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Re: 80 MGB LE - Brake Bleeding problem

To: prsmith@navnet.net, mgs@autox.team.net, members@batans.ns.ca
Subject: Re: 80 MGB LE - Brake Bleeding problem
From: Ajhsys@aol.com
Date: Tue, 21 Sep 1999 11:01:12 EDT
I'll take a stab at some of this.  

No, the relative elevation of the car should have no effect on bleeding the 
brakes.  The brakes should work when you go up and down hills!

I also have the pres. diff. switch in my Midget and just finished replacing 
the brake pipes, flex hoses, and all the rest of the system.  I bled the 
system from the farthest cylinder (left rear) to the closest (left front) in 
order.  That way I was assured of getting the air out.  The reason (IMO) for 
the odd bleeding order recommendation in one manual is to keep the pres. 
diff. switch in the center when performing a routine brake bleeding.  When 
you are replacing the entire system, you have to get fluid in the entire 
system first, then you can center the PDS by bleeding the two front calipers 
individually until the dash light goes out.

I would start again with the longest brake line run, and work toward the 
shortest, then try to recenter the PDS after you have good pedal feel.

I don't like the "clunk" that you heard, though.  That may have been a seal 
exploding in the master cylinder.  If you have no pedal at all, that could be 
a problem.  It could also be that the reservoir just ran dry, which would 
explain the sucking sounds.  The rear is a long run and it could have had a 
lot of air.

Check the MC by removing the two lines and pressing the pedal.  If fluid 
comes out both openings, put it back together and try bleeding from the back 
to the front.

YMMV!  (Curious!  My spel chekr wanted to replace "cluck" with "clank."  
Maybe my computer knows something!)

Allen Hefner
'77 Midget
'92 Mitsubishi Expo LRV Sport

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In a message dated 9/20/99 5:46:44 PM Eastern Daylight Time, 
prsmith@navnet.net writes:

<< As part of finishing my front end rebuild, I replaced the front brake
 lines.  While the front end was still up on stands (with rear wheels on
 the ground), I decided to bleed the system without installing the front
 road wheels.  
 
 First question.
 Would the relative elevation of the front end vis a vis the rear end have
 anything to do with the problem which follows?
 
 My car has a Pressure Differential Switch.  Both Haynes and Bentley tell
 us to remove the wire from the switch and back it out three and one-half
 turns.  
 
 Second question.
 Does this mean turning the switch body or does the point of connection of
 the wires turn within the switch body?  I was turning the switch body.
 
 For cars equipped with Pressure Differential Switches, Haynes says to
 bleed the brakes starting with the front wheel closest to the master
 cylinder, then right front, then the rear (I did the left rear then the
 right rear).  Bentley says to do the farthest from the master cylinder and
 then work to the closest.
 
 Third Question.
 Which is the correct order?
 
 I bled, in turn, left front, right front and left rear.  These three
 wheels went OK but I noted while doing the left rear that while fluid was
 coming from the brake cylinder when the pedal was depressed the fluid
 level in the master cylinder did not seem to be going down as fast as it
 had while doing the front wheels.  In fact at one point (after 3 or 4
 pump-bleed cycles) I wondered if it had gone down at all.  As we finished
 each wheel the pedal got sequentially harder.
 
 I then tackled the right rear.  With my wife on the pedal, I opened the
 bleed screw a couple of times.  All of a sudden, per my wife, the pedal
 dropped a couple of inches further than it had for the other wheels.  This
 sudden new bottom for the pedal was accompanied by  what my wife says was
 a clang.  At the same time I heard what seemed to be sucking sounds from
 the area of the master cylinder and the servo.  After this event we were
 never able to get pedal again even though I tried to bleed all of the
 wheels
 
 Fourth Question.
 Does anyone know what the problem is?  And what can i do to solve it?  >>

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