Check also the flexible brake lines. If your '79 450 SL with 185K miles has never had them replaced, I would do that.

I read somewhere on this site someone was having similiar problems and after many repairs, it was internal deterioration of the hoses.

Keep us posted,

Haasman

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)