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Your problem is what we call a "dump valve". It's an ABS release valve and it is bypassing. In the illustration below, it's located under the truck and is called an Electro hydraulic brake proportioning valve.
There are only 2 place for fluid to "bypass" without hitting the ground and they are the master cyl and that valve. That valve does vent to the master. That's what it is put there to do, assuming you found the right valve. It has to look like the one in the picture. If I have answered your question sufficiently, please click on the accept button for my answer.
The "dump valve" is an ABS release valve. When the ABS energizes and releases the rear brakes, that where it releases the pressure.
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