Is this a new problem or has it been happening for a long time.
Some things to look at are:
1. Did someone recently replace the front brake pads or rear shoes? Manufacturers spend a lot of time and money designing a balanced brake system. If you install cheaper front pads, they could have a higher coefficient of friction, meaning they grab harder than the originals did. This was a characteristic of asbestos linings which are no longer available in this country.
2. Most higher quality trucks and minivans use a height-sensing proportioning valve near the left rear wheel or back axle. Proportioning valves limit rear hydraulic fluid pressure under hard braking to prevent rear wheel lockup and subsequent loss of control. These valves work fine for passenger cars, but not for vehicles where there can be such a wide difference in the amount of weight in the rear. If the valve was designed for no weight in the rear, the back brakes would be very ineffective when the vehicle was heavily-loaded. If the valve was designed to provide maximum braking strength when fully loaded, the rear brakes would lock up and skid when there was no weight in the rear. The answer is the height-sensing proportioning valve. Look for a mechanical link between the valve and the axle or leaf spring that has become disconnected or rusted apart.
3. Pull the rear brake shoes away from the backing plates and look for grooves worn in the plates. The sliding surfaces on the frames of the shoes can catch in these grooves and prevent the shoes from applying or retracting. Professionals will apply a lubricant to these six points on each side when replacing the shoes. Are either of the self-adjusting cables broken or missing? Shoes that don't adjust have to move too far before they apply. This will cause the brake pedal to go too far to the floor.
4. Did someone replace the rear wheel cylinders? Dakotas have either 9" or 10" diameter drums and they use different diameter wheel cylinders. If someone put on a wheel cylinder that is smaller in diameter it will apply less pressure to the shoes.
5. Check the rear shoes for proper installation. Many do-it-yourselfers think the shoe with the longer lining goes toward the front because you need the most braking when going forward. This is wrong. This is a duo-servo system identified by the movable bottoms of the shoes. The shortest brake lining goes toward the front. When the brakes are applied, the smaller front shoe grabs the drum and tries to rotate with it. It pushes the bottom of the rear shoe into the drum through the adjuster link. The wheel cylinder pushes the top of the rear shoe into the drum at the same time. If the two shoes are turned around, the shorter lining on the rear won't apply the proper amount of friction. In short, the rear shoe does the stopping when going forward; the front shoe does the stopping when going backward. Also check that someone didn't put both short linings on one side and both long linings on the other side.
6. Did someone replace a rusted steel brake line to the front brakes? If so, they may have inadvertently bypassed the combination valve located on the frame rail just under the master cylinder. It contains a metering valve that delays the application of the front brakes until the rear shoes have had time to expand out to the drums.
7. Check for brake fluid contamination by removing one of the two caps on the master cylinder. If the rubber seal under the cap blows up, is soft and mushy, and you can't get it to go back in, the fluid is contaminated with a petroleum product. Power steering fluid is often the offending product, but engine oil and transmission fluid will do the same thing. Petroleum products cause rubber parts to swell and expand. A variety of problems can result. Sometimes an unscrupulous seller will put some transmission fluid in the master cylinder to swell the cup seals to hide a problem. Internal leakage of the cup seals will cause the brake pedal to sink slowly to the floor when you hold steady pressure on it. Transmission fluid will expand the seals and make the brakes work real nice, ... For a few days! Then you got junk. The only proper fix is to flush and dry all steel lines, and replace ALL rubber parts throughout the entire system. That means all rubber hoses, both front calipers, both rear wheel cylinders, the master cylinder, the combination valve which has rubber o-rings, and the height-sensing proportioning valve which has rubber seals inside. If you have anti-lock brakes, the hydraulic unit must also be replaced. This is a very expensive repair.
There are two common ways petroleum products can be accidentally introduced into the brake fluid. The first is by performing a complete front brake job that includes repacking the front wheel bearings with grease. You wipe your hands on a rag, then with your fingers, you push the seal back into the cap of the master cylinder when filling it with brake fluid. As the front pads wear over time, the piston moves out of the caliper housing. This is the self-adjusting feature of disc brakes. When the piston moves out, brake fluid fills in behind it, so as the brakes wear, the fluid level in the master cylinder goes down. You should never have to add fluid to top it off. If you must add fluid, the disc pads are worn or there is a leak. Either one must be fixed. When the fluid leaves the master cylinder, the vacuum created pulls the rubber seals out of the caps. They are designed to do that to prevent a vacuum buildup while sealing out moisture in the air. When you push those seals back in with your fingers that have grease or oil residue, you just contaminated the brake fluid! You can do this too if there's oil on your fingers from doing an oil change.
The second, less common way to introduce contaminants is by using a funnel to fill brake fluid after it was used to add engine oil or transmission fluid. Some minivans have brake master cylinders that are impossible to fill, due to their buried location, without a funnel. Also, pressurized brake bleeder equipment used in professional shops is usually filled with a funnel. Simply wiping out the funnel after using it for petroleum products leaves a residue behind that will contaminate the brake fluid. Most shops use a dedicated funnel that is never used for anything other than brake fluid.
As far as the vehicle picking up speed, how fast does it idle in neutral? If higher than normal, check for vacuum leaks or a problem with the idle speed motor. The computer could be commanding a higher idle speed based on incorrect readings from the coolant and ambient air temperature sensors. If your truck has a carburetor, the idle speed could be misadjusted. If the engine is not coming back down to idle, it will try to drive the rear wheels in opposition to the brakes. If you can carefully shift into neutral while driving, apply the brakes and see if they work properly. If they do, you may not have a brake problem at all.
Friday, March 20th, 2009 AT 2:42 PM