From: Kupiec, Paul [PKUPIEC@imf.org]
Sent: Monday, July 16, 2001 8:40 AM
To: 928
Subject: [928] bleeding the brakes on a high mileage car

A couple of months ago, I decided to rebuild the calipers and replace the
brake hoses on a 1985 vehicle with about 164K miles on the clock.  Why?  No
problems, I  just thought it was time.

What can I say, I'm just that kind of a guy.

Despite relatively good brake hygiene habits (clean fluid every couple of
years), the inside of the calipers did have considerable gunk build up and
showed early signs of corrosion.  A just-in-time rebuild. BTW: this is
exactly the state of every brake system I have taken apart over the years on
15 year old cars, so no surprise.

For my periodic fluid flushes, I had been using a mighty-mini vac (or
whatever its called) for the past few years.

When undertaking the rebuild, I asked my long-time German parts guy about
the master cylinder and the possibility of ruining it by bleeding the brakes
on a high-mileage car.  The car has been in the family since new and the
master was original.

My parts guy confirmed that he too had heard the story about the potential
perils of bleeding brakes on a high mileage car but thought the story was an
old wives tale--or more accurately, an old car-guy's tale.  The story goes
that, when bleeding brakes using the master cylinder, the movement through
the full master cylinder piston stroke may ruin the seals by running them
over a natural wear ridge in the cylinder bore since the normal application
of brakes just uses a fraction of the total piston travel.

Remark: it is very difficult to use a mini-vac pump to bleed a
completely-purged brake system.  You would need hours of time and Hulk
Hogan's forearms to accomplish the job.  All right, I know that someone will
respond to say that that they do it all the time....yada, yada, yada.

The brakes rebuilt nicely as far as the calipers and hoses went, but after
getting it back together, the pedal never felt right.  I re-bled the brakes
three times, and the car stopped fine, but the peddle never felt as I
thought it should.

Over the last two months I have noticed that, after sitting for sometime
stopped in traffic (a constant state in Wash DC and suburbs) the pedal would
slowly sink.  Not enough of an issue to set off any warning lights, and no
signs of leakage.

Yesterday I replaced the master cylinder---what a difference in brake feel.
The brakes now are much more easily modulated.  The "feel" is back, and I'm
much happier, save the missing skin on my knuckles---you'd think they would
be more calloused being dragged on the ground all the time--but I diverge
from my story.

Moral:  The old-car-guy's tale about the potential perils of bleeding the
brakes on a high mileage car may have some empirical validity.

My recommendation:  change the master cylinder "while you was at it" if the
mileage is over 100K. (I'm not firm on what high-mileage should
mean---depends on type of driving I think. I've seen lots of people who
never seem to use their brakes---rather swerve in and out of traffic--I
guess they replace fenders, airbags and hoods, but save on master cylinder
replacement costs. Other opinions about the upper limit for "automatic"
replacement are encouraged.)   If it is worn enough to do damage to the
seals then it is time to change it.

Cash strapped/or do not want to replace something that may not be broken?
Then use one of the pressure bleeding techniques which bleed the system
without pumping the master cylinder.  The modified reservoir cap with the
garden sprayer pressurizing device comes to mind. (No I have not built one).

BTW the car was not a Porsche, but the SO's 1985 BMW 325e---the daily
commute vehicle.  An awesome 122 hp.

regards to all 
phk