Bosch ABS2, which is what our 80s non ASR 'benses have, do not have pressure accumulators. Just a return pump and valves. ASR equipped W126s do.

On non ASR cars bleed the brakes as you would normally. There is no extra bleeding needed to clear air from the ABS system.

The problem here is probably the master cylinder, but it might be possible for the ABS system to cause a sinking pedal if it were broken somehow. Unlikely though.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.