/transmission combination.​
And a V-8/4R70W add an additional 200 lbs to front and about 40 to rear.​

That "flow control" valve will will delay rear lockup, but it won't allow fluid to get there as quick, but it won't help if you're needing to seriously check speed as fluid/pressure will get there quick enough (unless you set to to point where rear brakes are doing nothing again).
 

Shran

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The fiberglass box (and associated tailgate and bumper) is actually 10lbs heavier than the equivalent styleside box, per Ford! Those fiberglass fenders aren't light.
Slightly heavier still if you leave the styleside bumper on as it is wider, but there are aesthetic issues with that.​
A styleside box which has been lined will be heavier as there is considerably more area to coat.​

Did your Ranger originally have 9" or 10" rear brakes? Ford's proportioning valve puts far more pressure to the 9" rear brakes than it does to the 10" ones.

Do you still have RABS working? It should prevent lock up of the Expo axle...

The wife's Explorer Sport is >500lbs heavier on rear axle than my daughter's B4000 SC (both are 4.0 SOHC, Auto, 4x4). And it not just raw difference, it's percentage difference. Explorer is almost 50/50 balance, B4000 is 60/40.
Front axle weight is surprising close between Explorer and Ranger with same engine/transmission combination.​
And a V-8/4R70W add an additional 200 lbs to front and about 40 to rear.​

That "flow control" valve will will delay rear lockup, but it won't allow fluid to get there as quick, but it won't help if you're needing to seriously check speed as fluid/pressure will get there quick enough (unless you set to to point where rear brakes are doing nothing again).
The fiberglass box is an aftermarket deal, not a Ford part. I wish I had weighed the truck with the old box on it just for curiosity's sake.

The truck originally had 9" brakes. I am using the original '88 master cylinder (IIRC) but I am certainly open to using a different one if that will help.

RABS has been deleted. The valve was missing from the frame I used and I didn't feel like tracking down a replacement.
 

don4331

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The fiberglass box is an aftermarket deal, not a Ford part. I wish I had weighed the truck with the old box on it just for curiosity's sake.

The truck originally had 9" brakes. I am using the original '88 master cylinder (IIRC) but I am certainly open to using a different one if that will help.

RABS has been deleted. The valve was missing from the frame I used and I didn't feel like tracking down a replacement.
All, I've got are '93+ boxes - but I could weigh the one on the driveaway for Monday.

Yes, Master cylinder/proportioning valve from Explorer or from Ranger with 10" rear brakes will help significantly.

Finding a good, functional RABS is getting difficult these days.
 

Shran

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All, I've got are '93+ boxes - but I could weigh the one on the driveaway for Monday.

Yes, Master cylinder/proportioning valve from Explorer or from Ranger with 10" rear brakes will help significantly.

Finding a good, functional RABS is getting difficult these days.
Blah... OK. I was hoping you wouldn't say that, the fittings changed sometime around '89ish so it's not quite a plug and play affair... I just love throwing new parts at it though so I guess this is my opportunity to make the master cylinder nice and shiny.
 

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