PeachParts Mercedes-Benz Forum - View Single Post - Getting back the missing vacuum 83 300SD

rhodes2010 · #1 11-09-2013, 07:39 PM

I still had low vacuum after replacing the one-way metal vacuum valve
screwed in to the vacuum pump on the 83 300SD.

I went after the debris from the previous one way valve that was open
straight through, that I thought must be there. (there was no debris
ever found - do not think there ever was any parts in the connector).

Learned a little. Will try and explain it. Did not bring pictures for exhibits.
But may add some later.

I fished for the parts through the opening where the one-way valve
is removed from the pump. This produced nothing.
So on to removing the pump front cover.

The vacuum pump front cover can be a real mess to get off for most people
because the screws are often rotted / seized and they have standard or slotted
heads. (I like the idea of replacing them with allen heads.

Because my car has been a California dry desert car its whole life,
the screws were able to come out without damage.

So here is where it gets good and could answer a lot about where the missing
vacuum goes.

There are three valves behind the vacuum pump cover, they appear identical to me.

They are positioned like this:

1

2 3

1 is the valve that pulls vacuum and leads to the brake booster.

2 and 3 are the exit valves that expel air on the up stroke and need to hold
closed on the stroke that creates vacuum.

In my case valve 2 was disintegrated and not doing its job.

In my opinion 2 and 3 must work for vacuum to work.
My next junk yard run will be me taking these out of the vacuum pumps as spares.

I removed the debris and small spring from the broken valve. Using the
remaining frame and put in position of valve 1.
Valve 1 was good so moved it to 2.

Keep in mind and watch carefully as valve 1 faces one direction poking
out of the pump and 2 and 3 are the opposite way and flush with the pump.
(sorry for no pictures).

So at this point I have valves 2 and 3 intact and working, valve 1 not working.
But because the one way screw in valve is there, and works, this makes up
for valve one having no guts.

After I put the front cover back on the pump (quite a challenge when
on the engine and vertical instead of on a bench laying flat).

All vacuum returned and brakes work and car shuts off.

I was almost ready to purchase a new vacuum pump.
Now I might just get a repair kit instead.
For sure I will start collecting these little valves from the junkyard.

Hope this post helps someone else who is missing vacuum.