Why Should You Bleed Your Brakes?
Well there are several reasons:
New Brake fluid is fairly incompressible. So when you squeeze your brake, the force in the piston at the Lever goes to the piston at the calliper with the fewest losses. Over time however, a couple of things happen. One is that air can get into the system, either by absorption directly into the brake fluid or more commonly by absorption of water, which contains air.
Air is much more compressible than Brake Fluid. Think in terms of childhood games with a needle-less syringe filled with water. If you put your finger over the end and squeeze the plunger, it doesn’t move at all, or not so you’d notice. With air in the syringe rather than water you can compress it a fair bit. Same happens when air gets absorbed into your brake fluid. It’s what makes your brakes feel mushy or squishy.
The second thing that happens is that the brake fluid can absorb water. This water very often gets into the system as a result of condensation due to heat, from when the brakes or hoses get hot. Water can have dissolved oxygen in it, which when hot expands into small air-bubbles. Enough of these can also make the fluid more compressible. The water, if left in the system, can cause corrosion of the Cylinder, which is what eventually ruins your seals. When you bleed your brakes and the old fluid is brown, part of that is the corrosion.
Water in the fluid will boil and change state from compressible gas to incompressible fluid during the brake application, the change in volume causing a change in brake force. Old brake fluid can cause a failure that is fast and dramatic. i.e. if they get hot from constant use down a hill, you can have a situation where you will have NO BRAKES. So change the fluid regularly.
Basically, if you ever do anything that results in fluid coming out anywhere except the filler, you need to think about bleeding.