The brake fluid reservoir should be topped up with new fluid if necessary. Use DOT 4 brake fluid. If the reservoir is allowed to get very low you will get air in the system and will need to bleed the brakes. Remember that the fluid is moisture sensitive so keep the cap on the bottle at all times.
You should replace the brake fluid every two years. Again, this is not something to worry about as long as you follow these guidelines. You will need to buy an 'easy bleed' kit. Mine is the cheapest one available and was under £2. It is simply a tube with a one way valve; the only other thing you need is an empty milk bottle. Although, you can bleed the brakes on your own, the job is a lot simpler if you have some help.
The secret is to keep topping the reservoir up as you remove the fluid so the system is always full. Gradually, you will replace the old fluid for new. The old fluid will be almost black in colour whilst the new stuff is golden yellow.
Start with the driver's side wheel. Find the bleed nipple on the brake caliper, which will have a rubber cover on it. The nipple itself is brass and is easily damaged ! so no mole grips or pliers please. Follow the instructions with your bleed kit. Generally, you place the open end of the tube over the bleed nipple so that it is a tight fit. Place the other end of the tube into the milk bottle and put some clean brake fluid in the bottle so that the end of the tube is submerged. Now the fun starts. With a suitable spanner, slowly loosen the nipple. It needs less than a quarter of a turn to release the ball bearing. Get your friend to sit in the car as you do this and push hard on the brake pedal. As you undo the nipple, the fluid should start to travel down the pipe and into the milk bottle. Your friend should gently pump the brake pedal while you keep topping up the reservoir up. After about 5 or 6 pumps, the new fluid should appear in the tube, and you can tighten the nipple. The brake pedal will go hard again. Remove your bleed kit and replace the rubber cover making sure to wipe up any spilled fluid. Repeat this exercise on the passenger side wheel and finally on the back. Note that there is only one nipple at the back (on the passenger side).
When finished, you should have a full reservoir of clean fluid. The brake pedal should feel nice and hard when pumped. If it has a lot of travel and feels spongy, you have air trapped in the system. Repeat the bleed process but watch the tube closely. Has the fluid coming out still got air bubbles in it ? Keep going until all the air disappears, remembering to top up the reservoir each time to keep the level up.