How do vacuum brakes work

Leave the archive and display this page in the standard design: Series IIa with compressed air brake ???

Dear Serians,

I've been working on my "new" Series IIa, 88, petrol, first registration in Switzerland 1970 for 3 days now.
Frame No. 318 07 599G
The car is said to have been in service with a municipality in Switzerland until 2000, then bought by a municipality employee, deregistered in 2004.

To this day, I was not very surprised that a compressed air tank is installed.
No abnormalities in the engine compartment apart from a pressure hose - without connections on both sides - therefore removed.
Mounting screws for pressure vessels cannot be loosened - Flex cuts the screws.

In my over-zeal, 2 steel pressure lines were cut off - only then I noticed that the right rear brake line has a T-connection to the pressure tank and an additional linkage for the handbrake leads towards the tank. A Graubremse compressed air regulator has already been removed.

At the rear of the rear cross member there is no compressed air connection for a trailer or the like.

Please help, I'm lost. Anyone have any idea about this stuff.
Yes, I know - pictures are in demand - come tomorrow or on weekends.
LG Gerhard

Great, Gerhard
You did it. My warmest congratulations and all the best to the new family member.
Just please don't neglect the FC now;)
.... and yes, I'm eagerly awaiting the pictures
lg Dirk

In Switzerland (in contrast to D) vacuum brakes were widespread. I also had a vacuum brake on my first series.

Look for it here, I think you'll find something there.

Hello Gerhard,

In Switzerland, vacuum air brakes were permitted on small trucks for braking trailers; this braking of the trailer enabled the trailer load to be increased to 1.7 times, sometimes more, the total weight of the towing vehicle. The negative pressure was generated by the negative pressure in the intake duct of the engine, just like the brake booster of the service brake. The vacuum brakes were activated by a hydraulic valve in the rear brake circuit of the towing vehicle.

If you want to shut down the system, then you have to blind the outlet to control the hydraulic valve (cut off the end and solder,! Provisional!) Or better, insert a straight piece instead of the T-piece in the brake line, then of course bleed the brakes. Likewise, the outlet must be separated from the negative pressure of the intake duct and sealed so that the engine does not suck in the wrong air and for which the brake booster, if available, can build up sufficient negative pressure. Then you can / should remove everything from this system. There's space under the car.
!!! The old air tank is not suitable as a compressed air tank !!!

That's how it was with my 109 Series III, but it should be the same with you.


Hi Gerhard,

Congratulations on the series!

Because of the air brake: mine had too. When I got the new one, I made a few assumptions about it and also took a few pictures:

I think it's now clear that mine doesn't come from the Swiss Post. But maybe you can tell whether yours also has a number like that on the side of the seat box, etc. - is that a license plate for municipal vehicles?



Hi Michael,
thanks for the information - that this should work with negative pressure is not yet clear to me.
Today I removed the boiler and the 2 regulators.
Separated from the normal hydraulic brake line at the T-piece - I still have to make a seal there.
I only removed a loose pressure hose in the engine compartment - I have not yet found out where it was previously connected.
I'll look again between Luffi and carburetor.
2 hoses lead to a pressure gauge with 2 pointers.
Photos will follow.
LG Gerhard

Hello Miles,

thank you for your contribution.
All parts are, as obviously with you, from the company Grau (Graubremse).
Image 1 - oT
Image 2 - T-branch from the brake line
Fig. 3 - Pressure vessel
Image 4 - As a rule, filled with brake fluid in the lower area, obviously Luftikus at the top

More pictures will follow.

LG Gerhard

Hello Miles,

to your question about the number on the seat box - I didn't find that on mine.
He has a few small marks on the steering column that a private individual would hardly be able to attach.
I also sent a request to the previous owner, who registered the car for the last time.
I wanted to send a few more photos but somehow it doesn't work anymore?


Hello Gerhard,

maybe someone from the previous owners upgraded the vacuum supply to standard and therefore can't find anything.

For the sake of interest: which measuring range does the measuring instrument have, the photo would be very nice, I am curious, because this was no longer available for me, would be an explanation for a drilling in my case.


On the picture you can see a hydraulically operated air brake valve and behind it a line filter in the air line.
The two pointers in the instrument show the supply and brake pressure.
Everything looks like an old single-line braking system.
However, the trailer brakes are said to have been operated with negative pressure in the Swiss. Perhaps it was generated in a similar way to a brake booster on the intake manifold.

Is there a blind plug or something like that somewhere?

Here's something else of interest on the subject:

Greetings Marc

Hi Michael,
no wonder, it's with me too;)

Hello Marc / Pernod

Unfortunately, I have problems uploading pictures - I have no idea why.
The picture No. 4 is definitely from my Landy - the same parts should be used as with miles.

The previous owner informed me at the WE that a compressed air brake was mandatory for trailers over 3.5 tons in Switzerland.
The Landy was registered with the municipality of Landquart - I suspect in the forest or building yard area. A rotating light was probably also installed - I have already dismantled parts with slip ring and cabling in the middle of the disk.
A request to the community is already pending.
The compressed air system has now been completely expanded - if the function is clear, it may be reinstalled.

The rear cross member has a round hole in the right area - the compressed air connection must have been there.

When removing the carburetor (Weber 34), I noticed that an aluminum spacer was inserted between the intake manifold.
There is a blind plug there, which would again speak in favor of the vacuum brake.
Hose and regulator from the engine compartment
Connection to the mechanical handbrake
Capped line to the pressure gauge
Manometer, above main switch

BG Gerhard


when I look at my boilers and compressors, I advocate compressed air, not negative pressure!
The Grau manometer also looks familiar to me.
With our bureaucracy, I dare to doubt whether this is permissible in Germany - unfortunately!
So: Remove everything and only mount the "normal" components of the liquid-operated brake.


Yes, the instrument looks a lot like pressure.
Mine is (was) clearly negative pressure, same company, same layout, measuring range up to -1atü.

Hello Gerhard,

The pressure gauge looks a lot like compressed air. Also, I had no connection to the handbrake, at least I didn't see one, just looked, found no evidence.

If this cannot be clearly identified from a technical point of view, the technology should be put in the archive; I advise against operating it.
But it is interesting to think outside the box a bit.


Hello screwdriver,

the components of the compressed air brake remain in any case temporarily stored for the TÜV / individual approval (currently on the loading area of ​​the FC).
Since I have not yet figured out the function, a test would be tempting - but not on the street.

Today, after 1 week in my possession, I made new discoveries - I come out as a greenhorn here.
The trolley actually has a heated windscreen LH u RH - now I'm not surprised that the battery was always empty so quickly and that there is a main switch.

The 4 rear seats have additional heating consisting of an electric fan behind the driver's seat. Is this via an air hose with a "heat exchanger"? connected. There is also a water expansion vessel and pipes that lead to nirvana.
I'll post pictures tomorrow - say more than words.
The professionals can certainly tell me what this thing can do and how it can be completed.

BG Gerhard

Hello Gerhard,
you apparently have the complete winter package. Take a look at the optional parts catalog, they are easy to see.

Hello Pascal

I've just downloaded the catalog and have already found some of the accessories.

Thank you, Gerhard

According to reports, there seem to have been both compressed air and vacuum braking systems.

You can completely bag the vacuum system here in D.
The discharge systems here in Germany were discarded at the time and are still only permitted to carry trailers up to 25 km / h.
Anyone who wanted to continue moving heavy trailers at that time had to convert their towing vehicle to dual-line brakes within a period of time.

Here is some more reading material:

Greetings Marc

Short status report,

had the day before yesterday TÜV appointment for individual approval.

Except for a small defect (rubber mounts on the right front damper had some play), no complaints.
The CO value was also okay, without having turned anything on the carburetor beforehand.
The brake behind the right was and is, despite the new brake cylinder, worse than with brakes that have not been replaced. Easy to see on the brake tester.
Despite multiple bleeding, I still have the feeling that there is air in the system somewhere, because the second time I pump the pedal is significantly harder and the pressure point is there much earlier.

Now one more question about the H approval.
Unfortunately, there is no motor vehicle expert in the vicinity who can assess whether the vehicle is H-suitable or not.

The responsible TÜV does not make an assessment - do any of you have a good tip?
At most an opinion from a German expert?

Thanks in advance.

LG Gerhard

The CO value was also okay without me having changed anything on the carburetor.

The fact that you have a shorter pedal travel the second time you pedal may also be because the brake shoes are not meticulously adjusted. Then you still have too much to overcome somewhere. The second time you step on it, the cheek is not completely back.

Greetings Reinhard

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