Be Very careful when pulling the plastic reservoir out of the Master, it will break easily and is really annoying to replace. it is also possible your master is leaking into the booster, you can pull the master forward to clear the booster with the lines intact, then shine a light in there to look for brake fluid inside. I have seen this type of leak on several different Mercedes of this vintage. water has a tendency to drain under the booster running between the firewall and inner fender and rust the booster from the bottom, I think its more of a W108/109 problem but none the less you may actually want to pull the booster and see how solid it is. its really just 4 bolts on the inside, and the pedal has a fat bolt with a shoulder on it, very easy to pull on an automatic car.

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R107.043 Euro 350SL (parts)(crushed)
W116.024 280se (crushed)
W114 280 (m110)
W108.067 280se 4.5
W108.068 280seL 4.5 (crushed)
W111 220SEB coupe
W110 200D went to the crusher
W110 190D sold sold sold

1970 Rover P6B

Used to own(1950 buick,1969 lincoln MK3,4G63t colt,87 300ZX, 79 F100, 92 XJ40)