Take one of your "failed" master cylinders apart, look for defects. MAYBE you find something "odd" on one or more of the seals. Or damage in the bore.
Sample master cylinder--yours won't have the bigass rear piston/seal. Should be the same size as the other seals.
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Pretty much all "new" master cylinders come from Communist China. Very likely that all master cylinder rebuild kits are sourced from the same folks as make the "new" master cylinders. Shameful!
As is so often the case, "intermittent" problems tend to be the hardest to fix. And your brake pedal only drops "sometimes".
I have a hard time believing that the hydroboost is to blame. Even if it was over-boosting the master cylinder, I'd expect brake hose failure before master cylinder seal failure.
I think this is in the ABS system
--there's various valves that block or allow fluid flow, and accumulator(s) that store pressure. A failure in the valve/accumulator/pump system could cause a loss of brake pedal height. I find it curious that this tends to happen at the same time that the wheel speed sensors are generating minimum signal voltage/frequency.
'Course, that brings us back to having a scan tool that can communicate with the ABS.
As soon as I get time I intend stripping the original MC.
Y'all please pardon me for asking what may be a dumb question, but do the 400 trucks still have a brake proportioning valve? Haven't been under my Burb in a while, so I don't remember seeing one like my square body trucks had.
Although none of them were hydro boost. One thing to remember about the OP's Burb is that it's low mileage, so it's been parked a high percentage of its life....mostly in cold damp climate. That would be rough on brake systems, evident by he's had to replace the brake hard lines already.
Pinger right. The proportioning valve is within the abs unit.
Now as far as looking for issues I would look at the pins. Mine actually leak a little when I brake hard or mess around. But there not constantly leaking. All I know is if you trap air your almost sol and you'll have to have the dealer work on it. Depending how you bled the brakes it's very easy to trap air. Knock on wood I'm gonna test my brakes once the fluid gets here and I can flush the brakes 1 last time. Before making sure there working normal.
Now since you've changed your mc multiple times I gotta ask how did you bleed the brakes from mc to abs module?