Upgrading Fox Body Brakes With Aerospace Components

Fox body Mustang brake systems were barely adequate when the cars were new and downright abysmal when you start adding performance. The Fox platform, born out of the budget-crunching late 1970s, utilized braking already old braking technology. With Ford planning to end the Mustang at least twice during the Fox’s production run there was little motivation to update the antiquated brakes.

Our Project Rehab was still wearing its stock braking system when we rolled it into Editor Creason’s garage nearly a year ago. The single piston front calipers and small by narrow rear drums weren’t going to cut it for our street/strip Fox. To give us the stopping power necessary and reduce some weight on Project Rehab we turned to Aerospace Components.

Left: The Aerospace Pro Street system we ordered contains everything we need to upgrade our brakes from the stock single piston caliper front, drum rear to five-lug four-piston 11.75-inch discs at all four corners. We're also converting to manual brakes which will save us more weight. Right: The front brakes are a three piece design that include this beautiful CNC machined billet front hub.

Four-Piston Stopping

This kit was designed for cars that are driven on the street but still see time making passes at the drag strip too. It’s a best of both worlds kit. -Matt Moody, Aerospace Components

Aerospace outfitted us with everything needed to upgrade Project Rehab’s brake system from mundane to monster. We’re utilizing the four-piston caliper Pro Street systems for our front, part number AC-220VR, as well as rear brakes, part number AC-546VR-LP. Coupled with those calipers is a manual brake conversion with master cylinder, part number AC-MBC-7993 and stainless steel braided brake lines for the front, part number AC-4100-M, and rear, part number AC-410-T. All of these parts take the place of the tired and inadequate stock pieces and complete our five-lug conversion.

“This kit is designed to handle cars up to 4,000 pounds,” says Aerospace’s Matt Moody. While Project Rehab won’t be anywhere near that weight, in our book you never can have too much stopping power. “The focus on this product is for cars that are driven on the street but still see time making passes at the drag strip too. It’s a best of both worlds kit, it’s a little heavier than our race kits but it gives you the brake cooling needed for street driving.”

The Fox’s Dull Claws

Stock Fox body front calipers used a single piston design along with rubber bushing slide pins to do the majority of the stopping. Their barely adequate 10.94-inch diameter rotors were also miserable in terms of providing adequate surface area for the brake pads to grab as well as dissipating heat generated by spirited driving. The stock front caliper design is also bad about dragging on release, this is due to the slide pin design and pistons that don’t fully retract when you release the brake pedal.

Our stock front brakes have seen better days, and won’t be even close to adequate once we really start pouring performance into Project Rehab down the road.

With the exception of the SVO and ’93 Cobra, Fox body Mustangs also relied on drum brakes in the rear. These brakes provide little in terms of consistent performance or heat dissipation, let alone stopping power measuring 9×2-inches. The drums also contributed significantly to rotating mass on the rear axle, adding more weight for the rear axles to spin as the car accelerates.

Stopping Better

Doing all of the work in this system are the awesome Aerospace Pro Street billet aluminum four-piston calipers. Machined from Alcoa sourced 6061 billets, they’re not only strong, but they look like gleaming jewelry hanging at every corner of the car. Each caliper is carved out on a CNC mill at Aerospace’s Florida headquarters. Inside each caliper are four 1.75-inch pistons made of a proprietary phenolic material.

Aerospace carves their calipers from 6061 billets on a CNC mill. They use an F1 technology inspired caliper piston material prevents heat transfer from into the brake fluid. This prolongs brake life and reduces the chance for pedal fade.

Heat transferred into the brake fluid can eventually lead to the fluid boiling, resulting in a soft brake pedal and brake fade. According to Moody that phenolic piston material is born out of Formula 1 technology and helps prevent heat transfer back into the brake fluid. Moody says these pistons are formed under extremely high pressure and then cylindrically ground to ensure a precise fit in each caliper.

Aerospace requires customers to provide several measurements to get the exact fitment for the rear brakes. The mounting plate installs behind the C-clip eliminator and takes the place of the stock brake mounting bracket.The rotor hats will determine the distance of the rotor from the mounting plate.

The front and rear calipers are identical, they’re actually the same part. This makes the system easier to service, and replace components on. You could even go as far as flipping the calipers one side to the other provided you removed the bleeder screw and installed the hose or line fitting in opposite positions so that the system can be properly bled. The Pro Street calipers are designed for drop in pads. This makes pad changes a quick and easy process.

Top Row: Left: The rear rotors are a two piece design. Be sure to clean all of the threads with acetone or brake clean and dry them prior to installing the bolts. We recommend mocking up everything before putting high-temp thread locker on all the bolts. Right: With the rotor in place the caliper goes on next. Note the position of the port for the brake line or hose as indicated by the red plug. The hose goes on the bottom, the bleeder on top so that air bubbles can rise out of the caliper for proper bleeding. The grade 8 caliper bolts hold it all together. Pay attention that the caliper does not contact the rotor, if it does the hat may have been ordered incorrectly. We recommend using three open lug nuts to hold each rotor while the caliper is mounted, this will ensure that the rotor is flat against the axle flange. Bottom Row: Left: With the caliper in position we tightened the mounting bolts by hand for mock up. Right With everything in place we check for proper fitment.

Clearance between the rotor and the brake pad tabs is tight but there should be no contact with the rotor secured in place. You should be able to see air between the rotor and the tabs.

The rotors on the Pro Street system measure 11.75 inches and are 13/16 of an inch thick. According to Moody there’s larger rotor vane thickness compared to the stock Fox body design to provide greater airflow through the rotor vanes to cool the brakes. These larger cooling fins also reduce the total weight of the rotor. The rotors are held onto their hats with grade-8 hardware and the fronts use a billet aluminum hub with screw-in 1/2-inch by three-inch wheel studs. This adds up to greater surface area for the Hawk brake pads to grab, better heat dissipation, and reduced rotating mass.

Moody also says that unlike the stock Mustang brakes the Aerospace pistons will retract fully when released. This keeps the pads from dragging on the brake rotors, further reducing rolling resistance on the starting line. They’ll also provide much better stopping when we hit the pedal at the end of a pass. “The lighter weight rotors and hubs coupled with brake calipers that can provide substantially more clamping force than the stock calipers provides a brake system that is significantly better than anything that came on a stock Fox body.”

This also converts the caliper to a separate mounting bracket that rigidly holds the caliper in position. There are no rubber bushings to flex in the Aerospace kit. This means that the Aerospace brakes will perform more consistently every time we step on the pedal. The pads will also wear evenly since there are pistons on both sides of the caliper acting on each pad, rather than the OE method of squeezing the caliper through use of a single piston and slide pins, which tends to wear the inside pads out faster.

Rear Considerations

When we built the 8.8 rearend for Project Rehab we had already planned to install disc brakes on the rear. As a result we sourced an SN95 housing. We had to provide some critical measurements to Aerospace in order to get the correct mounting brackets for our rear calipers. Since everything is built in-house Aerospace can accommodate a variety of different axle and C-clip eliminator combinations. We sent all of our measurements to our sales representative along with information on the brand of axles and C-clip eliminators we were using on the car.

Left: The final step of mock-up is to install the included drop-in Hawk brake pads. Center: This center bolt retains the brake pads. Right: We're now ready to take everything back apart and reassemble with thread-lock.

Going Manual

Aerospace’s manual brake conversion was something that we decided also made sense for Project Rehab. We wanted to upgrade from the funky stock three-port configuration. We also wanted to lose some weight and if we decide to run a very lumpy cam in the future, we didn’t want it to adversely impact the vacuum signal to our brake booster which could degrade braking performance and pedal feel. “If you’re not pulling at least 16-inches of manifold vacuum at idle then that vacuum booster won’t do you any good,” says Moody.

We installed the new manual master cylinder to the firewall using the supplied hardware. Aerospace has engineered this mounting bracket to place the master cylinder in proper position so that the correct pedal ratio is achieved for proper stopping power.

The 1.125-inch bore master cylinder will provide the necessary pressure when we press on the pedal. The stock master cylinder is a 21mm bore and when changing to manual brakes as well as converting to rear discs should not be used.

Making New Lines

Installing this system required us to make new lines to feed our line lock and all four corners of our braking system. We reused some of the stock lines, but all of our connections utilize a double flare brake line end. This requires special tools, patience and practice. The end result is a set of brake lines that won’t spring a leak, or worse yet leave us in the sand pit at the end of the track.

A unique feature that Aerospace has engineered into the Mustang manual brake conversion kit is their mounting plate designed specifically for the Fox body. This mounting plate moves the master cylinder up in its mounting position. “You have to raise the pushrod attachment point up so you get the right mechanical ratio, since now you’re using all mechanical leverage to stop – you don’t have the advantage of a power booster to create that leverage for you,” says Moody. It also keeps the master cylinder attachment point in line with master cylinder push rod, which is critical for proper operation. Aerospace includes instructions with the kit which outline making the necessary modifications to the brake pedal which must be removed and drilled. This kit does not include a provision for a brake light switch, but we found during our installation that the stock switch easily mounted back in place and works appropriately.

We’re also saving a tremendous amount of weight, Our entire front Aerospace system weighs just 38 pounds, this is compared to over 60 pounds for the stock parts. The difference with the loss of the master cylinder should make up another 10-12 pounds as well.

Left: Prior to installation we used the included rubber plugs to bench-bleed our new master cylinder. This removes trapped air from inside the piston bore. Right: We installed the master cylinder onto the new mounting bracket and then began plumbing our new hard lines for the front and rear brakes to it.

Front Modifications

Ford didn’t use this style of rigidly mounted caliper on the Fox body. As a result the spindles need some modifications to make them work with the Aerospace kit. The first of these modifications is to cut off the original caliper mounting tabs. Aerospace supplies detailed instructions on how to measure for this procedure.

The first step to installing our front brakes is to cut the OEM caliper mounting brackets off the stock spindle.

To cut the caliper tabs off there are two options. Take the DIY approach, or pay a machine shop to do the work for you. Depending on the tools available and the installer’s skill level this job could take anywhere from 30 minutes to all day. We suggest the use of an electric angle grinder or cut-off wheel attached to a heavy duty air compressor. Moody says an electric reciprocating saw can also be used. Some Fox body spindles had more nickel content than others, and as a result cutting the spindles may require several blades or cut off wheels to get through.

The second step is to drill and tap the three original dust shield mounting holes to accept the new caliper mounting plate bolts. This step will require the use of high quality drill bits, patience, and machine shop grade taps as well as a molly based lubricant for the tap. The spindles are typically drilled with a 5/16 bit. Moody suggested though that if you can find a letter O bit this is 0.004-inch larger and will make the process of tapping the spindle much easier.

Top Row: Left: The spindle after the tabs have been removed. Center Left: The three holes that were used to mount the stock dust shield must now be drilled with either a 5/16-inch or Letter 'O' drill bit. Center Right: With the holes drilled a 3/8-inch-16 tap is used to thread the new holes. Right: The Aerospace caliper mounting bracket is installed using the supplied Grade 8 bolts. Bottom Row: Left: The front hub and rotor assembly begins by installing the screw-in half-inch studs. Center Left: Aerospace provides a dual bolt-pattern on these hubs, we're using the more common 5x4.5-inch pattern. We marked each hole to speed up identification during assembly. Center Right: The rotor plate is attached to the hub next. Right: Finally the rotor is secured using the supplied grade 8 bolts.

We were able to successfully drill and tap one spindle with no issues. On the other spindle we broke three taps and ultimately pulled the part off our car and dropped it off at a machine shop. Two days and a mere $45 later they had removed the broken tap and machined the holes properly. Next time we do this we’ll just leave the spindles with the machine shop and save the aggravation.

Side by side, the small and heavy stock rotor (right) and the new lightweight Aerospace Pro Street rotor.

Top Row: Left: We installed our new braided steel brake lines. Center Left: We very lightly lubricated our spindle and bearings for a dry test fit. Center Right: Once we had verified proper front fitment we packed all the new wheel bearings which Aerospace includes with the rotor. Right: We installed the rotors with new bearings, new washers, and new cotter-pins. Bottom Row: Left: We mounted the new Aerospace caliper in place. Center Left: With everything tight and greased we gave the front rotor one final test spin to make sure there was properly clearance and no rubbing. Center Right: The front brake pads were then installed. Right: The caliper bolts were torqued to 50 ft-lbs per Aerospace's instructions.

Mock It Up

It’s critical to do a dry test fit. We even recommend going as far as putting a wheel on so you can make sure that there are no caliper clearance issues or wheel offset fitment issues.

It’s outlined in the instructions from Aerospace, and Moody strongly urged that anyone installing one of these systems should do a mock up of all the parts prior to torquing things down and putting any thread locking compound on any bolts. “It’s critical to do a dry test fit. We even recommend going as far as putting a wheel on so you can make sure that there are no caliper clearance issues or wheel offset fitment issues.”

Once everything is verified to fit properly Moody recommends cleaning all of the bolts and bolt holes with acetone, brake cleaner, or even carburetor cleaner. This will remove any oils and allow the thread locking compound to bite into the bolt and the spindle.

Our completed front installation.

Make it Bleed

With everything completed we bleed the brake system with a pressure bleed, cross bleed, and reverse bleed.

With our entire brake system in place our final step was to bleed the everything. We used an injection system to reverse and pressure bleed our system.  The end result is a firm brake pedal and brakes that respond consistently whenever we apply pressure with our right foot.

With our Aerospace Brakes installed we’re one step closer to getting Project Rehab back on the road and to the drag strip. We’ll be able to do both knowing that our Fox body will stop the way it should, quickly, reliably, and consistently thanks to Aerospace Components.

About the author

Don Creason

Don Creason is an automotive journalist with passions that lie from everything classic, all the way to modern muscle. Experienced tech writer, and all around car aficionado, Don's love for both cars and writing makes him the perfect addition to the Power Automedia team of experts.
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