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Fox Body Master Cylinder Upgrade For Disc Brake Conversions

Last Updated 11/20/2020 by Taylor Ward

When increasing the Fox Body’s stock horsepower numbers, the master cylinder will need to be upgraded for disc brake conversions. Follow LMR's guide to help!

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FOLLOW: 79 93 mustang , fox body , brakes

Fox Body Master Cylinder Upgrade For Disc Brake Conversions - Fox Body Master Cylinder Upgrade For Disc Brake Conversions


The brake system on Fox Body Mustangs were adequate for their factory setups, but as we all know, these cars did not stay stock for long. Throughout the 80s and 90s, Fox Body owners quickly learned the potential of the 1979-1993 Mustang and wasted no time adding more power. The 5.0 pushrod engine was a perfect platform for adding heads, cams, and other bolt ons to make the Fox a force to be reckoned with both on and off the track.

When increasing the Fox Body’s stock horsepower numbers, the factory brake system will need to be upgraded as well. There are a few different ways you can do this with the most popular methods being a Fox Body 5 lug kit or swapping over to a disc brake conversion kit. If you are converting your drum brakes to a disc brake setup, you will need to upgrade your master cylinder to handle the fluid volume change.


Before we touch on the different master cylinder options for your Fox Body Mustang, let’s first take a look at what your Fox Body came with from the factory, as well as some popular SN-95 upgrades.


Factory Fox Body Mustang Master Cylinder Bore Sizes

Mustang Bore - Inches Bore - MM Bore - Fractional Size Front Brakes - Thread Size Rear Brakes - Thread Size
1979-1981 Manual .08750 22.23 7/8 1/2-20 SAE 7/16-24 SAE
1979-1981 Power .08750 22.23 7/8 1/2-20 SAE 7/16-24 SAE
1982-1986 Manual .08268 21.00 53/64 9/16-18 SAE 1/2-20 SAE
1982-1984 Power .08268 21.00 53/64 9/16-18 SAE 1/2-20 SAE
1984-1986 Power .08268 21.00 53/64 9/16-18 SAE 1/2-20 SAE
1984-1986 SVO 1.1250 28.58 1 1/8 9/16-18 SAE 7/16-24 SAE
1987-1993 All .08268 21.00 53/64 10mm-1.0 ISO x 2 12mm-1.0 ISO
1993 Cobra/Cobra R 1.000 25.40 1 10mm-1.0 ISO 12mm-1.0 ISO
1994-1995 Cobra .0975 23.81 15/16 12mm-1.0 ISO 10mm-1.0 ISO
1994-1995 GT/V6 1.0630 27.00 1 1/16 10mm-1.0 ISO 12mm-1.0 ISO
1996-1998 V6 1.0630 27.00 1 1/16 10mm-1.0 ISO 12mm-1.0 ISO
1999-2004 V6 w/o ABS 1.000 25.40 1 10mm-1.0 ISO 10mm-1.0 ISO
1999-2004 w/ ABS 1.000 25.40 1 10mm-1.0 ISO 12mm-1.0 ISO


1979-1986 Mustang Master Cylinder Options 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”
Fox Body Master Cylinder Upgrade For Disc Brake Conversions - Fox Body Master Cylinder Upgrade For Disc Brake Conversions



1987-1993 Mustang Master Cylinder Options For Disc Brake Conversions


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”
Fox Body Master Cylinder Upgrade For Disc Brake Conversions - Fox Body Master Cylinder Upgrade For Disc Brake Conversions

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

  • Bore Size: 1 1/16”
  • Push Rod Depth 1.40”
Fox Body Master Cylinder Upgrade For Disc Brake Conversions - Fox Body Master Cylinder Upgrade For Disc Brake Conversions

1994-1995 Mustang Cobra Master Cylinder | Buy Now!

  • Bore Size: 15/16”
  • Push Rod Depth 1.40”
Fox Body Master Cylinder Upgrade For Disc Brake Conversions - Fox Body Master Cylinder Upgrade For Disc Brake Conversions



Why We Use A 1993 Cobra Master Cylinder In Our Kits


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.



Gutting The Proportioning Valve


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.

Fox Body Master Cylinder Upgrade For Disc Brake Conversions - Fox Body Master Cylinder Upgrade For Disc Brake Conversions

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. Fox Body Master Cylinder Upgrade For Disc Brake Conversions - Fox Body Master Cylinder Upgrade For Disc Brake Conversions

  3. Remove the spring and shuttle valve from the hole and discard.
  4. Fox Body Master Cylinder Upgrade For Disc Brake Conversions - Fox Body Master Cylinder Upgrade For Disc Brake Conversions

  5. 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.
  6. Fox Body Master Cylinder Upgrade For Disc Brake Conversions - Fox Body Master Cylinder Upgrade For Disc Brake Conversions


How/Why You Need To Adjust Brake Bias With An Adjustable 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.

Fox Body Master Cylinder Upgrade For Disc Brake Conversions - Fox Body Master Cylinder Upgrade For Disc Brake Conversions

We at LMR hope that this master cylinder upgrade guide helps with your decision for your disc brake conversion on your Fox Body Mustang. As always for all things 1979 to present Mustangs, Late Model Restoration is your one stop shop!