every two years or...(a) you get decresaed breakign effieiceincy (b) you can get corrosion in the braking system ...its hydroscopic and absorbs water over time...this is GOOD as it keeps the inside of you brake system dry...but its BAD because it can only absorb so much...so eventually it gets to the point where weird things like water condensing out in your calipers...or boiling out under heavy braking become an issue....hence the evey two years...

dont pay for it though do it yourself...use a presure bleader (about $20) and you need a spmnner anda socket ot fit the brake nipples...and asome clear plastic hose to lead off the nipples...useht socket to brake the nipples free and the spanner to actually undo.do them up....easy job and very satisfying...bet yyour brakes get better too!