[Mgs] Brake failure - What caused it?

[mgbob at juno.com]

  A plausible explanation is that, over the months when parked, the MC lost
whatever fluid had been in it and rust developed in the cylinder. When you
topped it up, the MC worked well enough initially, but the rust chewed away
the cups and now they cannot consistently make the pressure needed for the
brakes.
  The brake-light clue would be puzzling in earlier cars, but as the switch in
'69 is activated by pedal motion, not by hydraulic pressure, it does not
matter.
  What is interesting, though, is that yours is a two-section MC. One would
expect that either front or rear brakes would have worked.
  R&R of the MC is not difficult (have a 12" extension and 7/16 sockets for
the under-dash nuts), though rebuilding of this MC is not something I have
successfully accomplished.
  What do you think?
Bob


---------- Original Message ----------
From: Charley & Peggy Robinson <ccrobins at ktc.com>
To: Undisclosed-recipients:;
Cc: Mgs at autox.team.net
Subject: [Mgs] Brake failure - What caused it?
Date: Tue, 24 Aug 2010 08:16:31 -0500

  Here's a little trouble-shooting exercise for ya, gang.  I got my '69
B out of the garage after it had been sitting undriven for several
months.Backed her out, hit the brakes and the pedal went to the metal!
Wound up on the grass next to my driveway, fortunately an uphill slope.
So I look in the master cylinder and the front chamber is almost dry -
my bad, should have kept it topped up.  So I fill it with Dot 4, do a
little bleeding and get a nice firm pedal with no leaking down.  Drove
the car a few times in the cool of the morning with no problem.

Couple days later I'm pulling slowly into my garage, got to stop and the
brake pedal goes to the metal!  I almost yanked the handbrake lever off
the floor!  So after I changed shorts, I looked in the master and it was
still full.  Hopped in the car and tried the brakes again and they
worked fine with a firm pedal.  Brake lights work too and there are no
discernible leaks.

I think I know what's wrong.  What's your take?

CR
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