Hello to all. This is my first post. I have a 1982 204D with auto trans. I have had it about a year. I am learning a lot but have a looong way to go. A couple of months ago I noticed that the rear compartment of the master cylinder was always low on brake fluid. I checked under the car but found no leaks. I pulled the master cylinder off of the booster and looked inside and found it to be full of brake fluid. Obviously the master cylinder had failed. I sent the booster off for rebuid and ordered a new master cylinder. While I was waiting for the booster to come back I replaced all 4 rubber brake hoses and both rear rotors and calipers and pads. The front brakes look good. All is now re-assembled and I find I have a new problem. On the first brake application I have power brakes but that only last a couple seconds and then I have non power brakes. I checked the vacuum pump and find it gives about 20" of vacuum. All vacuum lines have been disconnected by a previous owner. Only the power brake booster and fuel shut off are connected to vacuum. As far as I can tell that is. Could I have a vacuum leak where the master cylinder bolts to the booster? I removed the hose to the fuel shut off and attached a vacuum gauge to the main vacuum line. At idle I have the aforementioned 20" of vacuum but on brake application the vacuum drops off to zero within a couple seconds. Releasing the brake pedal and the vacuum builds back to 20" in a few seconds. This is what makes me wonder if I have a leak between the master cylinder and booster. The rebuilder sandblasted the booster housing and applied some kind of clear coating on it. The area where the master cylinder sits on the booster has some roughness from rust pitting. There is an O ring on the master cylinder but I dont know if its enough to seal.

Any thought? Please forgive this long message.