Fox Body Master Cylinder Upgrade For Disc Brake Conversions

For 1979-1986 Mustang owners wanting to switch to a disc brake conversion, you will only have one option to choose from. Due to the 2 port system, you will have to run an SVO master cylinder. SVO Mustangs were made from 1984-1986 and were outfitted with a 4 cylinder, turbo equipped engine.

Due to the rarity of these cars, it is going to be quite difficult to find a take off SVO master cylinder, let alone one that is in good condition. Here at LMR, we are all about solutions. That is why we offer a direct replacement SVO master cylinder to help you restore your SVO or complete your 1979-1986 Mustang disc brake conversion.

1984-1986 Mustang SVO Master Cylinder | Buy Now!

• Bore Size: 1 1/8”
• Push Rod Depth 1.43”

For 1987-1993 Mustang owners who are wanting to add disc brakes to their Fox Body, you will get a wider selection of master cylinders to choose from. Since these year models have a 3 port system, you get to choose from the following master cylinders to complete your conversion:

1993 Cobra Master Cylinder | Buy Now!

• Bore Size: 1”
• Push Rod Depth 1.41”

1994-1995 Mustang GT/V6 Master Cylinder | Buy Now!

• Bore Size: 1 1/16”
• Push Rod Depth 1.40”

1994-1995 Mustang Cobra Master Cylinder | Buy Now!

• Bore Size: 15/16”
• Push Rod Depth 1.40”

The reason bore size is so important is that it dictates how much fluid can be pumped through the master cylinder to the brakes. However, bigger is not always better for everyone when it comes to pedal feel. While ideally, it would be great to test drive cars with each setup to see which you like best, we all know that this is not feasible.

That is why we have put our personal experiences to good use and have listened to feedback from Fox Body owners on which they prefer best. This has resulted in the 1993 Cobra master cylinder taking the vote for the most preferred master cylinder. The 1” bore size and 1.41” push rod depth not only give you great braking power, but has a great pedal travel as well. This is why we include the 1993 Cobra Master Cylinder in our Fox Body 5 Lug Conversion Kits.

On your factory Fox Body brake system, you will find a proportioning valve/distribution block located under the master cylinder. This valve allows for the correct amount of rear brake pressure bias for your factory brake drum setup.

For a disc brake setup, this valve will need to be “gutted” to allow for an adjustable proportioning valve to be installed.

To gut the proportioning valve, follow these steps:

1. Take a ¾” 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.


2. Remove the spring and shuttle valve from the hole and discard.


3. 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 LMR.com exclusive!). Install the new Solid Plug in the proportioning valve.

With the factory proportioning valve gutted, you will need to manually set the rear brake bias. You do this by adding in an adjustable brake proportioning valve. In the upper passenger side corner of your engine bay, you will find a union installed on your brake lines that head to the rear brakes. You will need to remove this union and install the adjustable proportioning valve. From here, you can adjust the valve to get the proper bias needed for your application.