Dandy. I think you're expecting to see something that isn't going to happen. It is very rare for both wheels to spin at the same time unless you have a locking back axle. "Sure Grip" was Chrysler's trade name, but most people go by "Positraction", which is GM's trade name. A single wheel spinning is the result of one side having slightly more resistance than the other. When both tires are on the ground, everything will be fine. If you block the wheel that spins, you'll see the other one starts to spin. That's what the differential does. That difference in resistance is due to one brake being adjusted slightly tighter, more drag on an axle seal, a tighter wheel bearing, etc.
To prove to yourself this is what's happening, you can stick something through the wheel that spins, or place a 2" x 4" under the tire, then slowly lift it up like a lever. Be real careful though. I've had tires catch the board and send it shooting out. That can be dangerous.
The different wheels rotating in different directions is surely a result of how the brakes are adjusted. Drum brakes are real effective even without a power booster because they self-apply. What I mean is when one shoe starts to contact the drum, it tries to rotate with it, and that applies the other shoe through the star wheel adjuster on the bottom. The first shoe sticks to the drum and tries to apply the other shoe harder and harder. That doesn't occur easily when there's just a little drag on them and the brakes aren't being applied, but it creates enough resistance that one wheel rotates one way and the other wheel rotates the other way.
Next, we gotta clarify the parking brake issue. There should be tension on the cables when it's released. Given the symptoms, it's real likely one or both rear cables are sluggish from rust and is not fully releasing. You need the tension of the brake shoe return springs to pull the parking brake pedal up. That will turn the switch and "Brake" light off. First, look at the rear cables where they come out of their outer casings. The front of those casings will be anchored to a bracket pretty near the front leaf spring mounts. If the first 1/2" of either cable, right where they exit the casing, is shiny, that cable is not fully retracted. Try flexing that casing to see if spring tension will pull the cable in. If it does, that cable should be replaced. Never try to lubricate a parking brake cable. That will never fix it, and with a customer's vehicle will almost always end up with the vehicle being towed back to the shop for a wheel that's locked up.
Next, with the drum removed, look at the two shoe frames where they contact the large anchor pin at the top. If either shoe is not against that pin, the parking brake cable is holding them partially-applied. Assuming you're able to get the drum on, that will cause real easy rear-wheel lockup under light braking. Even if this is okay, look at the parking brake strut bar between the middle of the two shoes. You should be able to push that bar toward the front of the truck, against the anti-rattle spring pressure, with your thumb, about 1/8". If the parking brake cable is stuck in the partially-applied position, the lever hanging from the rear shoe will be pushing that strut bar toward the front shoe, so it will be tight. Here again, the fix is to replace the sticking cable.
This all assumes a parking brake cable is indeed sticking. If this was a Ford product more than a year or two old, I could guarantee that is the problem because they have so much trouble with their designs. The clue that this is causing your problem is the sagged parking brake pedal. If it weren't for that, there's up to two other things that will turn on the red "Brake" warning light. One is a tripped pressure-differential valve. That valve moves when the front or rear half of the brake hydraulic system doesn't build the same pressure as the other half. That occurs when one system has a leak, which can be a rubber hose, a steel line, a leaking rear wheel cylinder, or internally leaking inside the master cylinder. On Chryslers and GMs, that valve is spring-loaded and should reset as soon as you release the brake pedal, but it's fairly common for them to stick. A good, hard jab on the brake pedal almost always gets them to release, then, once it's centered in its bore, the switch will turn off. To tell if this switch is causing the "Brake" light to turn on, follow the two steel brake lines from the master cylinder down to the brass "combination valve" on the frame rail. In the middle of that block you'll see a single wire plugged in. Unplug that wire. If the light goes off, that valve needs to reset. Try doing that on a Ford product where they aren't spring-loaded. That can lead to an afternoon of frustration!
The third switch, which not all vehicles have, is a low-fluid level switch on the reservoir for the master cylinder. If the fluid is low and it's not because you were bleeding the system or had a leak, don't fill it. The main reason for it to be low is worn front brake pads. As they wear, the pistons come out of the calipers to self-adjust. To install the new, thicker pads, you have to push the pistons back into the calipers, and that pushes all the fluid behind them back up into the reservoir. If you filled the reservoir previously, the fluid will spill over and make a mess. Brake fluid will eat paint too. If the fluid is low, add just enough for the light to turn off, then inspect the front brake pads.
Sunday, September 29th, 2019 AT 5:25 PM
(Merged)
