A big thanks to Nige, Pete, Stan and everyone else who gave me advice on this brake fluid. Seems dead simple now I know what's what but it confused the hell outta me for ages

Exhaustive Brake Fluid Explanations...

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There are 5 main types of Brake Fluid:

DOT 2: Used in old drum brakes and is obsolete
DOT 3: Used mainly in older cars
DOT 4: Used commonly
DOT 5: Used in military vehicles
DOT 5.1: Used mainly in high performance cars

DOT 3, 4 and 5.1 are Polyalkylene Glycol Ether based and will absorb moisture, DOT 5 fluid is Silicone based and will not absorb moisture. However Silicone based fluid gives the pedal a spongy feel. DOT 5 brake fluid does NOT mix with DOT 3, 4 and 5.1 fluids. You should never use DOT 5 fluid in a car which wasn't intended for a silicone based fluid. The silicone is very difficult to wash away. Generally DOT 4 and 5.1 fluids are fairly interchangeable, though if the car is designed to take DOT 3 fluid you should take a little more care.

As DOT 3, 4 and 5.1 fluids absorb water over time it is common to have figures for Dry boiling points and Wet (3% water) boiling points. obviously the more water in the fluid the lower the boiling point will be. If the brake fluid boils it'll induce brake drag which in turn will induce brake fade. Basically, you don't want your brake fluid to boil.

Here are some boiling points for common fluids. The DOT 3, 4 and 5.1 numbers are STANDARDS and for a fluid to be rated as DOT 5.1 a fluid must EXCEED these ratings:

DOT 3 Dry: 205°C Wet: 140°C
DOT 4 Dry: 230°C Wet: 155°C
DOT 5 Dry: 260°C Wet: 180°C
DOT 5.1 Dry: 260°C Wet: 180°C

AP Racing 551 Dry: 275°C Wet: 150°C
AP Racing 600 Dry: 310°C Wet: 210°C
AP Racing Formula DOT 5.1 Dry: 275°C Wet: 184°C
AP Racing PRF 660 Dry: 320°C Wet: 199°C
Ford Heavy Duty DOT 3 Dry: 288°C Wet: 143°C
ATE Super Blue Racing Dry:280°C Wet: 200°C
ATE TYP 200 Dry: 280°C Wet: 200°C
Motul Racing Dry: 315°C Wet: 216°C
Castrol SRF Dry: 310°C Wet: 270°C
Performance Friction Dry: 288°C Wet: 140°C

Typically it will take an average car around 2 years for the brake fluid to contain more than 3% water. Heavy use of the brakes cause them to absorb more water. I emailed a guy from AP Racing and he said that a good DOT 5.1 fluid should last a normal car around 4 years. However if you talk to some car enthusiasts they'll tell you to change your fluid after every track day. Do as you wish.

What you will generally find is that higher performance brake fluids needs replacing more often, and are much more expensive. 500ml of AP Racing 600 is £12 from Demon Tweeks, whereas you can buy 1 litre of DOT 5.1 fluid from Halfords for about £8.50. You'll find that something like Castrol SRF will be fantastically expensive and is simply not necessary for your average car. For the economists amongst you I found 1 litre of Delphi 5.1 fluid for less than £5 IIRC from my local motor factors. Its probably better than the Halfords stuff anyway.

I decided, looking at the cost of fluids and the huge difference in Dry and Wet temperatures that it would be more cost-effective to buy a 'cheap' (Halfords) DOT 5.1 fluid and change it about once a year.


Changing Brake Fluid

Brake Fluid is nasty stuff and doubles as paint stripper. I didn't buy any gay surgeon gloves but to be honest I wished I had as I got completely covered with the stuff by the end of the day.

Once you've decided on your brake fluid you'll need to completely replace your old fluid with new fluid. I chose to use "Gunsen's Eazy Bleed" pack as it was recommended by several people and turned out to be very easy to use. Costs about £13 from Halfords.

The instructions in the pack are quite straightforward, but basically:

Get a spare wheel and lower the pressure to < 20psi (I had it at 20psi but had to lower it further to stop leaks, I'd say try 15psi)

Start at the point furthest from the Brake Resevoir and work your way forwards (left rear, right rear, left front then right front) Connect the tube provided to the Brake Bleed Nipple and start bleeding some fluid from the left rear caliper (see Welly's Brake Bleeding guides) Ensure you have lots of kitchen towel under the brake resevoir incase the pressure is too high, then connect the tyre. LOOK OUT FOR LEAKS IN THE BRAKE RESEVOIR CAP!

New fluid will start to go through the system and old fluid will start to gently flow out the Bleed Nipple. You should see the old fluid is a 'deep yellow' colour, whereas your new fluid should be quite clear. After you've put in 200-300ml of new fluid you should see new fluid coming out the caliper. Look out for the change in colour, its quite subtle. Close the Nipple and move onto the next wheel (disconnect the tyre while you're messing about, and refill the bottle)

I found fluid came out the rear calipers much slower than from the fronts.

I only used about 900ml of brake fluid altogether, and I allowed a generous amount of extra fluid to come out. You might want to buy 1.5 litres of fluid just to be sure, but don't be tempted to store brake fluid: it'll absorb water even through the bottle.

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