This is a tool we have only just discovered ourselves; and we are bowled over by it. Brake fluid is hygroscopic, which means that it absorbs water. If it has more than a few percent of water it should be changed. But how do you tell? Obviously with one of these testers! If in doubt change it; it's not that expensive. Water in the brake fluid will corrode the working parts of your braking system, which isn't a good thing! The most common problem is that the aluminium of the caliper, behind the piston seal corrodes and pushes the seal too tightly against the piston which means that you need more pressure to move the piston against the pads. This effectively reduces your braking efficiency and will eventually seize the caliper altogether. Another issue with water in the fluid is that as the brakes are applied the friction makes them heat up and as water only reaches 100°C before boiling, the water in the fluid turns into steam making the fluid compressible and further reducing the efficiency of the brakes. As bikers, we are vulnerable enough at the hands of car drivers who seem hell bent on killing us off as a race, without having iffy brakes!