I have had an annoying issue with brake bleeding. The left front and right rear brakes would not work whatsoever. Bleeding just produced more air, under pressure.

I am an engineer with a system (not automobile) that deals highly with fluidics and pressures. I replaced a lot of items regarding my brakes with no resolution whatsoever. The problem presented as such (this is VERY common, and no COMPREHENSIVE answer has been posted anywhere I have looked). This pertains to all ABS systems, so distribute freely.

Soft pedal (compressing air). Pedal drops 3/4 of the way before brakes grab, then only one brake hits causing ABS to kick in (as it is supposed to). This problem will present with ONLY a soft pedal also, depending on severity of the valve contributing to the problem. There will be no fluid leaks!

The problems does NOT give an ABS warning, nor brake light warning indicator, for under the self diagnostic test, it functions fine (equal air proportions within lines typically (equals same degree of compressability and a uniform pressure))

Trying to bleed out the air, just worsens the problem. Here is why.

The valve body of the ABS pump system contains solenoid valves which may not remain fully closed when exposed to a negative pressure. This happens when the system is opened (crack bleeder valve) and fluid is passed near (not through) the valve (ABS input valve). The negative pressure created pulls down the diaghram and introduces air into the line (creates a negative pressure on the pressure relief valve also). When the bleeder is closed, the negative pressure is removed and the valve is allowed to re-seal. When the brakes are actuated and pressure is built, the valve holds up to the positive pressure and allows the air to be compressed.

The valve in question needs replaced, for most this will require replacing the ABS pump assembly. I pulled one from a neighborhood scrap yard for my 99 Caravan for 38.00! As far as bleeding the system........

The fluidics diagram of the ABS pump determines the order in which the calapers should be bled. Many of the ABS fluid routes within the valve block are not too picky on which are bled first, but as a general rule, do one compartment of the master cylinder at a time. This means, on a diagonal split, do the furthest then closest to the pump, then do the secondary master cylinder compartment.

The majority of these valves are passive (no input required to actuate), hence the "scan tool" that many dealerships state are required are really not required (except on the actual pump side of some valve blocks (prior to the ABs input valves (isolated from the normal braking system))).

Depending on which valve behaves in this manor, it can display as air in the line, a locked cylinder, and so forth. This example is just one of those mysterious puzzles solved.

 

Putting the ABS system through a bleed procedure with a scanner eliminates the majority of air bubble problems. However, negative brake bleeding (vacuum bleeding) is not the only procedure out there. If you use a Phoenix brake bleeding tool, you can force fluid up into the ABS unit and bleed the bubbles out through the top.

Replacing the ABS unit was pretty extreme. I don't think you used up all your other options first.

 

It is almost impossible to displace every bubble regardless of method used. The residual air left should not impeed brake function to any degree you can feel or measure. I could have replaced the valve itself, once finding the manufacturer (Kloen makes these types of valves, and any of these valve manufaturers tolerances leave room for improvement). In any regard, the repair was not extreme. Even with the most effective bleeding proceedure used, the venturi effect causing this valve to open slightly by passive operation would effect the brakes functions. Replacing the valve would have been much more costly than the 60.00 (including warranty) I spent on a unit pulled from another Caravan.