Master Cylinder Questions

[pajsh]

Hopefully someone can answer some questions on my master cylinder.

Having looked in the Haynes Destruction manual it looks like it's a Teves (ATC) and it says you should be able to get a "service kit " for it. I'm going to ask VW tomorrow but I'm not holding out much hope .

Question 1 is what are the "cylinders" on the bottom of the unit. My guess is regulating valves as there is no regulator anywhere else on the car yet the manual mentions a "regulating" unit. If so can I remove them and fit a "Tilton" bias valve or should I leave them on and still fit the valve.

Question 2 is regarding the size of the cylinder, it has 20 stamped on the side so my guess is it's 20mm (no sh1t sherlock!) and I plan to use this with the Mk2 8" servo and 280mm disks & conversion (if I can ever get a reply from Just Caddy's). Good enough or not I ask??

No probs if I get no replies as I'll do it anyway and see what I get.

[Nate]

i think they're basically pressure reducers to stop the rear end locking up before the front

[polov8]

Those are indeed pressure reducers, and you'd be wise to keep them if you're retaining the rear drums. If you fit rear discs, then you'll require different reducers, as the needs of the rear calipers are different. The drums require a little residual pressure to be held in the lines to the rear brakes to prevent the shoes backing off completely, and giving you very long pedal travel. The discs don't require that, but do require limitation of absolute pressure to prevent locking up.

Regarding the master cylinder, if you're going with 280 discs, are you relocating the current calipers, or fitting an adapter kit and G60 calipers? If the former, then your current master cylinder design will be ok, but the 8" servo is tiny! Best upgrade you can do is find a mk3 golf in the yards (plenty of them) and rob the servo and 22mm master cylinder off one of those. The threaded shaft needs shortening a little, but other than that, swapping your clevis on will make this a bolt on swap. doing this will reduce your pedal travel, and gives a great upgrade to brake performance.

If you're fitting G60 calipers, the pistons are bigger than the stock scirocco calipers, so the above servo and master cyl upgrade is essential to prevent long pedal travel.

Whatever you do, spend a little time cleaning and adjusting the cross linkage, as it rarely seems to receive any care, and can be the cause of stiff or sticky pedal operation.

[pajsh]

Thanks for that though I'm still a little unsure. I plan to go down the 280mm route but not finding it that easy.

I'm trying to get adaptors from Just Caddy's but they have no phone number and aren't replying to my emails. On the net people seem to be selling Mk3 Gti calipers cheap enough but carriers are hard to find. Might have to be a scrappy for that one. The servo I think it's 8" but not sure as the car is an 89 so I believe it should be 9". Pictures on my build tread along with nicely cleaned and sorted linkage:-



I'll keep the reducers but I'm planning to fit a "Tilton" bias valve so may find I don't need them.



I'm off this week so will try and get down the scrappy later on.

One other question, if I'm only running one of the rear circuits to the rear (through the valve and to a tee) what is the best way to blank off the other cylinder. I'm guessing it's simply a bolt and some sealing compound.

Thanks for the reply.

[polov8]

Nothing in the scrappy is going to allow you to fit 280 discs to a scirocco, there do exist caliper carriers from the scirocco 16v, and some 5 cyl Audi 80/90/coupes from the mid eighties, that allow you to fit 256mm discs, but they're getting hard to find and correspondingly pricey these days. Don't discount them though, they're more than up to braking a mk2 scirocco.

Being a later mk2, you should have a 9" servo, and I think a 22mm master cylinder will fit straight to it, which is a bonus.

On the proportioning valves, the best way to do it is to retain the dual circuit, as it's there as a safety feature, and fit 2 proportioning valves, one for each rear brake. just blocking off one circuit and running the other split between the rear brakes will be dangerous. The only other way would be to have one circuit of the master cylinder running to the front, and the other to the rear down a single line to your valve, and then splitting to the rear brakes, BUT, if you have a failure anywhere on the front, you'll have lost 90% of your braking, and when you do try to brake, it'll probably lock the rears and have the effect of spinning the car, and firing you into the scenery!

[DT1]

Is this for the track?

You will fall foul of Construction and Use Regs if you do this on a car in use on the public highway.

[pajsh]

Early days but primarily Intended to be road legal and for occasional track work.

Presumably it's the single rear circuit that is the issue although I've seen it done this way on some road going cars already and my Locost 7 was set up this way.

The twin valve idea doesn't appeal not least for the £180 cost of the valves but also as matching the braking from wheel to wheel does not sound right.

Not sure which way to go now.

I think I'll go and consult my garage guru and see what he says.

[pajsh]

Well I've been on Vagcat and down to VW and the S/N of my servo is not recognised but I'm now pretty sure it's 8" as Vagcat only has 9" on LHD. P/N should be 172 612 107C according to VW which is 8" but it's 105J. (that's nothing new as the 020 box is a 6A which no one has heard of either).

So, assuming I'm off down the scrappy I need to look for a Mk3 GTi servo and 22mm master cylinder. I want to go 280mm with adaptors but as I no longer have the original carriers (stupidly threw them away) I need to get carriers from a Mk3 GTi - correct??

I could still go back to 256mm if I get lucky and find an old Audi 5 cyl with the right brackets.

I need quite a few other parts that are no longer available from VW so it's going to be worth a trip with the spanners I think.

I thought I was re-building a car but I seem to spend more time searching for bleeding parts.

[marcus]

Whatever you do, spend a little time cleaning and adjusting the cross linkage

Be VERY careful doing this.

Brunty offers some advice but if you're not careful you can end up over-adjusting it with the consequence that the piston in the master cylinder can't fully retract and partially covers the return holes, leading to spectacular brake binding (I know because I did it, the glow and the heat from the disks had to be experienced to be believed).