[T3] Master Brake Cylinder

[Jim Adney]

First, check the fluid level in the brake fluid reservoir. If it's 
way down, your fluid is gone, which explains your symptoms.

The inside of the MC boot should be dry. If it's wet, that means that 
the first seal inside the MC boot is shot and the MC is leaking out 
the front (actually rear) end.

You might also want to check the adjustment of your rear brakes. If 
they are far out of adjustment (they are NOT self-adjusting!) this 
can mimic an MC failure.

Used MCs that have been stored, either wet or drained, are not likely 
to serve well. They probably have rust in the bores that may or may 
not be cleanable. Most of the time they can be honed well enough to 
work, but sometimes that just won't do it. They have to be honed, 
then the remaining pits examined to see how deep they are and where 
they are.

Brake fluid absorbs moisture out of the air. When this happens the 
brake fluid becomes corrosive and the cylinder walls are usually 
beyond redemption within a year or so. Brake parts that have had all 
their brake fluid removed and then rebuilt with silicone fluid on the 
walls have a long shelf life. So if you have spare parts that you 
hope to use in the future, you need to rebuild first, then store.

This is called investing in your future.

I rebuild ATe MCs for $45 and FAG MCs for $55, plus shipping. Unless 
you already have all the tools and skills for this work, this is a 
great bargain.

Jim

On 5 Dec 2016 at 19:58, J. Jonik wrote:

> Re/  '71 Std.
>     A couple of weeks ago, a good brake bleed job got questionable brakes back to Almost Normal.
>    Now, having worked half-decently (not perfectly) a few days ago, and having not been driven for those few days, there's barely any brake pedal resistance.  Pedal goes way down.   And pumping the pedal isn't bringing back any resistance.
> 
> 
> The last time I was checking things, there was wetness in the slightly-damaged rubber boot between pedal and master cylinder.   No drip puddles under the car at wheels or anywhere.
>    Is this evidence that the Master Cylinder is faulty?   How to test to be sure? Should that rubber boot be dry inside?  And should it definitely be replaced in any case? (It's just to keep dirt out, right?)
> 
> 
> Then, what's advice on picking a good spare master cylinder from my collection?  If they are still wet from brake fluid, not dry, is that a good sign?  I kept some wrapped in plastic bags.
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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