Brake upgrade issue......

[audiman]

Maybe if you are selling these kits as a bolt on option, the seller should have done his damn homework?? Just a thought?

[twoqu]

Right,
As I suspected there does appear to be two master cylinder sizes. Up to chassis 85-H-900 500 it is 23.81mm and after that chassis number it is 25.40mm. I guess that ties into the introduction of the RR.
Please bare in mind that this is only a suggestion. As other have stressed, brakes are a serious matter.

[Alistair C]

Alistair,
Don't think calculations are necessary. Evidently there is a considerable difference in piston size as I expected. The only option, if he wants to keep those calipers is to find a master cylinder with a smaller bore than the one he has. Actually, if you have an RR, perhaps an MB one would have a smaller bore. When they used the wider vented discs on the 200q they used a larger one. Perhaps this is also the case on the Ur.

I know what you mean...... but it'd be the only way to know if these were a suitable match for sure.
To be honest, I've no idea what the hydraulic system on the RR Quattro is like. If it is the same as on the S2 then there may well some problems.

Going back to basics. If you're saying he's now got a master cylinder which is proportionally too big...... surely that'd mean the calipers would lock up too soon rather than not at all ????

[twoqu]

I know what you mean...... but it'd be the only way to know if these were a suitable match for sure.
To be honest, I've no idea what the hydraulic system on the RR Quattro is like. If it is the same as on the S2 then there may well some problems.

Going back to basics. If you're saying he's now got a master cylinder which is proportionally too big...... surely that'd mean the calipers would lock up too soon rather than not at all ????

Alistair,

Increases in m/cyl size reduces pedal travel but also reduces the power available. I suppose it not the m/cyl that is too big but the caliper pistons are too small! Remember Audi forgot to stick the vented rears on the S2, might check to see whether they altered m/cyl size.

[Lee]

If your front calipers have unedrsized pistons you are not going to solve anything by installing a smaller master cylinder. You brake balance will still be up the creek. You may get better hydraulic leverage but it doesn't address the brake balance at all. Get the right calipers to start with otherwise you could end up spending lots of time and money without fixing the real problem.

[twoqu]

No!,

It seems the S2 uses the 25.4mm m/cyl as well. The fact that no one has complained about this before leads me to think that by accident or design, the 996 front monobloc is a good conversion.

[twoqu]

If your front calipers have unedrsized pistons you are not going to solve anything by installing a smaller master cylinder. You brake balance will still be up the creek. You may get better hydraulic leverage but it doesn't address the brake balance at all. Get the right calipers to start with otherwise you could end up spending lots of time and money without fixing the real problem.

Lee,

Yep, I would agree there. But realistically what are the chances of that happening!

[audiman]

Agree with Lee, i changed my calipers and havent had any need to alter M/cyl

[Lee]

Get an expert to do the job in the first place and these problems would not exist. To solve a problem you must first understand it.

[Lee]

So, Twoqu, what is the problem you are having with a 1" master cylinder piston size?

[twoqu]

So, Twoqu, what is the problem you are having with a 1" master cylinder piston size?

I don't have any problem at all. Clearly this size of m/cyl works very well with the piston size of the 996 FRONT caliper. The guy has three choices. Either replace the calipers, change something, or live with it!
Had the person who supplied the kit advised accordingly then perhaps he wouldn't be in this situation.
As I said earlier, I don't profess to be an expert. However if you look back over the threads you will see that I correctly identified that the conversion was done with rear calipers. I was able to do this via remote viewing, a technique borrowed from my psych-ops training!

[chancha]

hmmmmmm.........

Thanks again guys........ so from reading all of your posts here the upshot appears to be that the setup I have would lend itself to the issues I have wrt ie brake assistance negligible. It appears that I have an imbalance in front to rear brake bias again caused by incorrect sizing of caliper pistons.
Someone asked a while back about the greasy film on the disks...... yes I did remove the film before running.........


Rgs Kevin

[dan_b]

What pads are you using, did you mention that?
I would take the advice of Lee here- IMHO he's very much the brake guru.

Strange though, with all this talk of servo assistance, I drove a Lotus Elise recently, which of course has no servo assistance at all: beautiful brakes.

There was a post earlier about pumping the pedal with the engine off 20x odd times - isn't that a test for the Bomb, rather than the master cylinder, or am I confusing matters?

If you have doubts over brakes, don't drive the thing and get it sorted by an expert. They're life or death equipment if you ask me.

[Lee]

Hey Kevin, we will take a look when we come to Scotland for the RR meet. Doug mentioned that you would make the car available and we can do some checks to confirm the problem and any remedy that may be possible.

Lee

[chancha]

Thanks Lee, I appreciate it very much........

Kevin

Hey Kevin, we will take a look when we come to Scotland for the RR meet. Doug mentioned that you would make the car available and we can do some checks to confirm the problem and any remedy that may be possible.

Lee