One of the reservoirs MB uses camouflages the rear fluid level. It is actually surrounded by the front fluid and the rear chamber goes empty without being noticed. If you remove the cap and the black rubber fluid guide one can see the empty chamber.

If that isn't the issue, the other thing that happens is that bleeding procedures state to not fully depress the pedal when open. When the piston seal is driven into the muck at the bottom of the bore it can easily never seal again.

The issue is in the master cylinder not the booster.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician