I'm having an issue with excessive brake pedal travel. ( '91 300SL 110K miles) So, I put on new pads front and rear. New rotors in front and new brake hoses in the front. When bleeding the lines by pumping the pedal, I get a firm pedal with short travel if the engine is not running ( No vacuum to booster ). But if I run the engine, the pedal is soft and drops to the floor with less than moderate pressure. All those new parts didn't help at all. Ordinarily, I'd suspect the master cyclinder, but with no vacuum, the pedal feel and travel seem normal. The car does stop, but not like it should. Anyone have any insights or suggestions? Why does the MC seem OK without vacuum? Thanks for any ideas. (Also posting this in the SL forum.)

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Resistance is Futile.

I have always experienced good results if i slowly flush/bleed the brakes with engine running. I place a 2x4 for preventing full pedal travel too or the piston seals may get damaged as they move into corroded zone

If you test with no vacuum assist you will never get a satisfactory result. I would suggest you bleed it again but place the catch jar/bottle above the bleed bolt level to prevent air being backed into the bleeder when the pedal is released. Parts stores sell a one man bleeder that has a magnet on the catch bottle to stick to the car for easy work.

Hopefully your problem will be solved otherwise a replacement MC would be required

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)