928finn

My -78 (AT) has multiple problems which I think is related to each other on vacuum pressure on the lines (most of them on lines leading to the cabin). I have measured the vacuum from the line leading to the cabin (before and after the white vacuum tank left side on the drivers side footwell) with engine running and its holding only about 200mBar:
- Door central locking not working (has not enough vacuum to push/pull passenger side lock)
- Air conditioning control switch a bit strange, all positions not working properly
- Cruise control not working
- Brakes a bit spongy

May these several problems all be related to the same problem on the vacuum lines and pressure?

I already have tested the blue/black vacuum valve with my vacuum pump from the engine bay (left upper side near the brake booster) and that one is holding the vacuum like supposed.
Locks are working fine when I give manually 400mBar vacuum to them.

The lines on the engine bay (on the brake booster area) differs on my -78 from the posted pictures on the earlier threads here on Rennlist and are a bit difficult to reach as those lines are hiding quite deep behind the brake fluid tank.

Car is running and idling fine and seems like all other systems needing vacuum is working as supposed.

Any ideas where to search the problem? Lines, vacuum tank behind the left mudguard, or something else?
What part controls the correct amount of vacuum?
Can there be one leaking spot causing this all?

Mrmerlin

disconnect the hose to the brake booster and plug it see what happens
If you have an original booster the chances are good you need new booster and master cylinder

928finn

I removed the vacuum hose from the brake booster, plugged it and started the engine (I replaced the hose lines also from booster to manifold). Vacuum to the cabin is still about 250mbar, so a bit improvement but nothing major. Having leak somewhere else but quite mysterious this is. Planning to blow smoke to the vacuum system next to find the leaks or does anyone have straight solution for this?

Jadz928

You should have a 4-way splitter coming of the hose going to the brake booster. The splitter is fed from the that hose w/a check valve in between.
Of the 3 others on the splitter, 1 feeds the interior, 2 feeds a vacuum tank in the front-right wheel well, 3 is for testing the whole circuit.
Test for vacuum both circuits separately - interior and vacuum tank. Note if any leakdown and report back.
Note: also make sure the check valve is holding on It's own.

928finn

Hello,

Thanks Jadz928 for the tip. I made a short video from problems solved so far as well still existing ones.
Vacuum reserve tank was out from the system so I think that solved the brake booster and cruise control issue. HVAC issues still remaining, see the video.

So far:
- Door central locking not working (has not enough vacuum to push/pull passenger side lock) SOLVED
- Air conditioning control switch a bit strange, all positions not working properly NOT SOLVED, LEAKING
- Cruise control not working (high probability SOLVED, not tested though)
- Brakes a bit spongy (high probability SOLVED, not tested though).

It´s winter so due the snow I can not test drive yet to check the brakes and cruise control but plenty of vacuum was missing from the system due to reserve tank was out from the system so I think those are solved.

drooman

The whole roadmap is right here:

78-79 vacuum system