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MG MGB GT V8 Factory Originals Technical - Does enlarging the m/c bore make sense?

I've got a question about the bore on the master cylinder that might be of some interest to those of us who are building V8's with upgraded/improved brakes. I just picked up a Ford 8 inch posi rear end with disc brakes. My intent is to use the Ford's disc brakes on the rear and install Porsche discs on the front with an appropriate set of calipers. Finally the question. Given that the brake master cylinder bore of my 78 B will probably not be sufficient to operate the brakes at their peak performance, if at all, would it be reasonable or practical/safe to increase the bore of the orginal B master cylinder to compensate, thus allowing use of the original master cylinder? Any thoughts?
Dan Bogdanski

"Given that the brake master cylinder bore of my 78 B will probably not be sufficient to operate the brakes at their peak performance, if at all..."

What does this mean? Can't deliver enough pressure to the calipers? If you increase the m/c bore you will reduce the pressure even more.

Paul Hunt

If the bore of the m/c is not the determining factor at to the efficient mechanical actuation of the calipers and thus the amount of pressure applied to the brakes, then what is? the stroke? the volume of fluid being pushed? the pressure at which the fluid leaves the m/c? My initial take on the point was that the more volume one attempts to push through the m/c the more pressure, since the lines remain the same size, thus one wouldn't get a "soft" brake pedal or one that catches at bottom of its travel.
Dan Bogdanski

Paul is correct. Moving a greater volume with the same mechanical advantage and leg pressure will lower the hydraulic pressure, not raise it. Your leg strength will not magically increase to compensate. You would have to incorporate a servo system to reduce the effort, but then you lose feedback. Servo-assisted brakes also don't stop too well if you lose power, but everything is a trade-off.
George B.

The determining factor is the volume of fluid that the new calipers require to go from rest to brakes applied.

The master cylinder must be able to deliver this amount as an absolute minimum.

Within this limit the smaller the M/C bore the easier the brakes will be to apply but the greater will be the travel of the pedal.

I suggest you keep the M/C bore standard until you have the set up installed and then see whether you want to change the bore.
Chris Betson

The balance between master and slave bore sizes dictates the work achieved. Then the stroke will dictate the volume of fluid that can be displaced before the pedal is at the limit of travel. You have to match the two demands together and further add the degree of pedal travel you wish (allowing for a degree of free play for clearance of friction linings when not braking)

You then further have to consider the efficiency of the brakes at front and rear. There is a need for a very clear bias for more effort at the front and a big efficient rear disc set up is very easily going to lock the rear and create instability. A pressure restricing valve may be needed in the line(s) to the rear brakes.

This whole area is one that is full of potholes and trip wires, tread very carefully.

Roger Parker

Gee I'm not sure I should get involved in this at all...


you really need to get hold of Fred Puhn's The Brake Handbook and read this to build up your understanding of brake design before you go forward with this project.

Your 78 will have a tandem master cylinder with a direct acting bendix type servo. this will push out an equal presuure to to both the front and the rear circuits. Paul is right in that you should not assume that the original mc will not be able to produce the line pressure but you are correct to say that it may not have enough displacement should you you choose 4 pot calipers which will use more fluid during operation.There are other limiting factors:pad centre to wheel centre/wheel radius/brake torque,coeffecient of tyres,C of G of car and change of weight balance under braking , servo ratio,servo pressure curve, design pedal pressure,pedal stroke and pedal ratio .Once you change the calipers both front and rear you change the brake balance -given equal line pressure it is the caliper piston sizes that set the basic brake balance.
I could expand but I think that you should read the book and understand the chapters on brake design before proceeding any further. I would not like to comtemplate the lawsuit that you could get involved in should this go wrong and personal injury ensues....
John Bourke

Thanks to everyone who responded. This is the kind of information that makes this BBS so very valuable. John, I will pick up Fred Puhn's book and begin my formal education. Roger, I respect your concern for safety in all aspects of V8 conversion projects and will keep your conmments close at hand during the build up of the braking system. I will keep you informed as the project develops. Now, back to school.
Dan Bogdanski

I still have too much pedal travel for my liking using 4 pot Wilwoods up front. It has been suggested to me that this may be caused by the lips on the calipers actually being stronger than on a two pot systems and thus requiring more effort for the brakes to be applied. Has anyone experienced this problem.

Also I know in the archieves Roger has already set out calculations for bore size, but has anyone uprated their master cylinder and what was the effect? Must order my copy of Fred Puhn's book.

Paul Wiley

I went already up that way with my brakes. I have the Porsche 944 rotors installed on the front and used Wilwood Dynalite 4 pots calipers with the original MGB master cyl. This set up works great! In the rear I still have the drum brakes but IMO if you go with discs you should not have to add much more oil in the system than with drums as the discpads are in very close contact with the rotors.
I am working on a Volvo rear end disc brake set up.
As other members told. Keep the original M/C and see what it does on the new set up and make than your mods.
Hope this helps
werner van clapdurp

The smallest M/C as possible should be used. A larger M/C will increase brake pedal travel.
The point that is important is the pedal ratio. Pedal ratio can be changed but will also increase pedal travel.
Balancing the brake bias between the rear and the front, top quality brake fluid, frequent flushing of the system, the use of light wheels, good tires with correct tire pressure, and good cooling of the rotor, can make a bast improvement to the brakes, this alone will improve the brake efficiency of any B.
Heat is the worst enemy of any brake system, so anything done in this area is a plus.
If you are using Ford rear disc brakes and Wilwwods at front, the m/c must likely is going to be the same size as the B, the question is, is travel of the m/c cylinder appropriate for the propose set-up? And the answer is yes.
Bias valves can be purchase from Tilton, Wilwood and others, they come in inboard mounting (lever type) can be adjusted from inside the car or engine bay mounted with a screw type adjusting capability.

Bill Guzman

To reiterate, a small m/c bore results in a longer pedal travel than a large m/c bore on the same system. However a small m/c bore will develop a greater line (and hence wheel cylinder/caliper) pressure than a large m/c bore for a given pressure at the pedal, at the expense of a longer travel.

Paul Hunt

See also the thread here on Princess calipers.

Roger Parker

This thread was discussed between 20/01/2001 and 28/01/2001

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