No fluid from proportioning valve to rear brakes

  • 1 POST
  • 2001 ISUZU RODEO
  • 3.2L
  • 6 CYL
  • 2WD
  • 110,654 MILES

Oil was in brake fluid for quite some time. Replaced master cylinder, front hoses, front calipers, rear hose, rear wheel cylinders. Front brakes work fine. Abs works fine. No codes. No fluid going to rear wheel cylinders. Seems to me problem is the proportioning valve which would need to be replace. I've just never ran into this before. I'm an assistant with 1 year exp. Can you please help.

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Wednesday, January 20th, 2016 AT 4:12 PM

1 Reply

  • 29,781 POSTS

You aren't going to want to hear this but you only did half of the only proper repair, and now you need to do all of your work over. I hate reading about brake fluid contaminated with a petroleum product because the repairs usually cost much more than the car is worth.

The only way to solve this is to remove every part that contains a rubber part, flush and dry all the steel lines, then install all new rubber parts. That includes the ABS hydraulic controller, master cylinder with reservoir and rubber bladder seal under the cap, all rubber flex hoses, and the calipers and wheel cylinders. If any part is not replaced, in this case the ABS controller with its rubber seals and o-rings, the contamination will leach out of them when you add the new brake fluid. At this point all of your new parts will have been contaminated again.

Few mechanics are experts with ABS hydraulic controllers because we typically don't repair them, so I can only share what I've learned in classes I've taken. There are rubber o-rings and seals, and if your system uses an accumulator, that may have a rubber bladder inside too.

For future reference, when you're doing brake work that involves brake fluid, most professionals even wash their hands with soap and water before reseating the rubber bladder seal in the reservoir cap so they don't get fingerprint grease in there. Never use penetrating oil to loosen rusted steel parts unless they are going to be replaced.

There's two things to consider for the no-brake-fluid-to-the-rear problem. Had you not replaced the master cylinder already, surely the lip seals in it would have grown past the fluid return ports and blocked them. For the front disc brakes that means the brake fluid is trapped, it expands when it gets hot from braking, and since it can't return to the reservoir, it applies those brakes even harder and they lock up.

For rear drum brakes the shoes have to move quite a bit before any heat is generated, and even then that heat doesn't transfer into the brake fluid very well. If the lip seal in the master cylinder has grown past the port, no more fluid can enter the system. Each time you press the brake pedal you just move a little fluid back and forth. No more fluid can leave the master cylinder.

The next possibility is a seal or o-ring in the ABS hydraulic controller has grown past a port and is blocking fluid flow. The way you'd have to determine that is by loosening or removing steel lines at various places to see where you can pump fluid out and where it won't come out.

Be aware too that with any master cylinder more than about a year old, you should never ever push the brake pedal more than half way to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pushing the pedal over half way runs the lip seals over that crud and can rip them. That results in a pedal that sinks easily to the floor or sinks slowly to the floor as you hold steady pressure on it. Often that doesn't show up for two or three days. The clue to this is you will not be losing brake fluid.

That problem occurs most often when do-it-yourselfers replace brake parts, then use a helper to pedal-bleed the system and the helper pushes the pedal all the way to the floor. Most mechanics never use a helper. Simple gravity-bleeding is much safer.

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Wednesday, January 20th, 2016 AT 4:49 PM

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