When doing a 5-lug, 4-wheel disc swap on a Fox Mustang the number one question we field is “What do I have to do with the master cylinder & proportioning valve?”. Unfortunately, there is no one size fits all answer, as it depends on what brake setup you’re using. For our SVE Brake Conversion Kit we tried a few different options and landed on the 1993 Cobra Master Cylinder as it provided a pedal feel that appealed to the most drivers. Follow along as we swap out master cylinders and modify the proportioning valve on Lee’s coupe!
Start by removing the stock master cylinder. You’ll want to pack a bunch of paper towels and/or rags in the area underneath as brake fluid will be going everywhere. Unplug the low fluid sensor on the master cylinder reservoir. Remove the two nuts that retain the master cylinder to the booster. Then, using a good set of flare wrenches (preferably name brand, like Craftsman), loosen the two line nuts that attach to the top of the proportioning valve/distribution block and also the one line nut that goes in to the bottom of the master cylinder. Carefully remove the master cylinder from the car. Quickly clean up any brake fluid with soap and water so it doesn’t damage your paint.
Now, it’s time to “gut” the stock proportioning valve/distribution block. Probably a good time to swap out paper towels! Take a 3/4″ wrench & remove the large hex head plug on the front of the proportioning valve. Be careful as there is a spring behind the plug that will cause the plug to shoot out if you don’t watch it. Remove the spring and shuttle valve from the hole and discard. Remove the larger o-ring from the stock plug & transfer it to the solid proportioning valve plug included in the kit (The plug is a LatemodelRestoration.com exclusive!). Install the new Solid Plug in the proportioning valve. Basically, what you just did, was defeat the stock mechanism for rear brake pressure bias. This is why you have to add in an adjustable proportioning valve. On the passenger side of the firewall, you’ll find a union in the hard brake line that runs to the rear of the car. Remove this union and install the included adjustable proportioning valve. Screw the knob on the valve all the way in for bleeding purposes.
Remove your new master cylinder from the box & bench bleed it. If you don’t know how, there are several good articles and videos that pop up via a google search. With the new 93 Cobra Master Cylinder bled, transfer the stock front hard line from the stock master cylinder to the new one. For the rear port, install the 3 port to 2 port conversion as shown below. The hard line on the car that runs to the left front will screw in to the bottom of the T-fitting of the conversion kit. Install the new master cylinder & tighten all connections. The stock retaining nuts can be reused.
I’ll take a minute here to discuss brake boosters. Many people will tell you that a booster replacement is required. It isn’t. Pedal effort is very relative & dependent on the driver of the car. If you feel the pedal effort required to stop is too great, then adding a 1993 Cobra Brake Booster may be in the cards for you. As stated earlier, we chose the 1993 Cobra master cylinder as the pedal effort required to stop the car was acceptable to the most people who drove the car.
Add fluid & bleed your brakes! Start with the passenger rear caliper, then move to the diver rear caliper, then the passenger front, and finally the driver front. Here is a great source for info on properly bleeding your Mustang brakes: How to Bleed Brakes – The Right Way A quality DOT 3 brake fluid, like Motorcraft PM1C, would be a great choice for your Mustang. Periodically check the fluid level in the master cylinder while bleeding and top off as needed. You should also check for leaks and tighten any fittings if a leak is present. Now, go back to the adjustable proportioning valve. Unscrew the knob all the way to its outer limit. Once there, screw it back in four complete revolutions. This is your starting point. Based on your feel, you can increase pressure or decrease pressure as needed. The valve comes with instructions for adjustment, & it’s really not too hard to do.
Once you’re done, put the wheels back on & enjoy!
Lee’s Coupe Build Links:
Check out the rest of this project:
- Mustang SVE 5 Lug Disc Brake Conversion: Part 1 – Front Brakes
- Mustang SVE 5 Lug Disc Brake Conversion: Part 2 – Rear Brakes
- Mustang SVE 5 Lug Disc Brake Conversion: Part 3 – Master Cylinder & Proportioning Valve
- Mustang SVE 5 Lug Disc Brake Conversion: Part 4 – Track Testing