Are you sure that the caliper is sealed properly?

Unfortunately, bleeding the brakes by pumping the pedal, the age-old way of doing it, also makes the main piston in the master cylinder travel through parts of the bore that it may never have been before. The result of this is, or can be, a master cylinder that's rubbish.

The brake fluid is supposed to be changed every (2 years?) because it absorbs water (hydrostatic? That doesn't sound right I'll have to look it up), so if the PO did not do that, there could be corrosion in the bore of the M.C. which will hasten its demise (hmmmm, talk about a bore).

I would get a mighty-Vac, or make a pressure bleeder, to bleed what's left of the brakes to see if it will bleed properly and to see if the master cylinder has had it. Pressure bleeding is the only way to go, apart from saving a dodgy M.C., but its much faster and much less fussy.

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1991 300 SEL
218,000 Km

"Xsbank's rules of mechanical intervention: Always go for the easiest solution first; 90% of what ails a gasoline engine is ignition; After that its all a WAG."