QuestionHi Howard,
I am converting a 1957 MGA to four wheel disc brakes, and I would prefer to use the original pieces, and DOT 4 brake fluid, but I have some questions:
1. I recognize with four wheel disc brakes the need for increased brake fluid reservoir capacity. Is it possible for me to bolt on the larger capacity reservoir cover from the later MGA models onto my 1957 master cylinder? Should I use an aftermarket master cylinder instead?
2. Is DOT 4 fluid compatible with the seals (Moss Motors' rebuild kit parts) in the MGA's master and clutch slave cylinders? Would I be better off with using aftermarket master and clutch slave cylinders?
Thank you in advance for any and all information you can give me.
Regards,
Dan
AnswerHi Dan,
A larger volume of reserve fluid is not the problem. A larger volume of fluid from the brake master cylinder piston will be the problem. Even though the pistons in a caliper don't move very far they do displace and use a large volume of fluid and if you have a master cylinder that can not produce the required volume in one pump of a pedal you can end up with a system that requires more then one pump of the pedal to get any braking.
A caliper piston is not up tight against it's brake pad, but is slightly retracted by the caliper piston seal after the brakes are released. Even though the movement is slight, the volume of fluid that is moved is relatively large.
You have a additional problem in that your master cylinder is a dual cylinder, having the clutch cylinder in the same casing. I don't have any of the bore diameters of the early drum brake "A" verses the later disk front brake "A" but you should check with Moss or other parts suppliers to see if the master cylinder was a different bore for the early "A" vs the later "A".
I believe the later "A" disk front brake system is a "Bolt-on" so all you need there is a later model MGA brake and hub with disk. If you find that the later "Disk brake" MGA had a different master cylinder, put one of them in also and then if you are going to modify the rear brakes to put disks there too be sure to get a smaller bore caliper so as not to require a large bore master and help keep the front/back braking ratio correct. (about 80% of your braking ability is on the front brakes and they must lock up first or your car will be very difficult to handle when braking hard.)
The ratio of braking will be different and if you end up with a system that brakes more on the rear then the front you will have a very dangerous system, but even that can be corrected by use of an adjustable differential valve available from most hot rod and racing parts suppliers.
It is a complex design to try to do it mathamatically using the diameter of master cylinder, leverage on the master cylinder piston by the pedal, by diameter of caliper piston, number of pistons, diameter of disk vs weight distribution of the car itself. It would take an experienced design engineer to come up with all the correct sizes of everything you will need.
But by looking at other working systems, you will have a rough idea of what sizes are needed and if the end result gets you an over powered rear brake you can just add an adjustable ratio valve in the line.
I am working with this problem now with a MGB-GT V-8 conversion I have built as I have switched from the standard 2 piston MGB disk front to a 4 piston Willwood caliper on the front and a US Brake 2 piston caliper on the rear replacing the standard MGB drum.
The Dot 4 fluid is the better fluid and can handle higher temperatures then DOT 3. Dot 5 is ok for show cars that are not used much but friends in the business advised me not to use it in a every day driver.
Let me know of your progress,
Howard
