Obviously if you suspect a leak in teh brake system you would look the whole system over. A leak can be obvious however some of them are not. If you suspect air in the system the pedal will feel " spongy" like not hard when applied or it will go to the floor on first application and then come up on the second or third application. It will probably need fluid change unless the car has recently been worked on. It takes DOT 3 brake bluid.