Intermittent power brakes
 

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.

No to vacuum leak between master and booster. I'd be suspicious of the check valve in the supply line, a leaking booster or failing vacuum pump that makes insufficient volume.

I hope the booster is not leaking, it is freshly rebuilt. I will test the check valve. Is there any way to test the volume capacity of the vacuum pump? I have a hand operated vacuum pump. If I can find a way to attach it to the booster I will test the booster with it. Forgive my slow responses.

Remove the vacuum mainline where it attaches to the vacuum pump. Unscrew the fitting that is in the vacuum pump. That is a check valve. Look inside and see if the parts of the valve have fallen out of it or if it is full of sticky gunk.

If it is full of sticky gunk hose it out with WD-40 to clean it. I had that happen and the WD-40 fixed the issue. Do not use brake cleaner as the valve inside is plastic and brake cleaner makes some plastics sticky and soft.

If the check valve inside parts is gone you need a new check valve. Thos parts fall into a passage in the vacuum pump, but I have never read of them damaging anythng in the vacuum pump.