[T3] MC failures

["Jim Adney" <jadney@vwtype3.org>]

Keith wrote about experiencing losing pedal pressure if he pumped 
without letting the pedal all the way back up.

I think this is just a sign of one of the typical MC failure modes. 
This happens when standard brake fluid absorbs water from the air and 
this hydrated fluid tends to collect at the bottom of the MC just 
under the drilled ports--for reasons that I really don't understand.

This moist brake fluid promotes corrosion of the MC wall, or pits, 
exactly where the primary seal might stop if you didn't let the pedal 
all the way up. If this happened, the fluid could leak around the 
outside of the seal because the lip of the seal could not reach the 
bottom of the pits and do its job. This would allow the pedal to leak 
down.

This is not normal behavior and a MC in good repair will not do this. 
While the best repair would clearly be to just replace the whole MC, 
I have actually honed and rebuilt many MCs that were slightly pitted 
and had them work as well as new. I assume this works because the 
honing removes the high spots and slightly reduces the low ones, thus 
reducing the difference. This can be done for a fraction of the cost 
of a new MC.

In the end, however, this is just another chance for me to recommend 
Silicon Brake Fluid, especially if you have just fixed up your old 
brake system and don't look forward to doing it all over again in a 
few years.

-
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Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA
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