Me wonders if the booster is working.

Run the engine for a minute or two without pressing the pedal. Shut it off then press the pedal several times. It should be relatively easy then as it looses vacuum with each press it should be harder and harder till it's darn near impossible to press. If it's never easy, the booster isn't working. Could be connected to the wrong port, the check valve could be missing or non functional, the hose could be leaking or the diaphragm could be torn. I'm not sure how to test for a torn diaphragm, a hand vacuum pump won't work well if at all due to the volume and applying pressure is fraught with danger as the thing is only designed for maybe 20" Hg of vacuum which is roughly 10 psi which is hard to regulate.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120