Car brake bleeding guide by micrabits

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Bleeding brakes on a car hints and tips!


Best method?
Everyone has their own theories but from personal experience I always start at the O/S/R wheel then the N/S/R wheel, N/S/F and finish at the O/S/F. Many pub experts and workshop manuals suggest starting at the furthest away bleed screw, typically the N/S/R but having tried both methods I find my method easiest!  

Do not pump to hard or fast.
If you are bleeding brakes in the normal way (Wife/mate/apprentice pumping the brake pedal while you slacken & tighten the bleed screws), it is easy for them to assume they should pump the pedal hard and fast as if they are trying to kick a tramp down a drain! This can cause the master cylinder seals to flip round rendering it useless. When I bleed brakes with an assistant I will insist they pump the pedal slowly and steadily. When I bleed the rear brakes with wheel cylinders I insist they only press the pedal half way down. Some cars are more likely to suffer from this than others are. Old Vauxhall Astras (Astra F models) are like this. If you try bleeding the rear brakes pushing the pedal all the way down the seals will almost certainly flip.

What to use?
From my own experience nothing bleeds to brakes as well as a decent air powered brake bleeder. This is fine in a big workshop but impractical to have at the house! I have had good results with Gunsons Eesi bleed device, especially useful if you are working alone. I have used the Mityvac hand pump a few times too. It is good on cars which can be prone to flipping master cylinder seals if bled in a conventional way but it gives you a sore arm!

Planning.
If you know you are going to need to bleed the brakes on a car give all the bleed screws a spray with some loose Juice beforehand. (WD40 or similar) . Penetrating fluid works best if left to soak in so apply it the night before if you can. At the very least, give it an hour or so to soak in.

Stiff nipps!
If you start to slacken a bleed screw and it is very tight to turn or does not want to move at all do not be tempted to heat it with an oxy acetylene torch (or similar) as you risk damaging the seals in the calliper or wheel cylinder. Instead, boil the kettle and pour it slowly on the area around the screw. If you pour it fast the water will run off without heating the area but if you trickle it out it will heat the area and make it easier to get the bugge…bleed screw slackened.

All the bleed screws?
If you have to bleed the brakes make sure you bleed them at all the screws. Many larger cars have an additional bleed screw on the load-sensing valve, older cars sometimes have an extra bleed screw near the master cylinder.

Multi bleed screws?
Some callipers have more than one bleed screw. Many 4-pot callipers have 2, sometimes 3. There are 2 ways to bleed these. Method 1, get a T piece and make up a pipe for your bleed bottle so you can bleed 2 at a time, too much mucking about for my liking! Method 2. Start at the lowest screw, bleed it, then do the one above it, work your way from bottom to top. Far easier!

Bleed, road test & bleed.
Some cars with ABS can trap air in the ABS unit. When you bleed the brakes everything goes well until you road test it. Not good! If you are bleeding the brakes on a car with ABS you should ALWAYS road test it afterwards and do a few ‘emergency stops’ activating the ABS. If there is any air trapped in there it will be released and it may be necessary to re bleed the system. Early Vauxhall Vectras and BMW 1 series are bad for this. It is much better to take 10 minutes extra bleeding and testing than it is to spend 30 minutes in the boss’s office getting your ass kicked!

If you overfill it, empty it!
Most cars maximum brake fluid level is slightly below the top of the master cylinder reservoir. If you fill it right to the top and leave it the fluid seals up the small breather hole. Because the brakes cant ‘relax’ overnight the fluid will suck the pistons in on the brake callipers so the first time you press the brakes in the morning nothing happens! Argh!

Dot 3? DoT 4? DoT 5.1?
Some cars specify DoT 3 brake fluid (Some Suzukis, Mitsubishis and Protons) Most specify DoT 4. The DoT rating mainly refers to the boiling point. DoT 3 fluid is cheaper but boils at a lower temperature. To meet the DoT standard (Department of transport) minimum standards it must safely mix with all other DoT approved fluids. Most do, common exceptions are LHM (as used in Citroens and Rolls Royces) which should not be mixed.
DoT 5.1 is typically purple and will mix safely with normal DoT 3 or 4 but for best results it should be used on its own. DoT 5.1 is not hydroscopic like normal brake fluid (means it does not absorb water) so in theory never needs changed. (DoT 3 and 4 usually need changed every 2 years or so) DoT 5.1 also has a higher boiling point so it is good for fast road and racing applications. Its only downside is it does not have the same damping properties so the brake pedal may feel slightly wooden. (Hard to explain but you would know if you had tried it!)
Certain specialists sell racing grade brake fluid. This is best avoided as it absorbs water like nothing on earth and offers no real advantage over properly bled DoT 4 systems. It is dearer too!

Why change it anyway? Most people only bleed the brakes after repairing the hydraulic system. Replacing a wheel cylinder or brake pipe but manufacturers recommend the fluid is changed every 2 years. The reason is nice clean new DOT4 brake fluids boiling point is 290 degrees celsius. Even driving like I do does not generate that much heat! But if the brake fluid absorbs just 1% water the boiling point drops to roughly 245 degrees, 2% down to 220. Over 2 years your brake fluid can absorb up to 10% water whick significantly increases the risk of your brake fluid boiling and nothing happening when you brake! ARGH!

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