1972RedNeck

When I first got my '93 W250 a couple months back, the ABS and Brake warning lights were on and the back tires liked to lock up really easy. After I adjusted and bled the brakes, the lights went out and the brakes worked perfect. Fast forward to last weekend, and I had to dynamite the brakes to keep from smacking an elk on the highway.

The truck stopped well and I didn't hit the elk, but afterwards, the ABS and Brake light came on. I eased on the brakes a little and the lights went out.

But ever since, whenever I use the brakes, the back tires lock up with very little effort. It's kinda fun when turning corners on the gravel roads, but it's a major nuisance everywhere else as well as being a danger.

I checked for codes by grounding the wire behind the glovebox but there aren't any (which is to be expected as the warning lights aren't on).

So why would the RWAL brakes not be working and what do I need to do to fix it?

diesel402004

So.... never had a prob with my 91 (maybe because it was much newer) but the 93 had been a constant issue with one side locking up and glazing. Tried all the mechanicals: shoes, springs, cylinder, drums, etc. Had bleed the brakes annually, but it always came back. Tried bleeding brakes again last week, but really spent time on RAWAL first, then RR, and LR and they've been working great ever since. My theory is that, somehow, a tiny bubble of air gets trapped in there and takes extra effort to get out.

twostep

This is how I remedied the RWAL:

mknittle

I did the same thing only a slightly different method. I hated that thing.

cougar

If you want to get rid of it, I'll take it. I need a spare to fix.

1972RedNeck

I did that to my last first gen but I would kinda like this one to work.

BigIron70

I would pull the drums most likely contaminated lining from seal leaking or wheel cylinder, brake dust also. Out of round drums will also cause problems.
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1972RedNeck

I just did all this last go around. My uncle (the previous owner) had just rebuilt the brakes including turning the drums.

Like I said, it all worked perfect until I had to slam on the brakes. Now the pedal feels a little softer and is much more "touchy".

BigIron70

Then I would read this;
COMBINATION VALVE OPERATION
METERING (HOLD-OFF) VALVE
The metering valve is used to balance brake action
between the front disc and rear drum brakes. The
valve meters (holds-off) full apply pressure to the
front disc brakes until the rear brakeshoes are in full
contact with the drums.
The valve is designed to maintain front brake fluid
pressure at 3-30 psi until the hold-off limit of 117 psi
is reached. At this point, the metering valve opens
completly permitting full fluid apply pressure to the
front disc brakes.
PRESSURE DIFFERENTIAL SWITCH AND
VALVE
The pressure differential switch is connected to the
brake warning light. The switch is triggered by
movement of the switch valve. The purpose of the
switch is to monitor fluid pressure in the separate
front/rear brake hydraulic circuits.
A decrease or loss of fluid pressure in either hydraulic
circuit will cause the switch valve to shuttle
forward or rearward in response to a pressure differential.
Movement of the switch valve will push the switch
plunger upward. This closes the switch internal contacts
completing the electrical circuit to the warning
light. The switch valve will remain in an actuated
position until repair restores system pressures to normal
levels.
COMBINATION VALVE TESTING
TESTING METERING VALVE
Metering valve operation can be checked visually
and with the aid of a helper.
Observe the metering valve stem while a helper
applies and releases the brakes. If the valve is operating
correctly, the stem will extend slightly when
the brakes are applied and retract when the brakes
are released.
If the valve is faulty, replace the entire combination
valve as an assembly.
TESTING PRESSURE DIFFERENTIAL SWITCH
(1) Have helper sit in drivers seat to observe brake
warning light and to operate brake pedal.
(2) Raise vehicle on hoist.
(3) Connect bleed hose to left or right rear wheel
cylinder. Then immerse hose end in glass jar partially
filled with brake fluid.
(4) Have helper press and hold brake pedal all the
way down and observe warning light.
(a) If warning light illuminates, switch is operating
correctly.
(b) If light fails to illuminate, check circuit fuse,
bulb and wiring. Repair as necessary and repeat
test steps (3) and (4).
(5) If warning light still fails to illuminate, check
brakelight and park brake switches (and wiring)
with test lamp. Repair or replace parts as necessary
and test differential pressure switch operation again.
(6) If warning light still does not illuminate,
switch is faulty. Replace combination valve, bleed
brakes and verify proper switch and valve operation.
COMBINATION VALVE REMOVAL/INSTALLATION
VALVE REMOVAL
(1) Raise vehicle on hoist.
(2) Mark or tag brake lines connected to valve for
assembly reference.
(3) Disconnect lines at valve (Fig. 8).
(4) Disconnect wires from differential pressure
switch.
(5) Remove bolts attaching valve to frame bracket
and remove valve.
VALVE INSTALLATION
(1) Mount new valve on bracket and tighten valve
and bracket screws/nuts securely.
(2) Connect brakelines to valve. Tighten fittings to
16 NIm (145 in. lbs.)
(3) Connect wires to pressure differential switch
terminal.
(4) Bleed brakes.
(5) Lower vehicle and verify proper brake operation.
BRAKE LINES AND HOSES
BRAKE LINE AND HOSE INSPECTION
Flexible rubber hose is used at both front brakes
and at the rear axle junction block. Inspect the hoses
whenever the brake system is serviced, at every engine
oil change, or whenever the vehicle is in for service.
Inspect the hoses for surface cracking, scuffing, or
worn spots. Replace any brake hose immediately if
the fabric casing of the hose is exposed by cracks or
abrasion.
Also check brake hose installation. Faulty installation
can result in kinked, twisted hoses, or contact
with the wheels and tires or other chassis components.
All of these conditions can lead to scuffing,
cracking and eventual failure.
The steel brake lines should be inspected periodically
for evidence of corrosion, twists, kinks, leaks, or
other damage. Heavily corroded lines will eventually
rust through causing leaks. In any case, corroded or
damaged brake lines should be replaced