MG Sports Cars

engine swaps and other performance upgrades, plus "factory" and Costello V8s

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worn
Warren Bond
Toronto
(26 posts)

Registered:
06/28/2013 09:56AM

Main British Car:
1973 MGB 305 Chevy

Brake Servo
Posted by: worn
Date: December 08, 2013 10:28PM

My MGB has a TR7 servo and master cylinder installed by the PO. The MC works fine but I doubt ever rebuilt, and the servo is leaking and brake fluid is getting sucked into the intake manifold. Simple, replace or rebuild but I'm looking at aftermarket master cylinders and 7" servos made for hot rods. They look good, are inexpensive and I can't see any real problem to adapt, but they have 1" bores. Anybody running 1" bore master cylinder with stock brakes? I believe the pedal pressure required will be quite high, am I right in thinking the servo will even things out and give good pedal feel and braking?
Thanks



Edited 3 time(s). Last edit at 12/08/2013 10:48PM by worn.


88v8
Ivor Duarte
Gloucestershire UK
(682 posts)

Registered:
02/11/2010 04:29AM

Main British Car:
1974 Land Rover Lightweight V8

Re: Brake Servo
Posted by: 88v8
Date: December 09, 2013 04:06AM

No, it will be a disaaaaaaster...
Depending which master you have, the bore diameter is 0.692" (early) or 0.75" (late model).
Effort is proportional to the piston surface area, which is 0.38"sq and 0.44"sq respectively.
A 1" master has a piston area of 0.79"sq.

So, that's 0.79 compared to 0.38 or 0.44, say around double the pedal effort for the same braking. No, the servo will not sort that out. You could perhaps fit a second remote servo in series, but there you're into unknown territory afaic.
Sorry !!

OTOH it would do wonders for your leg muscles.

Ivor


mstemp
Mike Stemp
Calgary, Canada
(197 posts)

Registered:
11/25/2009 07:18AM

Main British Car:
1980 MGB Rover 4.6L

Re: Brake Servo
Posted by: mstemp
Date: December 09, 2013 07:57AM

Warren,

I am running a hot rod booster and Ford 7/8" mc wo issue. Not sure why the booster would force you to use a mc of 1". They are a standard size where it protruded into the booster I thought.



Edited 1 time(s). Last edit at 12/09/2013 11:55AM by mstemp.


worn
Warren Bond
Toronto
(26 posts)

Registered:
06/28/2013 09:56AM

Main British Car:
1973 MGB 305 Chevy

Re: Brake Servo
Posted by: worn
Date: December 09, 2013 11:41AM

The reason I was asking about 1" bore master cylinders is because so far all I've come across are 1" dual line master cylinders and the 3/4" have all been single line.
But, if the size where it mounts to the servo and the bolt spacing are the same, gives me a wider range to look at.


worn
Warren Bond
Toronto
(26 posts)

Registered:
06/28/2013 09:56AM

Main British Car:
1973 MGB 305 Chevy

Re: Brake Servo
Posted by: worn
Date: December 09, 2013 04:08PM

This looks like the answer
[www.britishv8.org]


88v8
Ivor Duarte
Gloucestershire UK
(682 posts)

Registered:
02/11/2010 04:29AM

Main British Car:
1974 Land Rover Lightweight V8

Re: Brake Servo
Posted by: 88v8
Date: December 10, 2013 02:26AM

Changing the pedal leverage has the same effect as changing the hydraulic leverage by fitting a different master. Except in this example the change is in a helpful direction, compared to that m/c change which wasn't.

In my 74 Landrover (V8) I fitted larger wheel cylinders with a stock m/c, and that had the effect of increasing the hydraulic leverage so I had less pedal effort, but, more pedal travel. The factory could have done that but it meant more frequent brake adjustment to keep the pedal off the floor and would have conflicted with their desire for longer service intervals.

Just to mention, the same applies with that pedal mod you flagged; more leverage means more pedal travel.

One can do the same sort of thing with the handbrake lever to increase the power of the handbrake, but the brakes have to be adjusted more frequently so as not to run out of travel.

Ivor


classic conversions
bill guzman

(294 posts)

Registered:
01/09/2008 01:58AM

Main British Car:


Re: Brake Servo
Posted by: classic conversions
Date: December 10, 2013 11:36PM

Changing the pedal ratio has different effects. that is nothing new.

If you have the late pedal box then you are home free, you can use a 7/8 MC from Wilwood or an jap import.

!" MC would be ok with 4 piston calipers front and rear. The pedal ratio of the MGB with a servo is 4:1 the ideal is a 6:1 or a 7:1 which would be the same as using a booster. For racing purposes the booster is replaced with a change in pedal ratio it saves weight.

This is my RD and the one I am building. I would recommend using a GM type equalizer valve for the brake bias which is important.

Here is the formula:
Pedal ratio is the ratio of leverage you brake pedal applies to the master cylinder. To determine the pedal ratio you need to measure the height of the pedal to the pivot point then divided the measurement of the pivot point to the lower arm that controls your rod to the master cylinder.
A = height of pedal
B = center to center measurement of the lower arm
C = pedal ratio
A divided by B equals C
Or example 9" divided by 1.5" equal 6 to 1 ratio.

Also there is a difference between Master cylinders; A drum master will differ from a disc brake master in two ways: The amount of fluid that a drum brake master moves is less than that moved by a disc master and drum masters have 10 lb. residual valves to the drums. If you use a drum master for disc brakes you would move an insufficient volume of fluid and the disc brakes would drag because of the residual valves.
Pedal box and air cleaner 005.jpg



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