I worry about the brake reservoir level sensing switches, you know the two black buttons atop the reservoir. I saw fluid blow out of one of them once with pressure up 10 or 15 psi since then I have kept the pressure under 6 or 8 psi. There can be quite a head of air above the fluid in the Power booster, that is what is compressed.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!