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MG MGB GT V8 Factory Originals Technical - opinions on this brake mc

I will be using an 8.8 with disc brakes at the rear, and stock mgb brakes in front for now. My stock ('74) master cylinde is shot and I've been looking at this aftermarket unit:

I like the way it looks, and I also like the integrated proportioning valve. Can anyone give me an opinion on whether the 1" bore will work with the rest of my brake system? I have a rubber bumper pedal box and I think that this mc will mount with little trouble, I just am concerned about the bore size.

Ryan Reis

Ryan, too large a bore m/c will result in a hard pedal with very little travel, too small a bore will give you a soft pedal with perhaps too much travel. Try to stay with the same size bore as original unless you increase the pistion size on the calipers to larger than original. I noticed quite an increase in pedal travel just by changing the rear axle and using wheel cylinders that were only 1/8" larger in diameter over stock.
Bill Young

Is 7/8 the stock bore? I would just find a m/c with the stock bore, but what effect will changing from stock rear drums to the ford discs have? I don't have the funds to do trial and error at $100+ per try!

If Graham Creswick sees this message, I think his brakes are similar. If you're out there, what m/c are you using?
Ryan Reis

Ryan, you would need to calculate the total piston area of the drum brake cylinders and compare it with the piston area of the discs. If you come up with a significantly higher area with the discs then you probably would need a slightly larger master cylinder bore. What you need to think about is the total volume of fluid movement needed to apply the brakes. Way too much math for me to figure out here, but you can do the math and get a pretty good idea if you need a larger master cylinder or not. You need to find the pedal ratio for the pedal box you are using and then get the distance you want the pedal to travel. If you want 3" of pedal travel and the ratio is 11:1 then you would be getting only about .27" of travel in the master cylinder. This travel multiplied by the area of the piston will give you the total fluid volume displaced. Since it's a dual system, both front and rear will have the same volume available. If the rear disc pistons require more than this volume to engage the rotors from rest then you need more m/c volume.
Bill Young

Ryan, Bill is correct.
If you are using 4 wheel disc, do not use a MC smaller than 1 inch with 6:1 ratio. The stock RB pedal box pedal has 4:1 ratio. By re-drilling the hole 1 inch higher (closer to the pivot) will give you a 6:1 ratio
You will not need the power booster.

You will need to install a equalizer valve to control the bias from front to rear and re-flare the brake line pipes to fit the new MC.
Bill Guzman

Please let us know if you elect to try this MC and if it works. I too am installing rear disk and have uprated Wilwood calipers on the front so am very interested in your results as well as Bill Guzman's response.

All of the above comments are quite correct. I have the Ford 8.8/disc brake axle, a 1" bore aluminum M/C from an early 80's Lincoln and a separate Ford proportioning valve hooked up with a std MGB late model pedal box @ a 4:1 pedal ratio....The effort is higher than I would like although the basic system performs great!
This Winter I will be reworking the pedal ratio to a 6:1 as per Bill's must have decent brakes while attempting the Dragon next June.
A word of caution: when you relocate the pushrod higher on the pedal arm to get the 6:1 ratio, make sure that you don't raise the M/C up into the hood/bonnet - there isn't much clearance to play with......
Graham Creswick

Thanks for the info guys - I think I'll try the CPP m/c. I'm also going to price a m/c like Graham's, it is a nice looking unit. I probably won't have any feedback for anyone for quite a while! Lot's of work left to do.

Ryan Reis

I am using a early corvette mc same as Ryans internally. I have Disc/Disc which are Wilwood.
Pedal is great, pedal feel is great, etc. I also use a equalizer valve made for Disc/ Disc. Room is limited as Graham pointed out. The diference can be split by the angle of the rod, 1/2" angle is not a problem.
You may also use a 1 1/8 mc which will give a higher pedal.

Pressure as tested is as follows

mc 1 inch with 4:1 ratio and 75 lb pedal pressure= 390 psi out

same as above with 6:1 ratio 580 psi

with 1 1/8 mc 75 lb pedal pressure 4:1 ratio= 322 psi

same as above with 6:1 ratio 462 psi

When I drive the car on the track for testing I change the pedal rod location to the 5:1 ratio to better control pedal pressure. On the street is a matter of preference between 5:1 and 6:1
The brake system can be taylor to your liking.

This mc's are inexpensive $69 an up.

One concern is where do the lines exit the mc, passenger side makes it easy to install the line, driver side.....a bit dificult.
Bill Guzman

Bill Guzman,
I am very interested in your experience with regards to the no brake booster. I have a 79 MGB that is supercharged and if I could get rid of the brake booster as you suggest that would open up more options to me with regards to a cold air intake set-up to the face of my carb allowing more room between the carb and the firewall/bulkhead. Can you expound some more on the pluses and minuses of having versus not having the booster. Thanks for any info you can impart to further enlighten me on this subject.

Frank, the idea of power brakes in the MGB was more of a sales point of wiew. The MGB small booster works pretty good, but changing the pedal ratio from 4:1 to 6:1 has the same effect as power brakes.
The power booster helps on the pedal pressure thus making it easy on your leg to apply 75 to 100 lb of pressure.

If we were talking about a big heavy car, then the booster is a much needed item due to the pedal would have to have at least 7:1 pedal ratio. To accomodate this pedal to give you the needed pedal pressure, you would need to make a hole in floorboard to accomodate the poedal etc.

If using dis/drum you will need a equalizer valve for the dis/drum set up in your car. You will eliminate the present valve in the B Think about Wilwood mc they have one that is 7/8 that will work great with the disc/drum.
Bill Guzman

This thread was discussed between 18/11/2005 and 22/11/2005

MG MGB GT V8 Factory Originals Technical index

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