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Hydroboost Conversion Kit, 1999-2004 Hydroboost in 1979-1993 Mustang

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MM's Hydroboost Conversion Kit installs a 1999-2004 Mustang Hydroboost unit into a 1979-1993 Mustang

Why install a Hydroboost in a Fox Mustang?
  • Two words: engine swap! A Coyote or 4.6L modular engine swapped into a Fox Mustang leaves no room for a vacuum brake booster. Most people prefer power-assisted brakes over manual brakes, making a Hydroboost conversion the best solution.
  • Excellent brake response regardless of engine vacuum. Vacuum-assisted brakes provide varying levels of assist, depending upon the available vacuum supply. The changing assist causes inconsistent pedal feel. This is especially noticeable in a Mustang that's open-tracked, due to the wide fluctuations in vacuum from extended periods of wide-open throttle followed by hard braking zones.
  • Not enough vacuum at idle to operate a vacuum booster. Some pushrod engine cams don't allow the engine to create enough vacuum to operate a vacuum brake booster.
  • Low brake pedal effort, even with small-diameter brake rotors.
What's included?
  • Firewall adapter block. MM's billet aluminum design mounts the Hydroboost unit at the correct angle, the correct height, the correct fore/aft location, and the correct lateral location to operate exactly as it was designed to operate in a 1999-2004 Mustang. The 4 mounting studs are located to match the Fox firewall pattern of holes.
  • Brake pedal. The pedal arm is made of high-strength alloy steel, with the pushrod pin in the correct location to match the brake pedal ratio and pushrod geometry of a 1999-2004 Hydroboost Mustang. This gives the brake system the correct pedal travel and effort, and keeps the master cylinder from being over-stroked. The separate pedal pad is adjustable via multiple mounting holes to accommodate different driver preferences.
  • Pedal box spacer. A billet aluminum MM-designed spacer replaces the breakage-prone stock plastic spacer located between the firewall and the pedal box.
  • Tools. An MM-designed special socket removes the giant nut securing the Hydroboost unit to the stock firewall mounting plate, and then reinstalls it on the MM Aluminum Firewall Block. A second MM special tool easily modifies the firewall hole for the master cylinder pushrod.
What does it fit?
  • 1979-1993 Mustang
  • 1999-2004 Mustang Hydroboost unit
  • 1999-2004 master cylinder for Mustang with Traction Control
  • 1999-2004 master cylinder for Mustang without Traction Control
Which front and rear brake setups does it work with?
  • The Hydroboost is simply a type of power assist, just as a vacuum booster is a type of power assist. It will work with most of the front and rear caliper setups used on SN95 and Fox Mustangs. A limitation is the lack of master cylinder sizes available for the Hydroboost. This makes it important to confirm that the volume of fluid required to move the caliper pistons is compatible with the volume of fluid the master cylinder can deliver. If the caliper piston requirement is excessive, the brake pedal will have an overly long stroke, which is very disconcerting for the driver. At the extreme, if it exceeds what the master cylinder can deliver, the piston in the master cylinder will reach the end of its stroke and bottom out, preventing full application of the brakes.
  • While you're confirming that your Mustang's calipers are a match to the Hydroboost master cylinder, it's also a good time to confirm your Mustang's front to rear brake bias is in the range of what works well. Fox and SN95 Mustang brakes should have between 70% and 75% front bias (also referred to as 70/30 and 75/25). The more heavily modified the suspension is for improved handling, the closer to 70% the front bias should be. An exception is a street and/or strip Mustang running an extreme 'big-and-little' combination of fat rear tires and skinny frontrunners, where substantially more rear bias is required to offset the lack of front tire grip.
How do I confirm my brake calipers are a match to the Hydroboost master cylinder, and that my bias is good?
  • You can calculate the fluid requirement of your calipers from the piston area, and then segue into calculating the bias by taking into account the rotor diameters. Or, you can take advantage of MM's Technical Services and let MM's setup engineer do the math. Select the Fox Hydroboost Conversion option, and follow the instructions.
  • Most of the aftermarket big brake kits listed for SN95 Mustangs also work well on a Fox Mustang. There are some exceptions. Brake systems we've done the calculations for are listed on the Fitment tab. If you don't see your brakes listed, go to the Technical Services page, select the Fox Hydroboost Conversion option, and we'll run the numbers for you.
  • The brake setups most likely to have poor bias are those put together by DIYers. These systems are often done following dubious Internet directions, or simply because the parts were readily available and easily bolted up. Good DIY mechanical skills don't always coincide with good brake system engineering.
What's required, but not included?
  • 1999-2004 Mustang Hydroboost unit
  • 1999-2004 Mustang Hydroboost master cylinder, for either Traction Control (TC) or non-TC master cylinder
  • Hydraulic hoses connecting the Hydroboost unit to the pump and steering rack
  • Power steering pump
  • Brake hard lines connecting the master cylinder to the car's brake system.
  • Brake fluid
Which of the two Hydroboost units should I use?
  • For best pedal feel use the 1999-2004 Hydroboost.
  • See the Parts ID tab for photos to help identify your Hydroboost unit and master cylinder.
Why the 1999-2004 Hydroboost?
  • Ford's Mustang engineering team learned from their first Hydroboost experience. For the 1999 model year they significantly increased the pedal ratio to reduce the initial pedal effort required to activate the Hydroboost. Compared to the earlier unit, the later Hydroboost has lower initial pedal effort, helping to make precise brake modulation easier.
Tech Notes
  • Two different Hydroboost units were OEM equipment on Mustangs. One unit was fitted to 1996-1998 Mustangs and the other to 1999-2004 Mustangs.
  • Each of the Hydroboost units has its own set of unique hydraulic hoses.
  • Mustang front brake calipers changed in 1999. The GT front calipers went from single-piston to dual-piston. The Cobra front dual-piston calipers had the piston diameter increased.
  • There are two different master cylinders for 1999-2004: One for Mustangs with Traction Control, and another for those without TC. They look identical externally, except the outlet ports are different diameters. The bore diameter is the same for both.
  • The MM Firewall Adapter Block and MM Pedal Arm only work when used together. You will get bad results otherwise; see the DIY Fails tab.
MM's Recommendations
  • If you have the choice, use the 1999-2004 Hydroboost.
  • Use the power steering pump from a 1996-2004 V8 Mustang.
  • When swapping engines your life will be easier if you mount the power steering pump on the driver side of the engine, as the hoses will be shorter and easier to route.
  • Install a fluid cooler in the return line from the steering rack, between the rack and the T-fitting for the Hydroboost return line. Recommended for all applications, and a requirement for track-driven Mustangs to prevent failure of the Hydroboost unit from overheated fluid.
  • Match the hydraulic hoses to the year of the Hydroboost unit.
Related Parts
  • Master cylinder brake line installation kits
  • MM power steering hose kits (NOT YET AVAILABLE)
  • MM Adapter fittings steering rack to -6 AN hose
  • Brake fluid
  • MM Swap kit for 1996-1998 Hydroboost unit
  • Technical Services: Fox Hydroboost Conversion option
  • MM Brake Proportioning Valve Eliminator kit


  • Allows installation of a Hydroboost unit so you can have power-assisted brakes on a Fox Mustang with an engine swap that leaves no room for a vacuum booster. No need to settle for manual brakes!
  • Provides excellent brake response and consistent pedal feel regardless of the engine vacuum.
  • Low pedal effort, even with small-diameter brake rotors.
  • Mounts the Hydroboost unit/master cylinder and the brake pedal arm in the correct relationship to function as it was originally designed.
  • Provides the same pedal feel (effort and travel) as a stock 1996-1998 Mustang GT or Cobra, when 1994-1996 GT front and rear brakes or 1994-1996 Cobra front and rear brakes are used, and matched with the corresponding 1996-1998 Mustang master cylinder.
  • No modifications needed to the Hydroboost unit, making future service and replacement easy.
  • Firewall Adapter Block bolts to the firewall, with only minor firewall modifications needed. It's still easy to return the car to stock.
  • We include a new billet aluminum pedal box spacer to ensure your Hydroboost installation project isn't stopped in its tracks while you try to repair the stock plastic spacer you didn't know was broken until your Mustang was taken apart.
  • Pedal pad location can be adjusted to suit the driver.
  • Includes the socket required to tighten the giant nut securing the Hydroboost to the MM Firewall Adapter Block.
  • Includes a special tool to easily and quickly modify the upper edge of the hole in the firewall for pushrod clearance.


  • MM's billet aluminum Firewall Adapter Block mounts the Hydroboost unit at the correct angle, height, fore/aft location, and lateral location.
  • 4 mounting studs on the Firewall Adapter Block match the Fox firewall hole pattern.
  • MM Brake Pedal Arm is made of high-strength alloy steel, with the pushrod pin in the correct location to match the brake pedal ratio and pushrod geometry of a 1999-2004 Hydroboost Mustang.
  • MM Brake Pedal Pad is adjustable to suit different driver preferences.
  • Billet aluminum MM Pedal Box Spacer replaces the breakage-prone stock plastic spacer.
  • Includes MM Socket to tighten the giant nut securing the Hydroboost unit.
  • MM special tool to modify the firewall.
  • Retains stock brake light switch.


  • 1979-1993 Mustang
  • 1999-2004 Ford Mustang Hydroboost unit

For compatible brake systems, see the list below.

Does NOT fit
  • 1996-1998 Ford Mustang Hydroboost unit
  • Any other Hydroboost unit
  • 1994-2004 Mustang
  • MM Strut Tower Braces MMSTB-1 and MMSTB-2

See the Parts ID tab for photos to help identify your Hydroboost unit and master cylinder.

Hydroboost Compatibility and Brake Bias
MM's Engineering Team has done the math to determine the compatibility of a Hydroboost conversion with many brake systems typically used on Fox Mustangs; see the table below. As a bonus, this table also includes the front/rear brake bias for those systems.

If your Mustang's brake system isn't listed, we can calculate its suitability for a Hydroboost conversion, along with the bias ratio; go to the Technical Services page, select the Fox Hydroboost Conversion option, and follow the instructions.

Ford used brake proportioning valves with the slope and breakpoint pressures optimized for the various Mustang brake systems. This leads to some OEM Mustang exceptions to the general rule that front bias should be somewhere between 70% and 75%. The optimized OEM proportioning valve took care of the baseline bias being out of the preferred range. Aftermarket adjustable proportioning valves have more limited slope options, so it is quite important that the baseline bias be good. MM recommends installing an adjustable brake proportioning valve to fine tune the brake bias whenever any part of the brake system is changed from the stock configuration. Keep in mind that an adjustable valve is only for fine tuning; it cannot make up for a system that is grossly out of front to rear balance.

  1. Do NOT use with the early Hydroboost and master cylinder. May be used with the later Hydroboost and master cylinder.
  2. For a V8 equipped car, bias only works for a very light front end or a very heavy rear end.
  3. Bias works for drag race skinny front tires and very wide rear tires
  4. Bias only works for a track car with a very light front end, IRS, and road race stiff suspension.



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