As others and Best have stated, the circuit is the root cause of the failure, but once you pull the switches you see another example of the absolutely horrid wiring materials used for insulation on the wires in the switch. I have a 79 and 88 Jeep that the wiring is in better shape than what I typically find underhood on a Disco. Whomever the supplier was for the harness I hope they are no longer in business. The internal faults are electrical wiring and circuit board components always, it is never the solenoids or the valves. The switches that are part of the circuit are moved by the activated solenoids, a feedback loop device. Option b corrects the wiring faults. This is a case where internet deductive reasoning is no match for boots on the ground parts in hand field experience.

 

It's Option B, not plan B. Plan B is a whoooole other sumthin' ...

 

dammit! i read your post and was like, "who would call it Plan B?"

then went back and checked my post. sure enough. oh well, guess i'll leave it as is for posterity's sake.

 

I did wonder about that …

 

Ok, so it sounds like replacing the shuttle valve solves the problem temporarily, not because there was anything wrong with the valve per se, but because with a new part the ground will function normally, at least for some time. And for that reason Option B is the long term and only genuine fix. Is that right?

 

Yes Option B fixes the "weak link" within the Wabco Unit.

 

No one is replacing the shuttle valves. The shuttle valve SWITCHES get replaced when they fail testing. Otherwise the bypass is all that is needed.