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Re: Brake Trouble! UPDATE!

To: MG List <>
Subject: Re: Brake Trouble! UPDATE!
From: Max Heim <>
Date: Thu, 12 Oct 2006 15:26:56 -0700
Not so. Early Bs had single MCs but still had disc brakes. Post-65 American
cars with drum brakes use dual MCs. The dual circuit is a redundant safety
feature, not inherently related to disc brakes.

That said, manufacturers tended to take advantage of the dual circuits to
optimize the front circuit for disc brakes (by increasing the size of the
reservoir); and often adding a residual pressure valve to the rear circuit
(in disc/drum systems), to ensure quick actuation of the rear shoes.

I believe Saab used a diagonally-split dual circuit (left front/right rear),
which would have necessitated a different approach to the MC design.

My experience with failures in dual circuit systems is that a burst flex
hose in the front circuit will drop the pedal considerably and reduce
braking action enormously; the same failure in the rear hose has a similar
effect but not quite as drastic. Either one would leave you in "limp home"
mode, at best. Clamping off the open line might restore some pedal.

on 10/12/06 1:57 PM, Charles & Peggy Robinson at wrote:

>    I believe the reason for the dual master cylinder in the so-equipped
> MGB is that the car has disc brakes in front and drum brakes in the
> rear.  This necessitates a different size piston in the master for each
> type braking circuit.
>    That being said, I agree that each circuit should have a firm pedal.
>   My take on the OP's problem is that the rear circuit was clogged up
> and the front circuit wasn't fully bled.
>    CR
> Paul Hunt wrote:
>> My subsequent thoughts also.  As I say I have no experience of dual
>> circuit brakes with one circuit failed, but logic dictates that surely
>> the whole point of the system is that if one circuit is leaking and not
>> developing any pressure, or is full of air, the other circuit *must*
>> give a firm pedal albeit it with increased stopping distances

Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires

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