brake master cylinder question

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volkswagen50
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brake master cylinder question

Post by volkswagen50 » Sat Apr 21, 2007 5:40 pm

On the dual curcuit master cylinder, there is a screw inbetween the two pistons. It is called a stroke limiter screw in my book. If it's inbetween the two, does it only control the rear stroke? If so what does it do to the stroke in terms of fluid moved, volume wise? Does undoing it give a longer stroke and therefore more fluid to the rear? I have four wheel discs and the rears don't feel very effective right now. Thanks, greg

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Post by david58 » Sat Apr 21, 2007 10:51 pm

Image
See the screw near the middle? That's the stopper screw. It sets the rearwards travel limit of the solid spacer. It's what determines how far back the innards can travel. You can't remove the solid spacer until that screw has been backed out.
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Marc
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Post by Marc » Sun Apr 22, 2007 12:36 am

So, no, it's not intended to be any kind of adjustment you can make to change your front-to-rear bias.
You could plumb in a limiting valve to cut back on the pressure applied to the front brakes, These are commonly found on many cars hooked up the opposite way, to keep the rears from locking up first - many have active linkage connected to the rear suspension which limits rear pressure when the ass end comes up on braking, to which you could set up an adjustable link to allow you to dial in the amount of front pressure reduction.
Another possible solution would be to change to a master cylinder from some other car which has different bore sizes for the front & rear circuits, plumbing your rears to the "big" end.

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Post by volkswagen50 » Sun Apr 22, 2007 7:23 am

That's not the screw I'm talking about. Take a look in that middle spring, it's in there. It screws through that spring into the aluminum piston, the long way. I've got one taken apart so I can see it quite plain. In the Clymer manual, the cut-away photo has it labeled as in my original post. How about it? Greg

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Marc
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Post by Marc » Sun Apr 22, 2007 10:13 am

So long as there's no failure in either hydraulic circuit, both pistons will have the same stroke. Even if you could tweak the M/C to alter the relative strokes of the two circuits it would not change the pressure applied.

You could go to dual master cylinders with a balance bar that allowed adjustment of the leverage available to each from the pedal, but that's a rather ambitious modification Some folks have reported good results from inserting a residual pressure valve in the rear line to keep a couple psi of pressure on the caliper at rest - this keeps the pads in light contact, if you're having a problem with knockback due to rotor runout it'll give you a higher pedal.

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Post by volkswagen50 » Sun Apr 22, 2007 12:53 pm

What i was hoping for was that I could just make it so the rears started their stroke earlier, by pushing the piston towards the rear of the car while the cylinder is at rest. While the pressure would be the same, a longer stroke for the rear would move more fluid.

The pressure valve you speak of, where can I find one? I've heard of them and seen them mentioned in other posts.

Thanks Marc.

Greg

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Post by Marc » Tue Apr 24, 2007 11:11 am

http://www.wilwood.com/Products/006-Mas ... /index.asp
http://store.summitracing.com/egnsearch ... 4294843719
I don't know of anyone producing one that's a straight bolt-in for a VW. CB Performance sells one they claim will screw directly into the master cylinder, but it has pipe threads - it may work, but that's pretty Mickey-Mouse IMO. American cars use an inverted double-flare on the end of the tube and a straight thread (typically 3/8-24 for a 3/16" tube) while VWs use an "ISO bubble-flare" and a metric nut (10x1.0mm IIRC). If the RPV has tapered pipe threads you should be able to hunt down adapters to go directly to metric. Adapters from NPT to 3/8-24 FIF are easier to find but then you'd need to cut your line, put 3/8" nuts on it, and reflare the ends to match.

http://www.cbperformance.com/catalog.asp?ProductID=1130

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Post by volkswagen50 » Tue Apr 24, 2007 7:48 pm

The brake problem may have been a slightly more obvious problem. The calipers were flexing something aweful when i had an assistant press the pedal during bleeding. I got a good look at it and the calipers were not even close to being centered relative to the disc. I used my micrometer to measure many washers around the garage that would fit and found a good combination to get the caliper centered on the disc and the flex went away, and the pads are now slightly dragging as they should, instead of a gap on one side. I'm so mad i didn't see it sooner. Thanks all. Greg

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