Brake Bleed Procedure on Smart Fortwo and Forfour
Service interval: Whenever pedal feels spongy, after caliper removal, after pad/rotor work that pushed pistons back hard, or after a fluid flush
Tools you'll need
- 7mm or 8mm box wrench for the bleeder screws (verify per chassis)
- Clear bleed hose snug-fit on the bleeder nipple
- Catch jar — a clean mason jar works
- Penetrating oil for stubborn bleeders
- Helper for pump-and-hold, OR a vacuum bleeder, OR a pressure bleeder for solo work
- 6-point socket as a backup if a bleeder starts rounding off
- Shop rags and clean water nearby — brake fluid eats paint
Fluids & specs
| Fluid | Spec | Capacity |
|---|---|---|
| Brake fluid | DOT 4 — 453 generally calls for MB 331.0 / DOT 4 LV (low-viscosity) for ABS/ESP. 450 and 451 take regular DOT 4. Don't use DOT 3 (lower boiling point) and don't use DOT 5 (silicone, incompatible). Don't use bottles open more than 12 months. | Roughly 250-500 ml for a bleed; 0.5-1.0 L for a full flush |
What this is + why it matters
Bleeding brakes is just getting air out of the hydraulic lines. Air is compressible, brake fluid isn't — that's the whole story. When the pedal goes spongy, sinks slowly under steady pressure, or comes up high one stop and low the next, you've got air somewhere in the system. The fix is the same on every Smart from the 450 to the 453: open each bleeder in sequence, push fresh fluid through until what comes out is clear and bubble-free, close it, move on.
You'll need to bleed after any of these:
- A caliper came off and went back on (rebuild, replacement, or even just unbolted to swing out of the way).
- Pads or rotors got swapped and the pistons got pushed back hard against fluid in the line.
- You did a full fluid flush — bleeding is the last step of a flush.
- The pedal feels spongy, sinks under steady pressure, or comes up at a different point each stop.
- You opened a brake hose or hard line for any reason.
The procedure covers all three Fortwo chassis and the 453 Forfour with minor differences, mostly in bleeder wrench size and rear-wheel access. The bleed sequence and the technique itself are identical across the lineup.
One thing this procedure won't fully fix: air that's worked its way into the ABS modulator. A manual bleed gets you about 80% of the way. The remaining 20% lives inside the ABS pump and needs a scan tool — Star Diagnosis or compatible — to cycle the pump electrically while you bleed. If your pedal stays soft after a careful bleed and you've recently been into the ABS block, that's where the air is. Skip ahead to the gotchas section.
What you'll need
Listed in the tools section above. A few specifics worth calling out:
- Bleeder wrench size varies. Most 451 and 453 calipers use 7mm or 8mm bleeders. The 450 is similar. Test-fit before you crack anything — using a wrench that's a hair too big rounds the bleeder off, and a rounded bleeder turns a 90-minute job into a caliper replacement. A box wrench is better than an open-end here; a flare-nut wrench is better still if you have the right size in the kit.
- Fresh fluid in a sealed bottle. Brake fluid is hygroscopic. Once a bottle's been open more than a year, the fluid inside has absorbed water from the air whether you can see it or not. Don't reuse last year's half-bottle.
- Match fluid spec to chassis. The 453 with ABS/ESP generally calls for MB 331.0 / DOT 4 LV — low-viscosity so the ABS pump can move it fast in cold weather. The 450 and 451 take regular DOT 4. Mixing DOT 4 and DOT 4 LV is generally OK in an emergency, but for a full bleed match what your reservoir cap says.
- A pressure bleeder makes this a one-person job and keeps the master cylinder topped up automatically. A vacuum bleeder is the next-best solo option. A helper pumping the pedal works fine if you have one — that's the classic pump-and-hold method.
- Chassis access notes. On the 450 the rear bleeders sit close to the wheel arch and can be reached without pulling the wheel; the fronts are easier with the wheel off. On the 451 all four are reachable wheel-on but you'll want the wheel off if you also want to inspect the slide pins or pad backing. On the 453 the rear caliper bleeders are tucked behind the parking brake mechanism — wheel off is the right call.
Step by step
- Park on level ground, wheels chocked. Ignition off. If access is tight at any wheel, pull that wheel — on the 451 and 453 you can usually reach all four bleeders with the wheels on, but it's faster off. Cover the fenders with rags before you start.
- Top up the master cylinder reservoir to the MAX line with fresh fluid before you start. From this point until the job is done, check the reservoir between every corner and never let it drop below MIN. Running it dry pulls air into the master and means a much harder recovery bleed.
- If any bleeder looks corroded or the wrench doesn't seat fully, soak that bleeder in penetrating oil and walk away for an hour. Coming back to a soaked bleeder beats snapping one off and learning what a caliper costs.
- Bleed sequence: rear-passenger first, then rear-driver, then front-passenger, then front-driver. Longest hydraulic line first, shortest last. The 451 and 453 share this order; the 450 follows the same logic. If your workshop manual specifies different, follow the manual.
- At rear-passenger: clear hose snug onto the bleeder nipple, other end submerged in 1-2 inches of fresh fluid in the catch jar. The submerged end stops air being sucked back through the bleeder when it's open.
- Pump-and-hold (two-person): crack the bleeder open about a quarter turn. Helper presses the brake pedal slow and steady all the way to the floor and holds it there. You close the bleeder while they're holding. Only then does the helper release the pedal. The order matters — if the helper releases before you close the bleeder, air gets sucked back in past the threads. Repeat at this corner until what comes out is clear, bubble-free fluid — usually 4 to 8 cycles. Top up the reservoir between corners.
- Vacuum bleeder (solo): attach the bleeder pump to the bleeder nipple. Crack the bleeder open. Pull vacuum until you've drawn through about 250 ml at that corner with no bubbles. Close the bleeder before releasing vacuum. Top up the reservoir. Move on.
- Pressure bleeder (solo): pressurize the reservoir adapter to about 10-15 psi. Open each bleeder in sequence; let clean fluid flow until bubble-free; close. The reservoir stays full automatically — that's the whole point of a pressure bleeder.
- Move through rear-driver, front-passenger, front-driver in that order. Each corner takes 3-5 minutes once you're set up.
- After all four corners, top the reservoir to MAX. Pump the pedal a dozen times with the engine off. It should come up firm within the first inch or two of travel. Spongy means there's still air; bleed the worst-feeling corner again.
- Test drive at low speed with plenty of room. Confirm pedal feel and stopping power at 15-20 mph before you take the car onto a real road. If the pedal goes long after the first few stops, top up the reservoir, pump the pedal back up, and re-check the worst corner — there was still air.
A quick chassis-specific note: on the 453, if you've had the rear pads off and forced the parking-brake-integrated piston back, the EPB (electronic parking brake) sometimes needs to be put through a service-mode cycle with a scan tool before the pedal returns to normal. If you didn't touch the rear pistons in a way that involved the EPB, you can ignore this.
Common gotchas
The reservoir runs dry mid-bleed. This is the single most common failure mode. Air gets into the master cylinder and now you've got a much harder bleed — sometimes you have to bench-bleed the master with the lines off the car to recover. Top up between every corner. Use a pressure bleeder if you don't have a helper.
The classic recovery if you do let it run dry: cap the reservoir, push the master back in slowly with a screwdriver shaft on the pushrod (engine off, key out), and bleed each corner two or three times in sequence. Sometimes that gets the air out. Sometimes it doesn't and you're pulling the master to bench-bleed it on the bench with rubber lines looped back into its own reservoir. Avoid the whole thing — top up between corners.
Spongy pedal after a careful bleed. On any Smart with ABS/ESP — which is all 451 and 453 — air that gets into the modulator will not come out by gravity or pump-and-hold. The ABS pump has to be electrically cycled while you bleed, and that needs Star Diagnosis or a compatible scan tool. Manual bleed gets you 80%; the last 20% is in the ABS module. If you cracked an ABS line or let the master run dry, plan on a shop visit or a smart-specific scan tool.
Bleeder screw rounds off. Old bleeders corrode in place. Soak them in penetrating oil overnight before you start — twice if the car has lived in salt country. Always use a 6-point box wrench or socket, never a 12-point. A 12-point grabs the corners; a 6-point grabs the flats. If the bleeder is already starting to round, stop, soak more, and consider a bleeder extractor before it snaps. A snapped bleeder means caliper rebuild or replacement.
Brake fluid eats paint. A drop on a wing or fender will lift the paint within a few hours and you'll have a permanent blister to remember the job by. Lay rags over the fenders before you start. Keep a wet rag and a water bottle within reach. Wipe spills off bodywork, calipers, and the driveway immediately — water dilutes it fast if you catch it in the first minute.
Air re-enters through a loose bleeder. If the bleeder isn't closed snug between pedal cycles, air sucks back in past the threads. Close it firmly each time before the helper releases the pedal. The submerged-hose-in-jar trick helps here too.
Wrong fluid in the bottle. Cheap unbranded "DOT 4" from a discount auto parts store has burned more than one Smart owner — the boiling point comes in low and the pedal goes long after a hot stop. Buy a recognised brand (ATE, Bosch, Pentosin, Mercedes, Castrol React) and check the bottle date if it has one. For 453 ABS/ESP, look specifically for the DOT 4 LV designation on the label.
Forgetting to fit a fresh dust cap on the bleeder. The little rubber cap keeps grit out of the bleeder cone seat. If it's missing or torn, replace it — grit on the cone is how you get a bleeder that won't seal next time you open it.
When to skip DIY
If the pedal stays soft after a careful manual bleed and you've recently been into the ABS lines, that's a scan-tool job — find a shop with Star Diagnosis or take it to a Smart specialist. The ABS pump needs to be cycled electrically with the bleeders open at each corner, and that's not happening with hand tools.
If a bleeder snaps off in the caliper, the caliper is coming off the car for either a thread repair (Heli-Coil or Time-Sert) or a replacement; that's not roadside-fixable and most owners send the caliper out. Reman calipers run $50-150 each — usually cheaper than the labour to repair a snapped bleeder.
If you've never bled brakes before and you're working solo without a vacuum or pressure bleeder, find a helper or buy a $25 vacuum kit before you start. Running the master dry on your first attempt is the classic costly mistake — it turns a 90-minute job into a recovery bleed that some owners can't recover at all without a scan tool.
And if the pedal was rock-hard before but is sinking slowly to the floor under steady pressure now, before you bleed — check the master cylinder. A bypassing master cylinder feels exactly like air in the lines, but no amount of bleeding fixes a worn master. If your fluid level is steady and there are no external leaks but the pedal sinks, replace the master before you spend another afternoon on bleeders.
Parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| DOT 4 brake fluid (1L) | $10-20 | Amazon · eBay · AliExpress · Google |
| DOT 4 LV brake fluid (1L) — 453 ABS | $15-25 | Amazon · eBay · AliExpress · Google |
| One-person vacuum bleeder kit | $25-50 | Amazon · eBay · AliExpress · Google |
| Pressure bleeder (Motive-style) | $60-90 | Amazon · eBay · AliExpress · Google |
| Replacement bleeder screws (set) | $8-20 | Amazon · eBay · AliExpress · Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread. Marketplace links are non-affiliate.
Manual references
Top reference manuals for this chassis (from our catalog of 88 Smart manuals):
- 1998-2007 smart fortwo (450) - Communication & Breakdown Supplement — Breakdown Supplement, 33p, 2.3 MB
- 1998-2015 smart fortwo (450 451) - Technical & Service Reference — Technical & Service Reference, 977p, 92.7 MB
- 1999-2005 smart (450 452 454) - Glasurit Paint Color Codes — Paint Color Codes, 13p, 0.2 MB
- 2007-2014 smart (450 451 452 454) - Workshop Repair Manual — Workshop Manual, 4602p, 290 MB
- 2008 smart fortwo (451) - US Introduction into Service Manual — Introduction into Service Manual, 122p, 41.2 MB
Need something specific? Browse all 88 manuals by chassis, year, region, or document type.