Rebuilt the left rear caliper on a 380SEL last week and put in a new master cylinder. Found out the hard way that the rear brake portion of the brake reservoir is surprisingly small. We could get maybe three pumps (the old "up" / "down") on the bleeding of the rear brakes before it sucked air into the system. After that we made VERY sure that the reservoir was kept full during the bleeding, which resulted in a very nice hard pedal.

Redo the bleed, making very sure you keep the fluid level up!!

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered