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Re: bad behavior in rear brake

To: MG list <mgs@autox.team.net>
Subject: Re: bad behavior in rear brake
From: "Eric L. Van Iderstine" <elv1@ra.MsState.Edu>
Date: Thu, 22 Jun 1995 00:52:22 -0500 (CDT)
On Wed, 21 Jun 1995, Jerry Rau wrote:

> 
> Yesterday while driving my 66 MGB, the left rear brake locked up when I 
> applied what I would consider moderate brake pressure. I dont' drive the car 
> very often lately, so I wonder if the behavior is related to it sitting 
> undriven.

Probably not, whatever has built up on the braking surfaces should be 
worn off after one or two stops from highway speeds.

> I'm not sure why, but I remember someone telling me that every once in a 
> while it is a good idea to tap the brake while driving in reverse.

This is because more modern drum brakes would adjust the piece (VB and 
MOSS call it the adjuster assembly) that seperate the rear brake shoes at 
the top, while the vehicle was moving backwards.  It is done by some sort 
of racheting spring??, my father's 84 Ford van has it.  Your MG does not 
automatically adjust this top piece.  If you look under the car at the 
a few inches from the top of the brake housing assembly, you will see a 
small objecting protruding through the housing.  This piece is the 
adjuster screw.  It forces the pads apart, compensating for pad wear.  

> I did 
> this and it seemed to help the lockup problem. However if I applied the 
> brakes hard while going in the forward direction, I was back to the lock up 
> condition after just two or three applications.

If you are talking about it eliminating lockup while the vehicle is 
moving backwards, this would be due to weight transfer over the rear 
axle.  This weight transfer would make the "traction" force at the rear 
of the car greater (requiring more braking force to locup the wheel) 
than while braking in the forward direction.  

This is in line with what you wrote.

> 
> Anyone have an idea as to the cause or what to do about it?
> 

Back to the adjusting screw, it has four flat sides at its other end (like 
a pyramid).  By loosening the screw 1/4 turn at a time, you might be 
able to fix the lockup problem.  BE CAREFUL when you do this, make sure 
you tire pressures are the same, and that the treads are wearing evenly 
before tampering with mother nature.

Some possibilities from Fred Puhn's Brake Handbook are
1 Caliper piston seizing in cylinder (on wheel that isn't locking up)
2 wheel cylinder seizing (I'm still working on what a wheel cylinder is)
3 oil or fluid leaking into brake (on wheel that isn't locking)
4 Bad brake-shoe return spring

Since my B is disassembled, I should be able to answer questions if you 
have any loose/missing parts if you decide to pull it open.

Eric

Eric L. Van Iderstine - ME - Mississippi State University 
elv1@Ra.MsStste.Edu     http://www2.msstate.edu/~elv1/index.html
74 MGB & 85 Buick (until I finish the MG)

"An object at rest cannot be stopped." The evil midnight bomber 
what bombs at midnight    FROM "THE TICK"


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