Vacuum Brake Servo

[Error404]

Just found this pic in his build thread, looks tight!

Thats loads of room by Audi standards

[rj scotty]

There is an A/C pipe runs in that area to the drier in the right hand wheel arch. If the servo is going to foul anything it WILL be that.

[Error404]

yup we had a chat about that at ADI, I think that's the only bit that will need to be majorly modified i think

[Pat D]

Just use the AC pipe off the car where the servo came from?

[Pat D]

Just found this pic in his build thread, looks tight!

Is this an earlier S2? I just had a look at the size of my vacuum servo on the B4 and it looks like it won't fit as my S2 has braces welded from the strut tower to the firewall on my S2 which are right in the way of where the servo will sit

[AndyP]

Is this an earlier S2? I just had a look at the size of my vacuum servo on the B4 and it looks like it won't fit as my S2 has braces welded from the strut tower to the firewall on my S2 which are right in the way of where the servo will sit

This car wasn't an S2, it was a coupe quattro with an ABY engine conversion, so the S2 strut brace will need modification.

[Pat D]

I have few questions in regards to the brake bomb and high pressure lines, hopefully someone might know the answers.

1. Is the high pressure line from the PAS pump to the steering rack a different pressure to the one going to the bomb? Hence two separate pump outputs?

2. Can the bomb be deleted all together if there is no need for assist in the event of the engine not running and just run the high pressure line direct to the master cylinder?

Reason I ask is that I would like to use an electric PAS pump without Vacuum servo brake assist, since the electric pump can run with the engine off there is no need for accumulated assist.

[Error404]

Hey Pat:

An electric PAS pump won't offer you brake assist aswell. You would still need to retain the dual stage PAS pump, making the electric PAS pump kinda pointless. That is why people switch to electric PAS pump and vacuum servo at the same time. Or just vacuum servo and use a single stage hydro PAS pump as per the factory setup

The bomb works off the front section of the dual pump, using pistons driven by an elliptical collar, which AFAIK is higher pressure but I assume much lower flow than the steering side (the brake assist consumes very little fluid by volume compared to the rack)
The steering circuit is about [100bar].

As well as an accumulator, the bomb acts as a pressure damper for the servo, without the bomb i think you would need to use something to smooth the pump output, otherwise you will feel the pump through the brake pedal and it wont be very pleasant (feels like the ABS is on continually)

So I would just cut that strut bracket and make your life easier

HTH & Merry Christmas

[driftin jo]

I have few questions in regards to the brake bomb and high pressure lines, hopefully someone might know the answers.

1. Is the high pressure line from the PAS pump to the steering rack a different pressure to the one going to the bomb? Hence two separate pump outputs?

2. Can the bomb be deleted all together if there is no need for assist in the event of the engine not running and just run the high pressure line direct to the master cylinder?

Reason I ask is that I would like to use an electric PAS pump without Vacuum servo brake assist, since the electric pump can run with the engine off there is no need for accumulated assist.

Yes they are two different pressures, the PAS line is much lower pressure.

No, you cant run the pump line straight to the servo, a couple of quick taps of the brake and you would have less pressure as the pump is not be able to keep up, that's why you have an accumulator, it's not just in case the engine stalls or the belt snaps.
You will find that even cars that use electric pumps have an accumulator, usually as part of the pump assembly, I looked at fitting one once, I think it was off a jag.

[driftin jo]

like this

http://www.ebay.co.uk/itm/JAGUAR-XJ6...item2a27474a48

[Pat D]

So I would just cut that strut bracket and make your life easier

Haha, that's what i was trying to avoid

I'm wondering how much support that strut bracket provides and if it's worth fabricating another one around the vacuum servo.

[Pat D]

like this

http://www.ebay.co.uk/itm/JAGUAR-XJ6...item2a27474a48

That looks like another idea, I guess I could use one of them as well for the brakes

[Error404]

Haha, that's what i was trying to avoid

I'm wondering how much support that strut bracket provides and if it's worth fabricating another one around the vacuum servo.

Not a massive amount, its just a pressed piece of sheet, i bet you could bend it in your hands with it off the car

[AndyP]

I'm wondering how much support that strut bracket provides and if it's worth fabricating another one around the vacuum servo.

That's what I'm going to do, if you ask Vas I'm sure he'll post up a pic of his which has a curve in it to go around the vacuum servo.

Edit : You can see it in this post here, LHD, but you get the idea.

http://www.s2forum.com/forum/showpos...&postcount=482

[Pat D]

Great, well I'll stick with the original plan and use an electric PAS pump and vacuum brakes. I'll cut the strut bracket out tomorrow.

I just removed the vacuum servo from my B4 and emptied out a good half litre of brake fluid, so I filled it up with degreaser and then hosed it out. Should hopefully still be ok.