Hi Scott. Welcome to the forum. Put this in your memory banks for next time. To retract the pistons into the calipers, use a large flat blade screwdriver to pry them in before you unbolt the calipers from their mounts. If that doesn't work, you can also use a c-clamp after removing the calipers from the mounting brackets but if you have to resort to that, the pistons are sticking due to a buildup of dirt or rust and they should be replaced to prevent future problems. You should never have to open the hydraulic system to get the pistons to go in.
There are four ways to bleed the air out of the brakes. I use the gravity method. Open the bleeder screws, and loosen one of the caps on the master cylinder reservoir. Leave the caps in place, just loose, to prevent a vacuum from building up due to the fluid flowing out. That vacuum will prevent the fluid from flowing freely. Brake fluid loves to suck humidity out of the air. That's why it's best to leave the covers in place, just loose. Moisture in the fluid lowers the boiling point and promotes corrosion of metal parts. The lower boiling point will cause a mushy pedal and brake fade when the brakes get hot.
When air bubbles stop appearing at the bleeder screws and fluid is slowly dribbling out, close that bleeder and wait for that to happen at the other one, then close it too. Irritate the brake pedal a little by hand to dislodge any air bubbles sticking to the sides of hoses and steel lines, then open each bleeder screw, one at a time, once more. You might see a couple of little air bubbles come out, then tighten the screws.
Another bleeding method is to use a " bleeder ball". This is a container of brake fluid that you pressurize with compressed air, and it gets attached to the reservoir through a hose. Fluid and air are forced throughout the hydraulic system. This method works well for flushing old moisture-laden fluid out.
Vacuum bleeding makes use of a hand or compressed air-powered vacuum pump attached to the bleeder screw. Hand pumps are used by some mechanics but they aren't a practical expense for a do-it-yourselfer. Bleeder balls are shop equipment and require a lot of setup time.
Most do-it-yourselfers use the pedal bleeding method but there are some tricks and precautions to be aware of. First of all, NEVER, NEVER, EVER push the pedal all the way to the floor. Only go half way or less. Crud and corrosion build up in the bottom halves of the two bores where the pistons don't normally travel. When pushing the pedal all the way to the floor, you run the seals over that crud and they can be torn. That will result in a slowly sinking pedal. The only fix is a new or rebuilt master cylinder.
Have your helper stroke the pedal a couple of times, then hold it half way down while you open one bleeder screw. Tighten the screw then holler to the helper so he doesn't release the pedal before the screw is closed, otherwise it will just suck air back in. Do that procedure multiple times until clear fluid is all that's coming out, then go to the next wheel.
Now that you have already lost all the fluid to the calipers, it will take a lot more fluid to get all the air out. Your van has a " split-diagonal" system which means one front brake and the opposite rear brake are on the same hydraulic circuit. Continued pumping of the pedal will start to send a little air to the rear brakes, but since the fluid to the rear isn't going anywhere, the air won't make it more than a few inches down the line. There should be no need to bleed the rear brakes. More on that later.
Once you have the air bled out, and even if you just replaced the pads, the pistons will have to be pumped out until the pads contact the rotors. If you forget to do that, you won't have any brakes and the pedal will go to the floor. You have to pump the pedal numerous times to work the pistons out before you will develop a good solid pedal, and by that time you will likely have hit something!
Here's a good time to include an important note about GM front-wheel-drive cars. So far everything I've described pertains to any car, but GM fwd cars have an unusual valve in the master cylinder. It trips to block fluid flow from two of the four ports when the pressures in the two hydraulic circuits are different. A popped rubber hose is a perfect example. The problem is this valve WILL trip when you pedal bleed the system if you press it more than half way to the floor. One front piston will always move out first and that system will start to build pressure before the piston on the other side reaches the rotor. That causes unequal pressures. When the valve trips, you will not get air or fluid to come out of one front wheel and the opposite rear wheel. You will not notice the loss of braking either until the pads on one side are worn out and grinding, and the pads on the other side look like new yet. The only way I've ever found to reset that valve is give a short burst of compressed air through the opened bleeder screw of one of the blocked wheels, then let it gravity-bleed.
That blocked port problem doesn't happen on Chrysler products, but if you have anti-lock brakes, you may not be able to bleed the hydraulic controller without Chrysler's DRB3 scanner. My '93 Dynasty uses the Bendix-10 system. That hydraulic system can be bled just like any brake system. For my '95 Grand Caravan, the hydraulic controller is a different design, and the scanner is needed to actuate some of the valves so they can let the fluid and air exit from some of the internal passages. If your van doesn't have anti-lock brakes, bleed the system like normal.
Just because you drained all of the fluid out of the calipers, bleeding is still a simple procedure if you never let the reservoir run dry. If it didn't, there will be no air in the upper part of the steel lines, and the ABS controller if you have one. To get back to my earlier comment about air going toward the rear brakes, if it indeed made it that far, you will have a slightly mushy pedal until that air comes out. While most customers won't accept the comment about " just drive it for a week", if you do that, the air will work its way out. When you press the pedal and hold it, such as when waiting at a stoplight, a little fluid will flow down the lines toward the rear brakes. As you sit there, the air bubbles will tend to float back up the lines. When you release the pedal, the fluid rushing back into the reservoir will wash the air bubbles back too. After a few days, the pedal will feel normal.
Another way to get any suspect trapped air bubbles out of the front lines is to pry the pistons back into the calipers. The fluid, and any air bubbles, will go back up into the reservoir. When you pump the pistons back out by stroking the pedal, (remember, no more than half way to the floor), watch that the reservoir doesn't run empty or you'll introduce air back into the system and have to start all over.
Wednesday, September 15th, 2010 AT 1:55 AM