[T3] bleeding brakes question

[Jim Adney]

On 2 Aug 2011 at 16:47, Jeremy Menzies wrote:

> On a related but different note, I wondered about something a friend
> told me to be careful about when bleeding the VW brakes.  He said that
> when bleeding by the 'pump method', the person pushing the pedal down
> shouldn't go all the way down but only like 3/4 or so.  His reasoning
> was that when the brakes haven't been worked on in a while (as mine
> hadn't), there is corrosion that build up on the portion of the piston
> inside the master cylinder that doesn't move a whole lot during
> normal, less than full stroke braking.  He was saying that what can
> happen is that when you go to the floor w/no pressure, it'll push this
> corroded portion beyond it's normal range of travel and either damage
> the seals inside the MC, get stuck in the extended position or crud
> things up as the corrosion is scraped off.  Or something along those
> lines.  Result being: need to rebuild the MC. 

It's a reasonable thought, but I actually never see corrosion in the 
MCs that fits that description. Most corrosion occurs right at the 
first (rearmost in the car) seal and that's the seal that causes the 
MC to leak. The other seals almost never show any wear at all.

I also often see corrosion just under the ports where the fluid feed 
lines feed into the top of the MC. This appears to be due to damp 
brake fluid, so this is a good reason to bleed your brakes every few 
years, regardless, unless you're using silicone DoT-5.

I guess my feeling about the 3/4 stroke thing is that if you think 
you may have this problem, maybe you should at least look at your MC 
and see if it needs to be rebuilt.

Finally, I recommend that people not slam the pistons into the end of 
their stroke when bleeding, simply because there are parts in there 
that may be damaged if you do this too hard.


> Of course when I bled them I didn't know this so we went to the floor
> with it.  Now, I noticed that my pedal travels farther than before the
> bleeding and generally feels softer and I've been wondering if his
> tale could be a part of it (and a little worried I may have messed up
> the MC).  I did change the old fluid w/fresh when I bled but I
> adjusted the rear stars tighter so I expected them to feel harder.  

Your MC is probably fine. You may need to go back and readjust your 
rear brakes after you've driven a bit, just because the shoes will 
have contoured themselves to the drums and now have more clearance 
than you think.

Also, if you installed new or rebuild calipers, those will retract 
more than your old ones which were surely sticky. That is a good 
thing, but it results in longer pedal stroke now. This is what it was 
like when the car was new, but over the years you've gotten used to 
the "wrong" feeling as the calipers rusted inside and tightened up.
This extra pedal stroke is often perceived as a softer pedal, but 
it's the way your brakes are supposed to feel.

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