Having to ask suggests a lack of familiarity with cars but it is something you can do yourself with some precautions and warnings. First of all, it is absolutely imperative you do not allow any hint of grease or petroleum product to get on anything that contacts brake fluid. I even wash my hands to avoid contaminating the brake fluid with fingerprint grease. Use a flare-nut wrench, also called a line wrench, to loosen and tighten the soft nuts holding the steel lines to the master cylinder.
I'll include a trick that will prevent you from having to bleed at the wheels. Loosen the line nuts. Remove the nuts holding the master cylinder to the power booster. Pull the master cylinder off the booster and use it as a handle to bend the steel lines upward just enough to keep the fluid from running out of them. Remove the nuts the rest of the way. Don't let dripping brake fluid get on paint; it will eat it away.
Put the new master cylinder in a vise to hold it, then "bench-bleed" it. The new part will come with instructions for doing that, and a pair of hoses. Run the line nuts into the ports by hand. Those nuts should spin at least two or three turns to be sure they aren't cross-threaded. Push down on it to bend the lines back the way they were, then bolt the master cylinder to the booster. At this point you'll need a helper to work the brake pedal. The line nuts must not be tight yet. Have your helper VERY SLOWLY push the brake pedal half way to the floor, (never all the way). It should take about 20 seconds. As he does, you'll see air bubbles coming out around those nuts. When he hollers that he's half way down, or when you stop seeing bubbles and only see fluid, snug the line nuts, THEN he can release the pedal rapidly. If he releases the pedal before the nuts are tight, air will be drawn back in. By pressing the pedal slowly, any air in the lines will keep floating back up as the fluid goes down to the wheels. By releasing the pedal quickly, any trapped air bubbles will just wash back up into the reservoir along with the returning fluid. Loosen the nuts and do that a second time to be sure no more air bubbles come out. Tighten the nuts and fill the reservoir according to the condition of the front brake pads. If the pads are fairly new, fill the fluid to the maximum line. If the pads are older, fill the fluid to a little more than the minimum line. There needs to be room left for the fluid that will return there in the future when new pads are installed. Wash any spilled brake fluid from painted surfaces.
May, 7, 2013 AT 1:00 AM
Usually, fluid purging from the vent in the cap is caused by a restriction in the system, overheating fluid or low fluid level causing aeration which will vent out the cap. I know this is a very generalized answer, but without some diagnostic testing it is not possible to give an absolute answer. If the fluid is dark or burnt looking then the rack and pinion is probably wearing and the aluminum suspended in the fluid is causing restriction in the pressure regulator valve. If the fluid is clean bright red then I would suspect a leak causing the fluid to aerate from being low. Leaks from the rack and pinion side seals will not show because the fluid leaks into the tie rod bellows on the sides of the rack. The bellows need to be pulled back from the rack to inspect for the presence of fluid. Before having any parts replaced I would recommend having the power steering system professionally flushed with a power steering flusher. Most repair facilities will have the equipment necessary to flush the system. This will often times cure many steering issues as the regulator valve is freed up by removing the contaminates in the fluid and restoring the lubricity of the fluid.