1996 Dodge Dakota 6 cyl Four Wheel Drive Automatic 170, 13 miles
with the truck turned off I have a perfect brake pedal but when I start it I loose it all I've changed the master cylinder, brake booster, front brake line's, back brake lines and a wheel cylinder and have bled and bled and bled I get straight fluid the only thing I can think of is the ABS control is bypassing but I dont want to spend almmost $200 on a hunch if you can help I'd appriciate it. Thank you
First of all, do you have 4-wheel ABS or Rear Wheel ABS? The RWAL system can be ignored as far as bleeding is concerned. You might need the DRB3 scanner to activate valves in the hydraulic control unit in the 4-wheel system. By not opening some valves, air pockets could be trapped.
A common cause of low brake pedal is rear shoes out of adjustment. Pumping the pedal, then holding it will cause the pedal to be high and solid. Each stroke grabs another bite of fluid before the previous bite has a chance to return from the wheel cylinders. Applying the parking brake will also result in a solid brake pedal if shoe misadjustmet is the problem.
Sloppy, misadjusted front wheel bearings will allow the brake rotors to wobble and push the pistons into the caliper housings. The brake pedal will go down further than normal because it has to move more fluid to push the pistons back out. This will only cause a problem when the vehicle is in motion.
If you saw any rubber parts that were mushy and grown larger than normal, the fluid is contaminated with petroleum product. This can be as innocent as using a funnel for engine oil or transmission fluid, wiping it out, then using it for brake fluid. Residue remains in the funnel, so always use a dedicated funnel if needed for brake fluid. At any sign of contamination, you must replace ALL rubber parts and ALL parts containing rubber parts or seals, and you must flush and dry all steel lines. This must all be done at the same time. If you only replace a few parts at first, they will become contaminated through the fluid that gets contaminated from other older parts.
Your old booster will not cause a low pedal, but the new one could if it has an adjustable push rod that is misadjusted. There are different diameter wheel cylinders available depending on vehicle variations. A larger diameter wheel cylinder will require more fluid resulting in a low pedal. Every part installed adds another variable into the mix.
It is possible for the RWAL valve to cause a low pedal if the dump valve sticks open or doesn't seal properly, but this isn't common.