2017 BMW M240 Brake Caliper Problems and Replacement

By Core Brake Parts Tech Team — Circleville, OH warehouse

The 2017 BMW M240i leaves the factory with M Sport brakes as standard: blue Brembo fixed calipers, four pistons up front and two in the rear, clamping a 340 mm front and 345 mm rear rotor. Those calipers rarely fail outright, but they do stick, leak and drag when the piston seals, dust boots or brake fluid are neglected. Most M240i caliper problems are fixed with a seal rebuild or a caliper swap, done together with pads, rotors and a fluid flush.

How the M240i Caliper System Works

Base 2 Series coupes use a single-piston floating caliper, but the M235i and M240i were built as M Sport cars. According to BMW brake specialists, the M240i carries the S2NH M Sport brake package from the factory, which means a very different caliper design from the base car.

Some cars were built with an optional larger 370 mm front brake, still with blue calipers. Check which one your car has before buying rotors or pads, because the rotor diameter changes and the parts are not interchangeable across sizes.

A fixed caliper is bolted rigidly to the suspension and has pistons on both sides of the rotor that push toward each other. Brembo explains that fixed calipers are generally aluminum alloy, lighter than cast-iron floating calipers, and give more even pad wear and less residual drag. The trade-off is that there are more pistons and more seals, so there are more places for trouble to start. There are no slide pins on the M240i caliper to seize; when something sticks, it is almost always a piston.

Symptoms of a Bad Caliper on a 2017 M240i

• Pull under braking or while cruising. A piston that will not retract keeps one pad dragging, so the car drifts toward that side.
• One wheel much hotter than the others after a normal drive, often with a burning smell and heavier brake dust on that wheel.
• Uneven pad wear across one caliper. On a four-piston caliper, an inner pad worn much thinner than the outer, or one end tapered, points to a slow piston.
• Fluid on the inside of the wheel or the caliper body. A leaking piston seal lets fluid out and air in.
• Soft or long pedal. Can be a leak, but on a car that is driven hard it is more often old, wet brake fluid.
• Torn or swollen dust boots visible with the wheel off.

To diagnose, lift the car, spin each wheel by hand, and compare how freely they turn. Pull the pads and look at thickness at each end and on each side. With the pads out, check that each piston moves smoothly when pushed back; one that is stiff or rough is the culprit.

What Causes M240i Caliper Problems

Old brake fluid

Brake fluid is hygroscopic: it absorbs moisture from the air. Brembo notes that this moisture turns to vapor long before the fluid reaches its rated boiling point, lengthening pedal travel, and that fluid also has to protect the system from corrosion. Water in the fluid collects in the caliper bores and attacks the piston and seal surfaces. BMW schedules brake fluid replacement on a calendar basis through Condition Based Service, typically every two years.

Damaged dust boots and seals

The dust boot keeps road grit and water off the piston; the seal holds pressure and pulls the piston back slightly when you release the pedal. Brembo rates these seals for high pressure and heat, but once a boot tears, dirt and moisture reach the seal, the piston surface pits, and it starts to drag.

Heat from hard driving

The M240i is quick, and owners use the brakes hard. Repeated heat cycles age seals and boil tired fluid faster. Cars that see track days or spirited back-road driving need more frequent fluid changes than the calendar interval.

Pads run too thin

Running pads down to the backing plate lets pistons extend past their normal travel into the dirty, corroded part of the bore. When new pads force them back, they can bind.

Rebuild or Replace the Caliper?

Because the M240i uses aluminum Brembo calipers, the housing is worth keeping when it is not damaged. Brembo sells caliper repair kits with piston seals, dust covers and pistons so you can replace only the parts that wear. A rebuild makes sense when:

• The pistons clean up with no pitting or scoring.
• The bores are smooth and not corroded.
• The caliper body has no cracks or stripped threads.

Replace the caliper when the bore is pitted, a piston is scored and no replacement is available, the bleeder is seized or snapped, or you want the car back on the road the same day. Service calipers in axle pairs so braking stays balanced side to side. Browse brake calipers and confirm fitment against your VIN or your current caliper before ordering.

What to Replace at the Same Time

• Pads. Pads soaked in brake fluid or worn unevenly by a dragging piston should never be reused. For a sharper street and track pad, the EBC Bluestuff NDX front pads for the 2017 M240 coupe are built for the 340 mm front brake. Other options are in our brake pad collection.
• Rotors. A dragging caliper overheats the rotor. If it is blued, warped or below minimum thickness, replace it; see brake rotors, and match the 340 mm or 370 mm front size.
• Hardware. Pad retaining pins, spreader springs and anti-rattle clips fatigue with heat. A brake hardware kit finishes the job properly.
• Brake hoses. If the flexible hose is cracked or swollen, change it while the system is open. See brake hoses.
• Brake fluid. Opening a caliper means bleeding anyway, so flush the whole system with the fluid type listed in the owner's manual or on the reservoir cap. Brembo warns never to switch to a lower DOT grade than the manufacturer specifies and to keep using that type for the life of the car.

How Hard Is the Job?

Swapping a caliper on the M240i is a moderate DIY job for someone who already does their own pads. The fixed caliper bolts to the hub carrier, and there are no slide pins to service. Plan on these steps:

1. Lift and support the car safely and remove the wheel.
2. Clamp or cap the brake hose, then disconnect it from the caliper.
3. Remove the pads and the caliper mounting bolts, and lift the caliper off the rotor.
4. Install the new or rebuilt caliper, torque the mounting bolts and hose fitting to the values in the factory service manual, and fit new pads and hardware.
5. Bleed the system in the order the service manual gives. A full flush on a modern BMW may call for a scan tool to cycle the ABS unit.
6. Pump the pedal until it is firm before moving the car, then bed in the pads.

A caliper rebuild is harder: the pistons have to come out without damaging the aluminum bore, and fixed Brembo calipers should not be split apart at home. If you have never rebuilt a multi-piston caliper, a fresh caliper is the safer choice.

Frequently Asked Questions

Does the 2017 M240i have Brembo brakes?

Yes. The M240i comes with M Sport brakes as standard, using blue Brembo fixed calipers: four pistons in front and two in the rear.

Can I replace just one caliper?

You can, but it is better to service both calipers on the same axle so both sides grip the same. At minimum, replace the pads on both sides.

Why is only one of my front brakes getting hot?

Most often a piston in that caliper is not retracting because of corrosion behind a torn dust boot or old, wet fluid. Check piston movement and pad wear on that side.

How often should I change brake fluid on an M240i?

BMW's Condition Based Service schedules it by time, typically every two years. If you track the car, change it more often.

Are 340 mm and 370 mm front parts interchangeable?

No. Rotors and pads are sized to the brake package on your car. Confirm the rotor diameter before buying front parts.

Can I rebuild the Brembo calipers myself?

Yes, with seal and piston repair kits, if the bores are clean and undamaged. A pitted bore or cracked body means replacing the caliper.

Sources

1. BimmerWorld - F22/F3X stock, M Sport and M Performance brakes - M240i standard M Sport brakes, 4-piston front and 2-piston rear blue Brembo calipers, 340 mm and 345 mm rotors, 370 mm option
2. Brembo - Differences between fixed and floating brake calipers - fixed caliper design, aluminum construction, pad wear and seal function
3. Brembo - Caliper repair kits - seals, dust covers and pistons, what to inspect
4. Brembo - Brake fluid replacement - moisture absorption, pedal travel, corrosion protection
5. Brembo - Choosing the right brake fluid - never downgrade DOT grade, follow the handbook
6. Rick Hendrick BMW - BMW maintenance schedule - brake fluid every two years via Condition Based Service