Have you checked the hydro-vac? I've serviced many a C500-80 with that complex brake set-up with the hydro-vac. I've had that booster do all kinds of weird things from not releasing the brakes to intermittently not working at all.
I don't think silicone brake fluid will make a difference unless you find the brake fluid boiling as this is the only advantage it has- ( Higher boiling point).
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