2017 Subaru WRX STI Brake Caliper Problems and Replacement

By Core Brake Parts Tech Team — Circleville, OH warehouse

Caliper trouble on a 2017 Subaru WRX STI usually shows up as a pull under braking, one wheel running much hotter than the others, uneven pad wear inside a single caliper, or a fluid weep at a piston. The STI uses fixed, opposed-piston Brembo calipers rather than the sliding calipers on most cars, so the causes and the fix are different: there are no slide pins to seize, and the problem is almost always a piston that no longer moves freely or a seal that no longer does its job.

How the Brakes Work on the 2017 WRX STI

Subaru introduced the current-generation STI for 2015 with what it calls the Brembo Performance Brake System: ventilated discs at all four wheels, 13.0-inch front discs with 4-piston calipers, and 12.4-inch rear discs with 2-piston calipers. The 2017 model carries that same generation of hardware. Subaru's own press release for the 2018 STI describes monoblock 6-piston front calipers and larger drilled rotors as the upgrade for 2018, which is how you can tell a 2017 from a later car at a glance: count the pistons, or look for drilled rotors. Brembo's own Subaru application list also groups the 2013-2017 STI separately from the 2018-2020 cars.

The system is tied into Subaru's Super Sport ABS with Electronic Brake-force Distribution. According to Subaru, that ABS reads lateral g, steering angle, yaw and brake pressure, and it can brake each rear wheel independently during hard cornering to help reduce understeer. That matters for diagnosis: a caliper that drags or a piston that sticks does not just wear pads, it changes how evenly the car stops.

Why Fixed Calipers Fail Differently

Brembo explains the difference plainly. A floating caliper has pistons on one side only and has to slide on pins to clamp the outer pad. A fixed caliper is a single body bolted rigidly to the suspension, with pistons on both sides of the disc pushing the pads toward each other. Fixed calipers are stiffer and wear pads more evenly, which is why they show up on sports cars like the STI.

Because nothing slides, the classic sliding-caliper failure (a seized guide pin that leaves the inner pad doing all the work) does not apply here. On a fixed caliper, the moving parts are the pistons themselves. Brembo notes that the piston seals do two jobs besides holding fluid: they pull the piston back slightly after you release the pedal (roll-back) and let it be pushed back if the disc knocks it (knock-back). When a seal hardens or the piston bore corrodes, roll-back stops working, the pad stays lightly on the rotor, and you get drag and heat.

Symptoms and How to Diagnose Them

• Pull under braking. One side grabbing harder or releasing slower than the other. Check that tire pressures match first, then compare how each wheel behaves.
• One hot wheel after a drive. After a normal drive with light braking, carefully compare wheel temperature side to side without touching the rotor. A wheel that is clearly hotter points to a dragging caliper.
• Uneven pad wear inside one caliper. On a fixed caliper, the inner and outer pads should wear close to evenly. A tapered pad, or one pad much thinner than its partner, means one or more pistons are not moving as they should.
• Wheel hard to turn by hand. With the car safely supported and in neutral, a wheel that will not spin, or spins and stops abruptly, is dragging.
• Fluid at a piston or bleeder. Wetness around a piston dust boot, the bleeder, or where the caliper halves meet, together with a dropping reservoir level, is a seal leak. That is a stop-driving problem.
• Burning smell or smoke after a normal drive, which is pad material overheating from constant contact.

To confirm, remove the pads and, with a helper pressing the pedal very gently, watch every piston move out evenly. Then push each piston back with a proper piston tool. A piston that will not move, moves only under heavy force, or travels unevenly is the culprit. Do not let pistons extend too far with the pads out, and never push a piston back at an angle.

What Causes STI Caliper Problems

• Old, wet brake fluid. Brake fluid absorbs moisture over time. Water in the fluid corrodes pistons and bores from the inside, which is behind most seized pistons. Follow the fluid change interval in your owner's manual and use the fluid type printed on the reservoir cap.
• Torn dust boots. Once a boot splits, road salt and grit reach the piston surface, which pits and then binds against the seal.
• Heat. Track days and hard back-road driving cook seals over time. If your STI sees track use, expect to service the calipers sooner than a car that only commutes.
• Pad hardware and debris. Pads that bind on their retaining pins or abutments, or a caked-on layer of brake dust and rust, can hold a pad against the rotor even when the pistons are healthy. Clean the pad seating areas at every pad change.
• Improper piston push-back. Forcing pistons back with a pry bar against the rotor can damage seals or cock a piston in its bore.

The Fix: Rebuild or Replace

Brembo's position is that correct caliper operation depends on workmanship, correct assembly and the right materials, which is why a rebuild is only as good as the parts and care that go into it. On a 2017 STI you have two paths:

• Rebuild. Remove the pistons, clean the bores, and fit new seals and dust boots. This is reasonable when the bores are clean and only the seals and boots have aged. Do not split the two caliper halves; the bolts joining them are not a routine service item, and resealing the internal fluid passage needs the correct parts and procedure.
• Replace. If a bore is pitted, a piston is corroded, or a caliper leaks from anywhere other than a boot, replace the caliper. Replace calipers in pairs across an axle so both sides clamp the same. Browse replacement brake calipers and confirm fitment for the 2017 STI before ordering, since the 2018 and later 6-piston front calipers do not simply swap onto a 2017 without matching rotors.

After any caliper work, bleed the whole system, not just the caliper you opened, and confirm a firm pedal before the car moves. Bed in new pads following the pad maker's procedure.

What to Replace at the Same Time

• Pads. Pads that have been dragging are usually glazed or tapered. Fit a new set on the whole axle from our brake pads.
• Rotors. A caliper that dragged long enough to overheat often leaves a rotor with uneven deposits, blueing or thickness variation. Measure against the minimum thickness stamped on the rotor, and replace from our brake rotors if it is out of spec or damaged.
• Brake hoses. If you are replacing calipers on a higher-mileage car, the flexible hoses have seen the same heat and age. New brake lines are worth doing while the system is open.
• Fluid. Always a full flush after caliper work.

How Hard Is the Job?

Swapping a caliper on the STI is a moderate DIY job for someone who has done a brake job before. Each caliper comes off with its mounting bolts and brake hose, and there are no slide pins to service. The parts that need care are bleeding (fixed calipers have fluid on both sides of the disc, so bleed each caliper fully in the order the service manual specifies) and torque: tighten caliper mounting bolts and hose fittings to the values in the factory service manual, and use new sealing washers on the hose fittings. A rebuild with piston removal is a more involved bench job and is best done with the caliper off the car.

Frequently Asked Questions

Does the 2017 WRX STI have 4-piston or 6-piston front Brembo calipers?

Subaru's launch material for this STI generation lists 4-piston front and 2-piston rear Brembo calipers, and Subaru describes the monoblock 6-piston front caliper as an upgrade for 2018. A 2017 STI in original condition has the 4-piston front caliper. Count the pistons if you are unsure whether a previous owner swapped anything.

Can I lubricate slide pins to fix a sticking STI caliper?

No. The STI calipers are fixed calipers with no slide pins. Sticking comes from the pistons, seals or pad hardware, so the fix is cleaning, a seal rebuild or a replacement caliper.

Why are my inner and outer pads wearing differently on one wheel?

On a fixed caliper both pads should wear nearly evenly. Uneven wear means one or more pistons are sticking, or a pad is binding in the caliper. Check that every piston extends and retracts smoothly.

Should I replace both calipers on the axle?

Yes, when replacing. Calipers in pairs keep clamping force and response the same side to side, which matters on a car whose ABS can brake individual wheels.

Is it safe to drive with a dragging or leaking caliper?

A leak is not safe: losing fluid can lead to a soft pedal or reduced braking. A dragging caliper overheats pads, rotors and fluid and can cause fade. Get either one fixed before driving further.

Sources

1. Subaru U.S. Media Center - 2015 WRX STI press kit - Brembo system, 13.0 in front discs with 4-piston calipers, 12.4 in rear with 2-piston, Super Sport ABS and EBD
2. Subaru U.S. Media Center - 2018 WRX and WRX STI debut - monoblock 6-piston front, 2-piston rear and drilled rotors new for 2018
3. Brembo - Differences between fixed and floating brake calipers - fixed vs floating design, piston seal roll-back and knock-back
4. Brembo - Subaru car solutions - 2013-2017 STI grouped separately from 2018-2020
5. Free ASE Study Guides - Uneven pad wear - sticking calipers cause pull, drag, overheated rotors and uneven pad wear