Brake booster for 1968 200 D?
 

I'm new to the site and new to Mercedes, I've been a VW mechanic all of my life. I acquired this 200D with a bit of a brake problem. It seemed hydraulic at first, and I found a leaky wheel cylinder. Replacing it didn't help anything. I checked the pump on the engine; it has plenty of vacuum and the check valves seem to be working fine. Before I removed the brake booster, the problem was that it seemed to be using the booster assist for initial braking, but upon slowing down, it lost braking power, and pedal got stiff and much more pressure was needed to slow down/stop. I wanted to ask someone with experience if this does indeed seem like the brake booster. I have another one out of a 1963 6 cylinder gasoline, but it's stuck. I'm having difficulty locating one online. Does anyone know which models came with this particular unit? It's quite larger than others that I have seen, if need be I can post a pic of it. I believe it's a T51, but I don't have much more info on it. Glad to find this site and any info is appreciated! Thanks guys!

-Tim