TonyB is right.

AND you have to replace EVERY rubber part in the system...First.

AND you have to "FLUSH" the system with Brake Cleaner...First.

BUT I would not attempt to "Flush" an ABS unit with Brake Cleaner , So I
think you're out of luck as far as DOT 5 is concerned.

DOT3 OR DOT4 are both fine...

"Mushy" pedal is air.

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'84 300SD sold
124.128