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Re: master cylinder or brake booster???





After replacing the front pads and flushing new fluid into an A2 Jetta
my buddy runs we pumped and pumped and turns out he drove away with
spongy brakes. Over the next day or two it got no better and we ended up
ordering a replacement M/C from Potter and he has been fine since.  This
car had MUD in the brake lines so I'm not surprised the new fluid
cleaned out too much gunk, leading to failure.

On my own 84 GTI Donor Mobile I had trouble with the brakes working
sometimes and other times (just when you need it) it would go to the
floor and need to be pumped up to gain any braking power. Even then It
was tighter on one side of the diag circuit than the other and led to
one lock up that was almost catastrophic.   In this case I mentioned it
while I had it in the shop (ND) for smog work I didn't want to do at
home and they just bled some new fluid into it and the trouble never
came back. I have to think there was a nasty, sneaky air bubble in the
lines and it was purged.

Now, My 81 Roc was off line for about a year and I wanted to flush the
fluid out. plus I was installing SS brake lines. After D. Wong (the
Master) power bled them I noticed that they had a slight mushiness to
them and on the cross country trip I experienced a slight trouble where
they would be soft and require a pumping up to get back to normal.
(Great! Too far from home...) After running at breakneck speeds through
the desert and Sierra Mountains I got home again only to discover I had
a firm pedal and the trouble has not come back. (Bought new fluid to
vent the system with and every thing).

So, My experience and brain picking is:

- When bleeding the brake system, Do Not push the M/C too far down
during the pumps. Damage to seals can result. Scraping of unused
cylinder wall can result. 

- When purging the system, and when using a pump-up pressure bleeder be
prepared for air to collect out of the fresh fluid over the next few
days and re-bleed the brakes to eliminate any pockets that have worked
themselves out.


TBerk

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