I've had this experience too on my '80 Volare, ever since it was new. Only happens at home on my granite driveway, never when out-of-town attending classes and staying in motels with asphalt parking lots. Chrysler instructor explained it was due to humidity and a light surface of rust forming on the rear drums overnight. That rust is ground off with the first application of the brakes. Since then I've proven this. If I apply the brakes lightly once or twice before I get to the end of the driveway, that easy lock-up doesn't occur. The other clue is it only happens once. If this was due to mechanical problems, it would happen more often, at various other times, or all the time.
Since then, this has also happened to my '88 Grand Caravan and '94 Grand Voyager, both with rear drum brakes. Lock up first time of the day, then never again until the next day.
There also was a real common problem with easy rear-wheel lock-up on the minivans when it's raining. This can apply to Durangos too when they have a rear height-sensing proportioning valve. Trucks and minivans usually use them because there can be such a wide variety of loading, meaning with or without rear passengers, groceries, etc. Manufacturers go to great lengths developing brake systems that are balanced front-to-rear. For the proportioning valve, one size doesn't fit all loading applications, hence the adjustable valve near the left rear wheel. On trucks that valve is usually near the rear axle's center section and is attached to it with a linkage. On minivans, the valve is right in front of the left leaf spring's front attaching point, and the linkage is connected to the leaf spring. You'll see four steel brake lines attached to it.
What happens is the springs sag with age. That makes it appear to the proportioning valve as though there's a heavy load in the rear. The valve adjusts so more braking power, by percentage, goes to the rear brakes. In fact there isn't that heavy load in the rear, so the braking power is too strong. On wet roads the tires skid easier. That's why this easy rear lock-up problem occurs in wet weather.
The fix on the older minivans was real easy. Kneel by the front of the left rear tire, and reach under the front of the leaf spring with a 5/16" wrench. The linkage is in two pieces bolted together. Loosen that bolt and let spring tension stretch that linkage about 1/8", then retighten the bolt. You'll see a shiny witness mark showing where the bolt was tightened before, so it's easy to tell how much you just extended the linkage. I did that on just about every minivan I did an alignment or brake job on at the dealership. The difference was easily noticeable right away. As I left the shop for the test-drive, the parking lot was sloped down-hill. On a wet day, the rear brakes locked with the lightest brake pedal pressure. I did the adjustment in the middle of the parking lot, then there was no more easy lock-up after that.
Trucks seemed to have a lot less trouble with this, but I'd still look for that proportioning valve by the rear axle. There will be some means of adjusting that linkage. In this case it will need to be made a little shorter.
Saturday, July 17th, 2021 AT 5:54 PM
