MG Sports Cars

Hello I am doing a V8 conversion on a 63 MGB and have been doing alot of research and reading on the how to and other info. But I have never really came across what brake master cylinder size I should use. I believe stock has a 1" master cylinder. I am sure that it is not that simple so I was wondering if any body out there has some info or a way to go about calculating what size my master cylinder should be. my V8 will have disc brakes on all fours and it is a 63 so I am using the old style pedal box so room is a issue and I know I will be using a porportioning valve to help. Can I use the original can? I am just learning on all aspects of what goes into a car so any info I recieve or routes I go to be able to find out what I need will be greatly appreciated. thanks in advance

Hey, NB welcome to the site. Another conversion in KC that's great. Have two running V6s now and two more V6s under construction in the area. Bill Guzman at Classic Conversion Engineering has the low down on a good master cylinder conversion that should work well for you, worked find on the Roadmaster with larger discs up front and Jag discs in the rear. I believe it's a Corvette unit, but he can tell you more or check his website.

It's probably too early to decide on m/c size. Until you've picked both front & rear calipers you don't know how much fluid you'll be displacing. Net, there's not enough info to guide a m/c choice.

Assuming the '63's pedal box is the same as the later non-servo pedal boxes, you're right it's a tight fit. Several people have used the later OEM chrome bumper (non-servo) dual circuit master with residual pressure valves. It works, but fluid volume might be marginal.

As an example, my front brakes are wilwood 4-pot calipers & my back calipers are from a camaro. The MG dual circuit m/c wasn't big enough. My pedal went right to the floor w/system fully bled. I found 15/16" works best for me. I have a '96 Mazda Protege master with an Isuzu reservoir like this one [www.rockauto.com]. You need a reservoir that is fairly low & even lower on the back half where it goes under the cowl. That Isuzu reservoir is used on a lot of their cars from the 80's & you can often find them new on ebay. The reservoir is a close fit but doesn't seat all the way on the Mazda master. I've had it on for several months and there"s no seeping around the grommets, but I've since found the Isuzu reservoir fits right on an '89 Toyota Camry m/c that's also 15/16. The Toyota mc has a triangular mounting flange that will need a little trimming to fit the pedal box, but if you end up needing 15/15, I think the Camry/Isuzu combination is the ticket.. I'll be swapping my m/c for that one before much longer.



Edited 1 time(s). Last edit at 01/15/2012 11:10PM by rficalora.

I am using 99 Mustang rear calipers and I am using the toyota 4runner 4 pot calipers up front. I have the engine finally placed, cuts made to the firewall, crossmember and I will be cleaning the engine bay here shortly so I was just looking ahead so as I am doing work I can keep my eyes out for a MC that was a good deal and save some money, plus when I remove the pedal assemble and clean it I would like to have everything mounted . Rob, I have read your thread reguarding the MC and thought this might be a pain so that is why I am asking now. With reguards to master cylinder size what would be the best choice and why?

While I am talking about MC, How would a Wilwood 3/4" MC and a 7/8 Push style slave cylinder work for a t-5 Clutch set up. I have seen alot of people using the 5/8 CNC, so how would the 7/8 speedway slave compare.

I have been taking pictures of my work and will try and do I project journal here in the coming weeks to document my progress, if anyone would like to follow along

Thanks again for any help or response given

I made a complex spread sheet some time back. It calculated things from pedal arm, MC movement, MC bore, volume, throught to tire diam, brake diameter, brake pad size , co of friction, weight of car, balance, and a dozen other things. Keep in mind my car uses larger Wilwood brakes then stock both front and back.

I came up with a 7/8" as ideal for mine, but I oped for a 1" Wilwood which was close. Brake is good but I think if I came up with a 7/8" it would be imporved a bit. I calculated balance and I was very close bassed on my cars configuration and weight distribution. I put in a balance valve but when it was done brake balance was/is near perfect.

The main things to remember, a larger MC will move more fluid....which means LESS pedal travel, more pedal force requires, less pedal feel.

A smaller MC will move less fluid with more leverage meaning easier pedal effort and more pedal feel or control.

Too small of a MC will mean not enough pedal to reach full braking...but darn the pedal moves easy!

If you calculate the total surface area of the stock brake cyinders and front calipers and the surface area of the stock master cylinder then keep that ratio in mind for the new system you should wind up with a comfortable pedal effort and good brake action. After you select the brake calipers you are going to use, calculate their total piston surface area then select a master cylinder bore that gives you as close to the same ratio as the stock system had. There's really no need to have to guess if you follow the orignal ratio with the new setup.

Bill, do you know what that number would be?

Also, when they went to the dual circuit M/C the pressure gets split between the front and the rear and it woulkd be good if we knew what the proportion of the split is. (50/50?) Then too, the resulting proportion is likely to be much different for drums and disc.

Jim

Jim, it depends on the car, length of the brake pedal, pedal ratio, etc. so it varies from car to car. But the principal is the same for all hydraulics, force coming out of the master cylinder in psi is transmitted to the calipers and wheel cylinders at the same pressure, the mechanical advantage comes in by the different surface areas between the master cylinder and the calipers or wheel cylinders. As long as you keep that ratio about the same your pedal pressure and travel won't change much.
If you have a ratio between the master cylinder area and the total front caliper area of 1:2 then 50 pounds of force on the master cylinder will result in 100 pounds of force on the calipers. If you increase the caliper area by 50% then you'd need to increase the master cylinder area by 25% to keep the ratio. Surprisingly most cars are very close in master cylinder bore size, usually somewhere between 7/8" to 1" so we tend to sort of automatically stay out of trouble when we upgrade brakes and the systems work ok, maybe a bit more pedal travel or such, but drivable and safe. Start getting into larger 4 pot or 6 pot calipers and you'll get so much piston area that you have to upgrade your master cylinder. MGB stock calipers have a pretty big piston area anyway, as do Midgets and other British cars with the older style 2 piston calipers so we have fairly large bore master cylinders. Where we can run into problems is when we change from an opposed piston type caliper to a single sided caliper. There the area is about the same, but you may need more volume to move the piston over enough to engage the pads, but in general those are usually pretty compatable. That's what we did on the front of the Roadmaster as I remember and the original master cylinder would do the job, but we wanted a bit better feeling pedal so we upgraded to the cylinder Bill G recommended.
As for proportioning in a dual master cylinder, I'm not sure on that one. Generally the bore is the same for both pistons, so essencially it should be a 50/50 split, the engineers compensate with cylinder surface area to tailor the brake bias. Most have some type of slip mechanism to allow for the different volume requirements between front discs and rear drums or even the smaller caliper area front to rear.

That's the part that I find a little hard to unravel. The older cars used IIRC a single 3/4" MC but then I'm pretty sure they went to a larger bore with the dual MC, which in and of itself seems quite odd. On the RM I seem to recall that the Dodge Dart calipers had more piston area than stock, but not a whole lot, maybe 1 or two square inches or somewhere around 20-50%. Of course the rear Jag calipers are entirely different and they are 2 pot but small bore. I believe Bill's GM MC was 1". It all worked out quite good, but the MC was for a front/rear disc application and that is probably why. The Dart probably used a 1" MC, and it's likely the Jag XJS did too. That makes it easy.

On my car, with a balance bar and dual 11/16" MCs and Mustang calipers (which again probably used a 1" bore for the MC) and stock MG rear brakes, I had to shift the balance all the way to the rear to keep the fronts from locking too soon. Jag rear brakes should improve that situation but I really won't know until I try it. However it should be pretty comparable to the RM.

So I'd say if you are going to change your brakes, try to be consistent with the bore size of the donor's MC, and if you can, use donors for front and rear that used the same size bore on the MC. Then if you use drum brakes on the rear use a drum brake MC and likewise for discs. That just makes everything much easier.

If you go beyond those guidelines you are really pretty much on your own.

Jim

Jim most went with the dual master cylinders when disc brakes came along, bore size increased due to the larger pistons in the disc calipers compared to the drum wheel cylinders. Brake bias was altered by using larger bore wheel cylinders in the rear. Similar thing on MGBs where the roadsters had smaller bore cylinders than the GTs which were used to compensate for the increased rear weight bias of the GT. I had a small mismatch problem on my Midget after installing the GM rear axle and brakes. The cylinders were slightly larger and the drums larger requiring more fluid volume to engage the brakes and my pedal had far too long a travel to feel comfortable even when the system was fully bled. I installed a residual pressure check valve to keep the rear drums expaned a bit more and it did wonders. The cylinders were a compatable size but the larger drums were the problem.
Unless you have a stepped bore master cylinder you really can't get pressure differences between the front and rear outlets, you only have one pedal pressing down and the piston surface area is the same so the pressure has to be the same as well. All bias is engineered into the system by selecting relative bore size on the calipers and cylinders. For a highly modified system where you've altered the factory balance in the extreme by installing larger rear brake cylinders or calipers then about the only way to really address brake bias is through a balance bar like you have on your car and even different bore master cylinders if necessary.

Luckily for those still using the stock rear axle, the slaves are available in at least 2 or 3 bore sizes. Often this can compensate for larger calipers in the front but still means there is a need for a larger MC.

Seems to me that split systems came into use around '68 or shortly thereafter, which was before discs were common domestically, and about the same time on the MGB which of course already had front discs. Around '71 or '72 front disc brakes became standard on many US cars. There wasn't that much correlation between the two, as the split system was created for redundancy in the braking system in case one brake went out. I had a '63 Plymouth that had one adjuster that kept popping out and let me tell you that was a real thrill ride! By contrast we had a '68 Impala that popped a brake line and still stopped. That dual master was worth it's weight in gold.

But it still isn't that clear cut when going from drums to discs in the rear during a conversion because , entirely independent of the pressure, the volume requirements are quite different as you've alluded to, and the friction characteristics of the disc and the drum are also quite different, as the drum shoes are "self actuating" and therefore require less force to generate friction. Getting all this to balance out when switching from one type to the other is anything but straightforward. In which case, as I mentioned it's best to stick with component sizes from a known matched system of the same type. So if you use RX7 calipers for instance, try to use the RX7 sized MC and similar rear brakes if possible. Later RX7s undoubtedly have 4 wheel discs but I think earlier ones had disc/drum like most of the cars of that class (and dual MCs). And the same would apply in other applications. Now if you're converting to something like Wilwood, then like you said, calculate the area and multiply everything by that ratio. Unless you are going to rear discs and then you'd want to find a similar system to emulate. In a complicated case you could use the sizings of an OEM disc/disc system, calculate the ratio difference between that and your front calipers, then calculate back using that ratio to determine ideal MC and rear caliper sizes. And then hope you can find suitable rear calipers in that size. It may or may not be easy to do that last step.

Jim

NB, The Toyota FourRunner calipers and Mustang rear calipers are very similar to Rob's Wilwood and Camaro setup in piston size. I think the 15/16" would be ideal. 7/8" would have more pedal travel/more pressure. 1" less pedal travel/less pressure. Goldilocks effect. Probably need a proportioning valve for rear. Oh,by the way chrome bumper dual master is 3/4",rubber bumper 7/8". You could also do a twin master with balance bar like Jim B. Jeff Schlemmer has an article on this site( [www.britishv8.org] )about changeing the pivot point(more leverage) and going a size smaller on the twin masters. What are you using for rotors? Looking forward to pics.



Edited 2 time(s). Last edit at 01/17/2012 01:38AM by mgb260.

They did change the pivot point, probably at the same time as the MC bore size increase and relocation of the MC. It's been so long since I've looked at a dual MG MC I couldn't remember the size. With modern brakes it makes sense to go larger.

Currently I still have the Mustang rotors on front, Jag XJ6 at rear. But I have a set of Outlaw calipers to fit the front once I find some light weight slip-on hats and rotors. Been looking for 24mm x 11-12" scalloped but so far no dice. Maybe RX7s will eventually have them.

Jim B, Here is an idea for you. Double up two solid scalloped rotors(.350) with a skeletonized 1/4" steel spacer. Spacer would look just like rotor but would have milled slots where the holes are. Mount that to a hat and you have a ventilated,scalloped rotor almost 1" thick(.95).




Edited 3 time(s). Last edit at 01/17/2012 11:57PM by mgb260.

Jim N., I can't image that combination not warping terribly with the first application of heat, but you never know. Jim B, why do you need the 24mm thick rotor? Just because of the caliper you're using? How about making some spacers to fit behind the pads to space them in so that you can run a bit thinner rotor which is available and would reduce unsprung weight at the same time.

Bill Y., I don't think it would warp but I don't know. I mentioned to Jim B. the stainless shims behind the pads to use a common .81 " rotor or 1/4" aluminum spacer in the calipers for the 1.25" thick rotor. He was pretty adamant that he didn't want to do that and he seems to like the scalloped rotor look.

Thanks Jim, that actually is a pretty neat idea. But there are a few cars that do use 24mm rotors. Mitsubishi, BMW, and Porsche, as well as some RX8s and Volvo S40 and V40s. So I'm still hopeful of finding an aftermarket supplier who has the pieces I want at a price I can afford.

Jim

re: Jim B, Here is an idea for you. Double up two solid scalloped rotors(.350) with a skeletonized 1/4" steel spacer. Spacer would look just like rotor but would have milled slots where the holes are. Mount that to a hat and you have a ventilated,scalloped rotor almost 1" thick(.95).


I seem to remember Ted Lathrop using a pair of these rotors on the rear brakes of one of the conversions he did several years ago.

I'm with Bill on this idea. It might work fine but why go to that much trouble if a 24 already exists. I'm ever more convinced it should be there somewhere, just the use by Porsche and BMW would indicate it.

Jim

Since you said you were looking for scalloped discs I've been doing some research. Seems that most of those are used by the Sprint car guys and are custom one off pieces for existing Wilwood or similar calipers where weight it critical. I've seen lots of drilled, slotted, and other types of rotors for BMWs etc, but not scalloped ones and even in the light weight classes in SCCA type racing no scalloped rotors found so far. You may well either have to use a thinner rotor or settle for a standard vented type.