Couple questions that may or may not be helpful, but figure I’d ask.
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What kind of hoses does Rich use? The front calipers are fed by a longer hose vs. the rear’s two shorter hoses, so maybe that makes it more susceptible to bulging? (Edit: Seems unlikely the more I think about it, but maybe?) If you’re seeing the front pressure flat-line despite adding pressure, I wonder if there would be a visible indication in the hoses. Is it worth measuring the OD of the hose with a caliper with no pedal/first moment it hits 700psi/subsequent pedal pressure?
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Have you tried replacing the ABS control box? I imagine a failure there could result in the valves in the pump not opening up predictably – and hence why maybe your 4 ABS units appear to not work right. Or is there some kind power sequence that needs to happen to have the unit “open” the valves? I can only find dotted line diagrams of the ABS control unit (meaning only partial wiring is being shown), and I can’t remember if there are any wires that run to the ECU. Curious if you’re powering on the car between your tests to have the control box inform the ABS pump to open up.
Som