Mercedes W108 280 SE 3.5 Bleeding the brakes

The second of a 3 part series on how to change the brake callipers on a Mercedes w108.

Before we get onto this, a word about running and blogging (my running and someone else’s blogging).

I’d like to say welcome back to the world of y-front wearing anoraks of the blogosphere to the subversive runner. He generally blogs what I am thinking. No need to say any more.

On the running front I am in the process of ‘tapering’ for a marathon in just under 2 weeks. This isn’t really taking much of a toll on me as I had cut out my hills over the last few weeks anyway which was leading to a drop in mileage.

I recovered from the Cork to Cobh run with little or no problems. It didn’t impact on last weeks running – which was topped off with a 20 miler before breakfast on Sunday morning (makes me feel fit to just type it).

I have started to wear a new pair of NB 1064s even though the old ones have about 150 miles left in them. the logic behind this is that the old ones would have had 450 mile on them by the time the marathon comes around and the marathon in them or brand new shoes would hurt more than was necessary.

All that is required now is to stay clear of the kids colds and coughs and hopefully I’ll have a bit of fun on the 25th.

And now onto bleeding the brakes on an old Mercedes.

If you’ve never done this or don’t even know how the brakes on your car work then it couldn’t be easier. It’s a real case of if you do it once you’ll be able to do it again and again -just like wanking/sex/riding a bicycle (delete as appropriate).

On a bicycle the brakes are applied by pulling a level on the handlebar that pulls a cable that pulls the brakes onto the wheel rim or disc.

On a car the handlebar lever is replaced by a foot pedal (I bet you knew that bit) and the cable to the brakes is replaced by a pipe that is filled with a liquid known as  brake fluid (tell me if I’m going too fast on this one). When you push the brake pedal it squeezes the brake fluid down the pipes (known as brake lines) and this fluid pushes the at the back of the brake pad. the front of the brake pad is pushed against the brake disc (which is attached to the axel/wheel) and this slows you down. There are bits and bobs in between this that make sure that you don’t need the legs of a sprinter to press the brakes and that the fluid in the pipes reaches the 4 wheels in the correct order (front, then back on a car, back the front on a truck).

Now the brake fluid is great at doing it’s job but it doesn’t work if 2 things happen to it – it is allowed to absorb water or there is air in the brake lines. Both of these will make the brakes feel spongy. Spongy is a technical/mechanical term for you pressing the brakes and not much happening except your brain going ‘fuck/shit/fuck/shit/fuck’.

If you disconnect the brake lines to change the callipers you’ll have allowed a bit of air into the brake lines  so, after going to the trouble of putting new hardware on the car it is obviously a good idea to ‘bleed the brakes’

There are two ways to bleed the brakes – the first one involves your wife sitting in the driver’s seat looking bored and you lying under the car and shouting ‘pump!, pump!’ to her.

The other one you can do on you own with a device you buy in a shop (no laughing down the back – this is serious!)

This is a picture of said device.


DIY Device


This works in a pretty straightforward way – you fill the plastic bottle with fresh DOT 4 brake fluid from a sealed container and connect the white plastic hose to the main brake fluid reservoir under the bonnet of the car. The black hose is connected to the air valve on a the spare tyre. The pressure in the tyre pushes air into the black hose; this, in turn, pushes onto the fluid in the white plastic bottle which pushes the fluid into the main brake fluid reservoir. This pushes fluid down the brake lines and pushes the air to the ends of the lines. You can then open a brake bleeding nut and this lets the air and other impurities out. The brake bleeding kit comes with a selection of hoses and caps to fit all sorts of brake fluid reservoirs and a a selection of hoses to bleed the fluid from the bleeding nut to a suitable container. Don’t forget to take the rubber nipple off the brake bleeding nut. You’ll need a 9 mm spanner to open and close this nut.

The reason you use the spare tyre is because once you have bled the brakes the tyre might not be at full pressure. This would not be smart if it was one of the main tyres on the car.

See if you can follow all of that by looking at the three photographs below.

Air in the black tube, brake fluid out the white tube
























Brake fluid and car paint are like your wife and your porn stash – they don’t go together very well.

So take care to put a bit of a cloth or rag between  the bottle and the car paintwork.

Another view showing the air connection to the spare tyre

























Use a 9mm spanner to open and close the brake bleeding nut.

















Now, just to make sure you do it properly you’ll need to bleed the brake lines in a certain order. If my memory serves me correctly you should bleed the ones closest to the reservoir first. I think the sequence is front left, front right, rear left and rear right.

After you’ve finished this you just have to disconnect the bleeding kit, re-attach the wheels, take the car off the stands, tighten the wheel nuts, put on the hub caps, make sure the brake fluid reservoir is topped up, dispose of the waste fluid responsibly (not on the driveway), put on the kettle, have a cup of tea and feel like a ‘real’ man.

Don’t forget to take the car out for a bit of a spin to test the brakes and to check the brake fluid reservoir one last time.

Easy peasy lemon squeezy.

You’ll have to do the same thing to the  other calliper but that is the stuff of the next post.


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