Rear drum brake brake lockup

[cstarbard]

Hi all,

I have a 1996 2.3 4x4. This truck has 4wheel ABS that has never given me an issue (no ABS light on, all fuses good, can feel ABS work) but recently it has been really easy to lock up the rear drum brakes. It is very erratic and does not take much pedal pressure to cause lockup. The brakes will unlock as soon as I get off the brake pedal, so they aren't sticking, they are just very easy to lock, which has never been the case before. The issue started in November and has become more frequent since then.

Here is what has been done to brakes this summer:

-Replaced ALL hard lines (yes, every single one, even ones between ABS unit and master)
-Replaced two front rubber brake hoses and rear splitter hose
-flushed brake fluid in the whole system several times
-New pads, rotors, calipers and wheel bearings up front
-Turned drums, new shoes, new hardware kits, and new wheel cylinders in rear

The only thing I didn't replace at this point is the master cylinder, should I suspect that it is the cause of the rear wheel lockup? I don't think the brake pedal feels soft or anything like that.

Other potential causes? I know there is no air in the system, I am not losing fluid, and I have re adjusted the drums several time to no avail.

Thanks

 

[rusty ol ranger]

The older trucks with rear only ABS used a proportining valve that would apply more pressure to the back brakes when loaded.

Yours with 4 wheel ABS im not sure if still used this or not, but if it does, it maybe stuck allowing to much rear brake bias.

 

[RonD]

Adjust the Parking brakes first

Hold out Parking brake release handle
Pump parking brake pedal until it stops coming up higher in pedal travel, you can't "over" adjust it

Parking brakes are self adjusting IF........... you use the Parking brake daily, or if you back up and apply the brakes daily
Backing up doesn't adjust them, backing up and braking to a stop does

If rear brake shoes are not adjusted this way then rear wheels will often lock up


1996 should have dual Master cylinder, two brake lines come out and run to the ABS controller(4WABS)
The two separate lines are the proportioning valve
Front line is for rear brakes, rear line is for front brakes
Rear gets 30% of braking force, front gets 70% in pickup trucks, cars and suvs use 60/40, 60 to front, 40 to rear

So yes it could be Master proportioning is at fault, very very last thing I would suspect though

There is a special way to bleed 4WABS to get air out of the valves, requires powering up the valves to pump in fluid and pump out the air

 

[cstarbard]

Thanks guys.

Ron, that is an interesting way to adjust the parking brake. I've never heard of that before. What I was always told was back up and stop (I try do that at least once a week, if not several times) and whenever I have the rear wheels off I spin the adjusters by hand until the drum is a little hard to put on, then back adjusters off a hair.

I'm not at the garage so I just tried the parking brake the way you described, for maybe 30 seconds straight or so, and I couldn't feel a a change in the parking brake pedal travel. What does this mean to you, if anything?

I did notice today that with the truck running in neutral, the brake pedal doesn't really get stiff no matter how hard you push it... you can always get a little more to it, though it does resist you. It is pretty subtle though if it's not just me being paranoid.

I will take the rear wheels off sometime this week and adjust the adjusters and see if that gets rid of the lockup. If it does not solve the lockup, should I then check for air one more time? And assuming there is no air, should I suspect the master?

Thanks as always,

Chris

 

[Ranger850]

Leaking rear cylinders? But said your not losing fluid???

 

[RonD]

Yes, pumping the parking brake pedal adjusts the Star Wheels same as doing it by hand, or backing up and stopping

If pedal didn't come up higher then star wheels are tight

Longshot.............there is a LEFT and RIGHT star wheel, if they are reversed then rear brakes shoe can never get tight.
If there is a gap between shoes and drum, when you apply the brakes the shoes start to rub drum and then rotate forward and GRAB locking up rear wheel(s)

Point of the star wheels it to prevent a gap, shoes should always be slightly "rubbing" the drum, no gap at all


Main Brake pedal should get tight and stop going down, its hydraulic so fluid will only compress so far and then front calipers have pads pressed tight against rotors and rear slave has shoes pressed tight against drums, so pedal can not go down any more

Air can be compressed more with more pressure on the pedal
Leaking fluid will, of course, allow pedal to go down more
Leaking seal around the piston in the Master, can allow pedal to go down more

 

[cstarbard]

Leaking rear cylinders? But said your not losing fluid???

Right, no leaks, and the wheel cylinders were new as of July 2018. If I hadn't replaced them that would have been my first suspicion. Not that they couldn't fail still but I am not losing fluid in the master and I don't see any on the backing plates, and haven't seen any leaks with the drums removed, but I'll be checking with drums off again this weekend.

Ron, as always thank you for your detailed response. I'm pretty certain I don't have air in the system, as I have bled it a number of times. But I am thinking I will check again. I know I did not put the adjusters on the wrong sides as the hard ware kit bags and adjusters themselves were labelled Left and Right

The slight squish of the pedal and what you are saying has me wondering if the master might be the issue... I will double check for air, adjusters, for leaks at wheel cylinders, etc this weekend and update this thread then.

 

[cstarbard]

RonD you saved the day yet again. I tried your method for adjusting the self adjusters again (holding parking brake release and pumping pedal) and don't you know, the lockup seems to be gone. Thanks for sharing that super useful tip. I'm surprised that they were that far out of adjustment since I do the back up and brake method often... I will be using your method instead from now on. Thanks as always

 

[RonD]

Good work

Thanks for the update