Did you bench bleed the master cylinder when you changed it?

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Prost!

Yep.

Some may not agree, but I've been doing the following to my 300-SEL for the last 20 years.

Fill master cyl reservoir and keep it full throughout this entire process. Open all 4 bleeders - hose attached - other end in a catch pan/bottle

Start engine(if ABS) then start pumping pedal. You do not have to pump the pedal far - shorter strokes are better. Pump several times. This gets the fluid flowing. Continue keeping an eye on fluid level in master cyl reservoir - keep it full of fluid. Let as much flow out each bleeder as you like.

If any bleeder is not flowing - something in that circuit is clogged - bleeder/caliper/hose.

Once done - tighten all 4 bleeders.

You may need to either pedal bleed - vacuum bleed or pressure bleed all 4 afterwards, but in my case, the gravity bleed method I just outlined has never failed me when bleeding/flushing my 300-SEL. Nice firm pedal afterwards.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"