Hydraulic to Vaccum servo conversion

[diesel des]

Quick update. The rs6 servo arrived and a quick suck test shows it seems to hold a vacume. Will confirm fitting and operation tomorrow but this is a real good start!

[jade]

Quick update. The rs6 servo arrived and a quick suck test shows it seems to hold a vacume. Will confirm fitting and operation tomorrow but this is a real good start!

Good luck ! I am looking at doing this up grade on my LHD RS2 so watching closely

What is the part number of your servo ?

Cheers

W

[diesel des]

So its fitted. Haven't road tested it yet but it appears to be working ok. The servo doesn't hold a complete vacume but is much more air tight than the others.

So the issues I had are as follows. The rs6 servo is bigger than the a8 ones I had been working with before. It's a real tight squeeze getting it in place but once in place it's fine. I had to take the engine and gearbox off their mounts and Jack it around to get the servo in. Then the a8 style master cylinder bolts on ok but the actuator pin in the servo is recessed further in the rs6 servo than the a8 ones so modifications are required.

From memory the a8 servo actuator pin is recessed about 45mm and the rs6 servo about 60mm. I don't have a rs6 master cylinder to match this difference so I modified the actuator pin. It just pulls out of the rs6 servo as it's held in with a spring clip. The actuator pin has a shoulder about 10mm down and so a piece of 10mm od stainless pipe I had presses over the actuator rod and stopped at the shoulder. To make sure it stayed there I tacked it with the welder and ground it back to remove sharp edges.





Then it's just a matter of putting the actuator rod back in the servo with its sprung clip.



Once that's done the extended actuator rod stops about 45mm from servo to master cylinder mounting face which is about right for the a8 master.

Other issues are that the a8 master cylinder fluid reservoir interferes with the servo and the strut brace. The interference with the servo is fixed by just cutting a small plastic tab off. The strut brace needs remaking. I have cut mine and welded some box section as spacers to do the job. It's not pretty but is functional. I will remake the strut brace out of pipe and bend it to fit and weld on tabs for the mountings. Just need to get a good pipe bender. On the brake peddle side of things the rs6 servo is m12 fine think it's 1.5mm the urs4 is m10. The a8 setup is m12 too. So I made an adapter and basically you want it adjusted so you have about 235mm from the servo mounting face to the center of the clevis pin hole for the brake peddle. You can fine tweet it exactly to suit on car.

Here is the part number of the a8 servo I seemed to get a load of leaking versions.



And the new rs6 one even boasts it's for a rs6.



Anyhow just some engine mounts to button up and might get a road test tomorrow. Im totaly nackered!

[diesel des]

Ok more updates. I had forgot to mention that the low brake fluid level cable needs extended too. Now the brakes work great with plenty of assistance. But there is a constant hissing coming from the servo in the cabin. It goes when you brake. Comes back when engine creates a vacume. So there must be something else to the puzzle. Some sorta regulating or control valve?? Maybe the check valve on these new cars isn't a simple check valve??

[Error404]

I had a suck on my RS6 servo last night, seemed to hold a decent vacuum. I'm going to hook it up to the vac line on the car over the next few days and see how it goes. I can't see that the servo can be designed to leak in any way unless it runs independently of the engine, or it would have a constant vacuum leak.
Are you sure you don't have a leak on your climate recirc valve or something Des?

[diesel des]

Mine also seemed to hold a good but not perfect vacume until it was fitted and used a few times. Still isn't a bad leak but it's there and more so on the overrun down a hill. It actually squeels sometimes on overrun. Nothing else connected now on the servo line from the manifold just 2 check valves. It stops leaking when the brakes are under pressure eg peddle down but leaks as soon as I let go.

Try yours connected to the intake off the car and see what happens. I can't believe they are ment to do this too but I have had 3 now with this issue. I'm thinking of binning this idea and fitting a tdi single servo setup. Just can't bare myself to get under dash of tdi to remove it and then drop engine again on my car to fit it. Really ****** off with it now.

[diesel des]

Another and hopefully final update on this. The air leak at the new rs6 servo got worse and was causing a high idle speed and it sounded like a dogs squeaky toy when you let go of the brake peddle. So I trailed it out and fitted a servo and master cylinder from a tdi parts car I had. It's working great! No noise and holds a vacume 100% wish I had of done this to start with. Slightly less assistance than the rs6 setup but it felt too much if honest.

An interesting thing I found was that 2 of the bolts need to be tight on the tdi audi 100 2.5tdi servo before it holds a vacume while bench testing.

See picture.





While testing i did a fairly hard launch while at a t junction and now I have drive train issues. I think it's the rear diff but could be a drive shaft cv. Bassicly there is a large clunk and intermittent loss of drive at the same time as the clunk while driving round a bend eg in a carpark. I just love work me...

[Az786]

Is it a straight fit from the tdi?

[diesel des]

On a urs4 it was a straight fit. No messing at all.

[Error404]

Are you sure you're not boosting the servo Des? I still can't get my head round the vacuum leak issue on the dual servos

[diesel des]

Definitely only giving it vacume. I have 2 check valves in series feeding it. It seems so unlikely that all 4 or 5 servos I got were faulty but yet that's what seems to be the case. There really is no need for the duel servo setup the single tdi one is fine.

[hiloran]

A8 servo fits nicely. Bolt pattern is same.

...edit. sorry after reading post again. Seem to be old info

[Error404]

Can you post pictures of the bolt pattern on your bullhead and the servo?

[marius555]

Dear friends. I am at this moment with the 4'th booster ( a8 d2 ). As mentioned above , it literally screams in the car....

I assume at this moment that the brake pedal angle isn't right so it's destroying a sealing or something .

Just talked with Pirvu Eugen on here and he measured that the mount on the pedal si 10mm down than it should be and this might be the cause...


https://www.youtube.com/watch?v=cY_XH9zcaS0

[Pirvu Eugen]

apparently...is an issue with these double diaphragm types into the c3 c4 chassis.
Is starange that into these chassis were also vacuum units with the same brake pedals. But single diaphragm type. The single type ,maibe, is accepting bigger rod angles.
The only thing can go wrong in this matter is the valve poppet.
What you think?