1978 Mercedes 240D Brakes less responsive b/c of vacuum system?

diesel
brakes
manual-transmissions
cylinder
mercedes-benz
transmissions

#1

When the brake pedal is first depressed, it sounds like air is being pushed out of it, sort of like flatulence, and are less responsive. When i pump the brake, it fills with air and tightens. The brakes work when I depress the pedal all the way down.



Also, I have to pull my driver-side door shut a few times before it will close and lock with the vacuum system. Otherwise it won’t close. Eventually the door closes and I can lock it when I shut the door just right. (My mechanic and I are working together to find replacement locks from junk yards. Any suggestions?)



Is this the master cylinder? Or could this be due to a leak in the vacuum system since my door does not readily close? Or is it something else entirely?



Many thanks in advance for your responses!


#2

If the brakes work eventually, it sounds like a vacuum problem, you need to get that system checked out for leaks, etc.


#3

Check the vacuum booster. You should not hear a noise when you step on the brake.


#4

There are specific steps for checking for vacuum booster related problems listed in Haynes manuals. You don’t need tools and you don’t have to be trained.


#5

Normally, pumping the brakes would cause the vacuum to be depleted from the reservoir, not increase it. If the vacuum system was at fault, normally the brakes would be hard to push from the get-go. I think you may have a failing master cylinder or air in the brake system, and when you pump the brakes it compresses the air, allowing for somewhat normal operation.

You could test this I suppose by finding a stretch of road with no traffic or surprises, such as a long, straight country road. Then get up to 40 or 50 and turn the engine off. (note that you will also lose power steering when you do this) Shift to neutral to ensure that the engine is decoupled from the drive train and not turning and thus providing vacuum. (there is another post on this forum where people have varying results with the engine still turning, but we don’t want to go there right now)

Pump the brakes gently until all the vacuum is gone, but don’t pump them enough to slow the car significantly. Wait a while coasting, then try to stop the car with one push of the brakes. Compare this to pumping the brakes and then pushing them to slow the car. If you still have to pump the brakes to effectively use them, the vacuum system is not at fault.