A Complete Guide To Check & Repair Brake Fluid

Do we really pay as much attention to what our brake fluid is doing as we should ?

Many bikers who look after their own steeds change the oil and filter, the spark plugs, check the disc pads and check that the lights work.

Very few seem to have a clue in a bucket how often other fluids need changing, nor the brake lines. Standard stock rubber brake lines should be changed every four years : fact ! Given the huge cost of stock items a sensible person will replace the cheap (to make) OE rubber lines with braided lines with stainless fittings.

They are less costly than OE and many, such as HEL, carry a lifetime warranty. Rubber lines swell under pressure, wasting lever/pedal effort which should be going into stopping the bike. If you lightly grip a rubber brake line, especially an old one, and grab a handful of anchor you will feel the line expand slightly in your fingers. The power to swell that line should be acting on the brake pads!

Another issue that few seem to appreciate is that brake fluid has a fairly short lifespan and should be changed regularly. It is hygroscopic, which means that it absorbs water. On the whole this is a good thing as it stops the water collecting in corners of the braking system, but you don’t want a very high concentration of water as it will attack the braking system components.

Watery fluid often sets up corrosion behind the caliper seals and the flaky aluminium pushes the seal tight against the piston, massively reducing brake efficiency. The water also attacks the chrome of the piston, eventually holing it and setting up rust underneath in the mild steel. For some bikes it is just the hassle and expense of new pistons, for others you really have a much bigger problem if you can’t get a new one.

At its worst the water loaded fluid will boil in the calipers from the heat generated from the pad to disc friction and that is really bad news. Laser Tools produces a cracking little professional tool for the trade which measures the water content of the fluid. We bought one years ago and have found it invaluable. We not only test the fluid in the bikes but also the new fluid.

When we first got the tool and were playing with it, we tested some brand new fluid from an unopened pot only to discover that it had a high-water content! We now buy our fluid from large high street autoshops, because they have a massive turnover and the fluid is always fresh.

It breaks my heart to pay retail, but it is better that than putting dodgy fluid in bikes! Given the ramifications of a brake failure, we now test all fluid before use as a matter of course.

With a MityVac Vacuum Brake Bleeding Kit, changing the fluid is a piece of cake. It’s best to wrap the threads of your bleed nipples with PTFE tape to stop it sucking air past the threads. Do this when you throw away the nasty BZP coated mild steel nipples to fit nice stainless ones!

Fit the vacuum pipe to the nipple, pump up a vacuum and crack open the nipple so the MityVac can suck the fluid out. Ensure that you keep topping up the master cylinder reservoir so that you don’t suck air into the system, or you will have to bleed it out again.

It isn’t hard to bleed most bikes with a MityVac, though in my experience Old Kawasaki fours are a nightmare for hiding little pockets of air for some reason, especially at the splitter. Keep sucking the fluid until you’re sure all the fluid is new.

If you’re really keen, empty out the MityVac pot and draw some more fluid out and test that. When doing front systems, I tend to get She Who Must Be Obeyed to keep the master cylinder reservoir topped up whilst I concentrate on the calipers. I always cover the tank with a new bin bag just in case SWMBO spills a bit of fluid on the paint work – although she seems to have a steady hand !

Always clear up spills ASAP as fresh brake fluid is a very adept paint stripper. I always wear disposable nitrile gloves as I find it hard to believe that something that melts paint is going to beneficial to my skin.