TOOL OF THE MONTH
Our Ralph looks at what’s stopping us on our classics!
Ralph on brake fluid.
Brakes that work well and tyres that grip are the two main things that keep us alive on two wheels. Today we look at brake fluid and sadly I see so many bikes with fluid that is in a terrible state and the owners seem unaware of the potential damage to their braking systems, not to mention their safety.
As we are interested in classic motorcycles in CMM, I shall mainly concentrate on DOT 4 fluid, which is really an upgrade on DOT 3, a glycol ether-based fluid which many of our older bikes would have started life with. DOT 4 has the addition of borate ester and has raised the boiling point of the fluid by around 15°C. If you have ever had the unpleasant experience of over-heating your brakes and boiled the fluid, then you will understand that anything that increases the boiling point has to be a good thing. Brakes work on friction, but with friction comes heat and plenty of it, so brakes have to be designed to deal with it. Harley-davidson used the silicone-based DOT 5 fluid in their bikes, which has the advantage that a spillage doesn’t lead to a respray and a higher boiling point. DOT 5, however, is likely to disappear because it is incompatible with
ABS (anti-lock braking systems), now mandatory on ALL new motorcycles following an EU ruling. Harley-davidson has now returned to DOT 4.
Because of the incompatibility with various seals and ABS, a new specification of fluid with an enhanced boiling point was required and would return to a base of glycol ether/borate ester. Rather than call it DOT 6, which would be logical, the new fluid, (that is completely incompatible with DOT 5), was named DOT 5.1.
It is important to understand that polyglycol fluids are hygroscopic, which means that the fluid absorbs water so that it doesn’t corrode the components of the braking system. It is imperative that you don’t have too much water in the fluid as above four per cent the water will start to cause corrosion. Brake fluid is described as being dry when it has no water in it and DOT 4 has a boiling point 230°C, but add 3.7 per cent water to the fluid and the boiling point sinks to only 155°C. For this reason, you should check the water content of your brake fluid at least every six months and the easiest way to do this is with an electronic tester like the Laser one I use.
Changing the fluid can be as easy or difficult as you want to make it, depending on budget. Photo 2 shows the most budget available bleeding equipment, which isn’t much of a step up from the bit of pipe and a jam jar. The pipe attaches to the bleed nipple and one’s assistant squeezes the brake lever whilst the man grovelling on the floor opens and closes the nipple. It is a laborious method, which I haven’t used since I was a teenager.
Vacuum bleeders are by far the easiest method and aren’t always expensive. The first step is to bind the threads of the nipple in PTFE tape, as this helps prevent sucking excessive air. Firstly, I will deal with my first vacuum bleeder, the Mityvac, which hasn’t changed in design for well over 30 years. You simply attach the pipe to the bleed nipple and actuate the hand vacuum pump, which sucks the air out of the pot and the pipe. Air pressure above the open master cylinder reservoir then forces fluid out of the master cylinder through the line and out of the open nipple into the collection pot. The master cylinder reservoir is kept topped up and when you feel enough fluid has been dragged through, you simply lock off the nipple.
Laser do a cheaper version that substitutes a syringe for the vacuum pump, which worked better than I expected in my test, though I would not have wanted to bleed an empty system with it.
If you have a compressor, even a small one, then the compressed air driven vacuum bleeder from Laser Tools is a cracking piece of kit. Plug it into an air-line, fit the pipe, the nipple, and switch on the valve and off you go. This bleeder actually has two pipes so you could do both calipers on a twin disc bike together (though were I keeping it I would ditch the second pipe.) For under £50 it’s a bargain.
The last new piece of kit Laser Tools sent me to test was their professional airpowered vacuum bleeder kit. It is primarily aimed at car garages and the kit comes with an automatic filler for the reservoir. I managed to attach it to my Z1000 master cylinder and at first thought it was great, until it started to overfill it and was dripping all over my bench, fortunately missing all the bike’s paint work; I had covered the tank with a bin bag! I think the problem is the small amount of fluid in the master cylinder was the problem and if I hadn’t filled it so much, I may have had a better result. The actual vacuum unit though is fantastic, and I have already warned Laser Tools they’re not getting it back, despite my already having a professional unit. This one is so much better made and is a sensible size to use on motorcycles with a two-litre capacity.
Basically, you have to change the fluid in your brake system at least once every two years. How easy you make it for yourself is up to your wallet. The cost of the cheaper of the two pneumatic bleeders is still way less than the cost of an hour’s labour at a regular small bike shop. Remember, brake fluid is cheap, brake components rarely are, and how much do you value your life?