Hydraulic assisted brakes - how do they work?

[mandic]

Hi guys!

First of all, I don't own a S2 so please bear with me.

If You don't know, Citroens use hydraulic pressure to power brakes, steering and for suspension.

But brake pedal travel and brake feel or better put applying brake force in emergency situations is a PITA.

So I was thinking of putting something like Audi's system on. To still use hydraulic (LHM) pressure for brake assist but have longer pedal travel and be able to more accurately apply the force.

So can anyone please explain (some photo material preferred too) how does it work?

Does the system work the same way like vacuum assisted boosters only instead of negative vacuum, Your system uses positive hydraulic pressure?

Can LHM oil be used in such system?

Thanks in advace!

Regards

Ziga

[S2central.net]

I dunno how similar the Audi hydraulic assist is to Shitroen, but yes... we have a hydraulic pump providing positive servo assistance to the brakes. The same pump assists the steering. We have a brake pressure accumulator as an emergency reserve of pressure for the brake servo in the event of engine failure/stall.

[S2central.net]

Oh and we have a weird mineral oil for this system - it is NOT the usual 'PAS fluid' as some have found out to their cost. DUnno what 'LHM' is...

[Rusty]

The PAS pump has a high and low pressure side doesn't it?

[Error404]

yarp. IIRC the brakes are the high pressure side, the PAS is the low pressure

[mandic]

We have a brake pressure accumulator as an emergency reserve of pressure for the brake servo in the event of engine failure/stall.

OK, but the brake servo is basically the same (black drum ) as in any other vacuum assisted brake system only here Audi utilizes positive hydraulic pressure?

Also, have seen pressure in the system is about 100Bar, right?

btw, LHM is fancy name for mineral oil.

Thanks!


Ziga

[S2central.net]

NO - on the S2/RS2 there is no 'black drum' for servo assistance on the master cylinder... You will find vacuum driven servos like that on other Audis - usually non turbo engines for obvious reasons. Even the modern TDi machines have a small engine driven vacuum pump to power the brake servo. The hydraulic assist is better IMO - it works on and off boost with right or left foot braking.

[johne]

In the Citroen system, the hydraulic oil (LHM) is pressurised and fed to the "brake master cylinder". The brake master cylinder isn't a master cylinder at all, it's a slide valve that your foot controls to regulate the flow of hydraulic oil to the brake caliper pistons, so in a Citroen there is no brake fluid at all, just hydraulic (mineral) oil throughout which is great, no water retention, no paintwork damage, no regular fluid changing and the brakes are a doddle to bleed. There is a hydraulic pressure reservoir (bomb) to provide braking if the engine dies. HOWEVER, big disadvantage is that once the engine dies and the reservoir is used up, no amount of standing on the brake pedal will make the brakes work as you're only opening a slide valve and if there's no pressure your foot can't make any. That's why Citroen's have the handbrake working on the front wheels.

In Audi's the brake system is conventional with normal brake fluid and a brake master cylinder that your foot can operate even if the presure assistance has gone. Most cars use a vacumm servo to generate additional pressure to help your foot out. Audi's work the same way but using hydraulic oil under pressure to help your foot out but without the hydraulic oil having any part in the actual braking circuit. Although it looks like one unit, the Audi master cylinder actually has two parts to it inside.

It may not be recommended but I've used LHM fluid in my Audi to no apparent ill affect when I didn't have any Audi mineral oil to hand.

So, if what you're thinking is to fit an Audi master cylinder to a Citroen, convert the rest of the braking circuit to normal brake fluid, changing seals etc., and then using the pressure feed from the Citroen hydraulic pump & reservoir to drive the asistance on the Audi master cylinder, then yes, you could do that.

John

[S2central.net]

Thats a nice explanation johne - and useful insight into the strange and bizarre working of a citroen... Personally I hate them - my Dad had one once and it was an absolute complete piece of junk - just my opinion like......

[mandic]

johne,

Thanks!

Does anyone have a pic of Audi's brake master cylinder? Would like to know how it looks like.

Many thanks!

Ziga

[mandic]

Have found a pic of supposed hydraulic master cylinder:

http://www.s2forum.com/forum/attachment.php?attachmentid=15997&d=1201891388

Is this it? But I don't see any entry for hydraulic assist.

Thanks

Ziga

[mandic]

aha, found this:



Cool...

So, has anyone any for sale?

Cheers

Ziga

[k.alle]

Where are you based if I may ask, USA?

I have one, PM for details, will be out of town till Sunday so reply will take a while.

So, has anyone any for sale?

Cheers

Ziga