2017 Subaru WRX STI Brake Fluid Type and Flush Interval

By Core Brake Parts Tech Team — Circleville, OH warehouse

The 2017 Subaru WRX STI takes FMVSS No. 116 DOT 3 or DOT 4 brake fluid, and Subaru's owner's manual says to use fresh fluid from a sealed container and never mix DOT 3 with DOT 4. Subaru's maintenance schedule calls for replacing the brake fluid every 30,000 miles or 30 months, and every 15,000 miles or 15 months if the car lives in high humidity or mountainous country. On an STI that sees track days or hard canyon driving, a fresh DOT 4 fill at that shorter interval or sooner is the smart call.

What brake system the 2017 WRX STI actually has

Before you buy fluid or plan a flush, know what you are bleeding. Subaru's official 2017 WRX STI specification sheet lists a Brembo Performance Brake System: power-assisted four-wheel disc brakes with a 4-channel, 4-sensor Super-Sport ABS with a g-sensor, plus Electronic Brake-force Distribution, Brake Assist and Brake Override.

Two things matter here. First, 2017 is the last year of the four-piston front Brembo setup; Subaru's 2018 update moved the STI to monoblock six-piston front calipers and larger drilled rotors, so 2018-up parts do not carry straight back. Second, every 2017 STI is a manual, and the clutch hydraulics run on the same brake fluid. The owner's manual describes a reservoir divided into separate brake and clutch chambers, with the brake level read at the front section. A proper fluid service on this car covers both.

DOT 3 vs DOT 4: which one to run in an STI

Both are glycol-based fluids and both are approved by Subaru. The difference is heat tolerance. The federal standard, FMVSS No. 116, sets these minimum boiling points:

The "wet" number is what the fluid does after it has absorbed moisture, and brake fluid absorbs moisture from the air every day it sits in the car - the owner's manual warns about exactly that. As water content rises, the boiling point drops toward the wet figure. When fluid in a hot caliper boils, it forms vapor bubbles that compress, and the pedal goes long and soft.

For a street STI, a quality DOT 4 is the practical choice: it is within Subaru's spec and gives more heat margin than DOT 3 at the same age. For a car that sees autocross or track sessions, a high-temperature DOT 4 racing fluid adds more margin, but those fluids generally want changing more often, so match the fluid to how you drive and how often you are willing to bleed.

Do not use DOT 5. It is silicone-based, it is not what Subaru specifies, and it does not mix with the glycol fluid already in the ABS unit and lines.

Flush interval: the Subaru schedule and when to go sooner

Subaru's 2017 maintenance schedule lists brake fluid replacement for all models at 30,000 miles or 30 months, whichever comes first. The same schedule shortens that to 15,000 miles or 15 months for cars used in high humidity or mountainous areas, and that note covers both brake fluid and clutch fluid.

Treat the short interval as your baseline if any of these apply:

• You live somewhere humid, or the car sits outdoors through wet seasons.
• You drive mountain roads with long downhill stretches.
• You do track days, autocross or spirited canyon runs. Many owners bleed the calipers before every track event.
• The pedal has gone softer or longer than it used to, especially after hard use.
• The fluid in the reservoir has turned dark amber or brown.

Mileage is not the only trigger. A low-mileage STI that sits most of the year still absorbs moisture, which is why the schedule has a months limit as well as a miles limit.

Signs your STI's brake or clutch fluid is overdue

• Soft or sinking pedal after hard stops. A classic sign of fluid near its boiling point. If the pedal is firm cold and fades hot, old fluid is a prime suspect - have the system inspected.
• Dark fluid. Fresh DOT 3 or DOT 4 is clear to light amber. Dark fluid has absorbed moisture and contaminants.
• Clutch engagement point wandering. Since the clutch shares the fluid, overheated or contaminated fluid can make the clutch pedal feel inconsistent.
• Level dropping. The owner's manual says to check the level monthly. A slow drop is normal as pads wear; a fast drop means a leak that needs fixing before any flush.
• Brake warning light. Low fluid can trigger it. Do not keep driving; find out why.

How to do the flush right on a Brembo-equipped STI

This is a moderate DIY job for someone who has bled brakes before. Fixed calipers often have a bleeder on each side, so bleed every bleeder on each caliper, not just the first one you find.

1. Buy enough sealed fluid of one type and one brand. The manual is specific: never mix brands, and avoid mixing DOT 3 and DOT 4 even from the same brand. Open a fresh bottle.
2. Protect paint and the exhaust. Brake fluid damages paint, and the manual warns that fluid on the exhaust pipe can cause smoke or even a fire.
3. Remove old fluid from the reservoir with a clean syringe and refill with fresh fluid to MAX.
4. Bleed each corner in the order the factory service manual gives, pushing fluid until it runs clear and bubble-free. Keep the reservoir from running low, or you will pull air into the ABS unit.
5. Bleed the clutch slave cylinder too, so the clutch circuit gets fresh fluid.
6. Torque bleeder screws and wheels to the values in the factory service manual, top the reservoir to MAX, and test the pedal before you drive.

If air gets into the ABS hydraulic unit, a scan tool that can cycle the ABS pump may be needed to clear it. That is the point where a shop earns its fee.

What to replace at the same time

A fluid service is a good moment to look over the whole brake system. Check pad thickness and rotor condition while the wheels are off. If you are refreshing the front, our DBA 5000 Series T3 slotted 2-piece front rotor for the 2017 and newer WRX STI is built for this car. Browse matching brake pads and brake rotors, and inspect the rubber hoses for cracking; braided brake lines firm up pedal feel on a car that sees hard use. Confirm fitment for the four-piston 2017 setup before ordering, since 2018-up six-piston parts differ.

Frequently Asked Questions

What brake fluid does a 2017 WRX STI use?

FMVSS No. 116 DOT 3 or DOT 4 brake fluid, fresh from a sealed container. DOT 4 gives more heat margin and is the common choice on this car.

How often should I flush the brake fluid on a 2017 STI?

Subaru's schedule says every 30,000 miles or 30 months, or every 15,000 miles or 15 months in high humidity or mountainous areas. Track or hard driving justifies going sooner.

Does the clutch use the same fluid?

Yes. The 2017 STI is manual only, and the clutch hydraulics use the same FMVSS No. 116 DOT 3 or DOT 4 fluid, from a separate chamber of the same reservoir. Flush both.

Can I mix DOT 3 and DOT 4?

Subaru says to avoid it, even within the same brand, and not to mix different brands. If you want to switch to DOT 4, do a complete flush rather than topping off.

Can I use DOT 5?

No. DOT 5 is silicone-based and outside Subaru's DOT 3 or DOT 4 spec. Stay within that spec.

Is a brake fluid flush hard on an STI?

It is a moderate DIY job with basic tools and a helper or pressure bleeder. The main risks are letting the reservoir run dry, which can put air in the ABS unit, and spilling fluid on paint or the exhaust.

Sources

1. Subaru of America - 2017 WRX STI Specifications - Brembo system, 12.8 in. and 12.4 in. rotors, four-piston front and dual-piston rear calipers, ABS and EBD, 6-speed manual
2. Subaru of America - 2018 WRX and WRX STI debut - 2018 change to six-piston monoblock front calipers
3. Subaru WRX STI Owner's Manual - Fluids - FMVSS No. 116 DOT 3 or DOT 4 for brake and clutch
4. Subaru WRX STI Owner's Manual - Brake Fluid - monthly level check, reservoir chambers, no mixing, moisture and exhaust warnings
5. Cars101 - 2017 Subaru Maintenance Schedule - 30,000 mi/30 mo interval and 15,000 mi/15 mo severe interval
6. 49 CFR 571.116 (FMVSS No. 116), Cornell LII - DOT 3, DOT 4 and DOT 5 dry and wet boiling point minimums