Do you have a spongy brake pedal, less than ideal braking force, or feel like you need to pump the pedal to get it firm? Chances are you need to bleed or flush your brake fluid. Brake fluid is hygroscopic, meaning it absorbs moisture from the air and while your brake system is pretty tight, water vapour still sneaks in over time. The problem is, water boils at 100°c and when it boils it starts to be compressible. That’s a problem as your braking system requires constant pressure to work. DOT 4, when it’s brand new, is generally rated for around 270°c which is plenty for regular driving (although track enthusiasts should get high temp fluid instead as I found out the hard way).
Also, if you’ve just replaced a part of your braking system, either lines, calipers or master cylinder, you’ll need to bleed your brakes to remove any air bubbles trapped in the system since they too are compressible. So, let me walk you through how to do it. I’ll be doing it on this BMW 5 series, but the process is practically identical on every car so you should be able to apply this to your vehicle.
On this F10 5 series that’s nice and simple. First, pop the bonnet with the latch on the drivers side footwell, then pop the three clips holding the plastic cowling on the left hand side by the windshield. You can then lift the plastic piece up and remove it, then you should see the brake fluid reservoir. Unscrew the cap and use some paper towels to make sure the float on the fluid level sensor doesn’t drip brake fluid all over your paint – DOT fluid is nasty stuff and removes paint rather well.
If you are flushing the system, which I am here, I like to remove as much fluid as possible from the reservoir before heading to the wheels. You can use a turkey baster or a syringe, this one I’ve got is for bleeding mountain bike brakes. Draw out as much as you can then you can use a funnel to pour some fresh fluid in. It’s best to fill it right up as we are going to be drawing a fair bit out through each caliper and we don’t want this running dry at any point.
Next is the dirty bit. Generally you want to start at the caliper that is the furthest from the master cylinder or ABS pump, but really either of the rear wheels is fine. Jack up that corner, remove the bolts and take the wheel off, then it’s a good idea to give the area around the bleed nipple a clean with some brake cleaner. Take the dust cap off, fit your spanner then your brake bleeder, and loosen the bleed nipple so fluid can flow out. Since this is the rear brake, don’t forget to take the handbrake off so the piston can move freely while we are doing the rear wheels.
If you’ve got two people, tighten the valve and have the person in the driver’s seat pump up the pedal, then while they hold the pedal down loosen the valve. The pedal should move towards the firewall as fluid is pressed out. Hold the pedal to the floor, tighten the bleeder then repeat. We did that about 5 times on the farthest wheel, and 3-4 on the rest.
If you are solo, but have this style of ‘one man’ bleeder kit you can just loosen the valve, then pump away at the pedal as fluid gets pushed out, but thanks to a one way ball valve in the tubing it can’t go back in.
Once you are done with a wheel, tighten the bleed nipple before you take the tubing off. You don’t want to introduce any air bubbles by accident. Then stick the dust cap on and give the area a quick blast with brake cleaner or soapy water. Now the wheel can go back on, and get torqued to spec. In this case that’s around 120 nm, done in a star pattern. I’ve found that running all the bolts in and getting them as snug as you can by ratchet then lowering the car slightly to get the final torque is the best way to not strip out your wheel hubs.
The fronts are the same process, just with a different position for the caliper. Remember to check and top up the brake fluid as you are flushing the lines out, if it runs dry you have to bleed all four wheels all over again.
If, like us, you are a little lazy and don’t fancy removing the front wheels to do this, if you turn the steering wheel all the way right, you can access the left hand side caliper’s bleeder without even jacking the car up. That’s what we did at the track day at Donington, and while it’s a little more difficult to find your way around just by feeling, it can be done and saves you some effort.
As a final step, check your reservoir and top it up so the fluid is about halfway up the basket. Then you can stick the sensor back in, screw the cap down, put the plastic trim piece back along with the three clips, drop the bonnet and that’s it, all done.
