280 vacuum pump diaphram replacement

My 1975 (114) 280 has gone though 2 replacement diaphrams in 11000 miles. When I got the car the diaphram was leaking causing hugh clouds of smoke when brakes were used and decelerating. Replaced it the first time, smoking problem resolved but only lasted about 7000 miles. Replaced again and worked for 4000 more miles, now puffing smoke again. Getting ready to put in the third one but wonder if I'm installing them correctly. All the damaged ones had similar size rips in the rubber (about 1 inch long) along the smaller diameter washer but in different orientations. I have been installing the diaphram with the "bump" in the rubber toward the cover plate, smaller washer on top (toward cover plate) and larger washer behind diaphram on actuator. Is this correct and is there anything else I should be looking for?

[stevebfl]

There is a proper amount of throw or stroke on the things. I can't tell you how to do it but if they are tearing that quick your stroke is probably too great.

I would invest in the closing plate and vacuum hose from a 76 model where MB decided that the engine on these cars had plenty of vacuum for the brakes. The 76 had a plate covering the head and a line straight from the manifold to the booster. Just pull the pump and chuck it.

[ctaylor738]

When my '73 280 started laying down a smoke screen, I did the exact thing that Steve B recommended and ran the booster right off the manifold until I could get the parts to fix the pump. I noticed no change in the brakes or performance. In fact the car ran a little better, which I attributed to the pump having a minor leak for a long time.

It sounds like you are doing the replacement diaphagm correctly. There are two kits to a rebuild, one with the diaphagm and one with the valves - you might try installing both if you do it again.

Partsshop has both.