The vacuum splits after coming from the booster supply line. The brown line is the supply for shut-off function. It goes to a vacuum switch on the steering lock. Once that switch is open the vacuum proceeds to the shut-off diaphram.
Montor vacuum in the brown line first while shutting off. Verify that the vacuum stays strong. If the source stays strong and the output line only has a small vacuum then either the switch or the coupling is leaking (based upon your statement that the shut-off diaphram worked).
Bosch Master, ASE Master, L1
33 years MB technician