Air can get into the brake hydraulic system if a line is ever disconnected, or when pads are replaced. That can lead to the pedal becoming low; which would improve if the air was purged out of the system. But I would still consider the possibility that the pedal height might not improve from bleeding the system. If that happened; it would reinforce my theory that the master cylinder has an internal leak and therefore needs to be replaced.
There are two simple tests which you can do to evaluate the condition of the brake system. One is to sit in the car with the motor idling, the parking brake released and the transmission in drive. Press the brake pedal down far enough to keep the car from moving, and keep pressing hard enough to prevent the car from moving for three or four minutes. Pay attention to the position of your foot during that time. If your foot does not move lower during that test; the master cylinder is probably OK. But if your foot gradually moves lower; the master cylinder is leaking internally.
The other test is also done with the motor idling and the parking brake off: Press the brake pedal hard enough to keep the car from moving, note the height of your foot; and then quickly lift your foot up and rapidly press the pedal to the point of resistance three times. At the end of the third stroke, hold the pedal down and note whether your foot is now at the same height that it was orginally. If your foot is now higher than it was before; there is air in the brake lines.
When bleeding brakes; bleed the cylinder at the right rear wheel first, then the left rear, then the right front, then the left front. It is important to prevent air from being drawn back into the system when the pedal is lifted up. For this reason, the end of the hose from the cylinder being bled should be submerged in a jar that is partly filled with brake fluid. That will both prevent air from being drawn back into the cylinder, and will make it apparent when bubbles come out while the brake pedal is pressed.
The way to bleed brakes is to open the bleed valve about 1/4 turn, and have someone in the car gradually press the brake pedal all the way down while you watch the fluid come out the end of the submerged hose into the brake fluid in the jar. Once the pedal has been pressed all the way down; DO NOT LET THE PEDAL COME BACK UP UNTIL AFTER YOU FIRMLY CLOSE THE BLEED VALVE. Then let the pedal up, pump it down and up a few times, and then open the valve again and do another full down stroke of the pedal. If any air bubbles or dirty fluid comes out during the down stroke, close the valve at the end of the stroke and then repeat the procedure. Keep repeating the bleeding sequence until no more air or dirty fluid comes out during a down stroke. Then close the bleed valve, remove the hose, and move to the next wheel. Be sure that the level in the fluid reservoir is kept up. If the fluid level goes below the bottom of the reservoir; air will get into the system and require the system to be bled all over again.
It is a potentially messy and demanding job to bleed brakes. If you don't have the proper size wrenches (6 point box or flare nut wrenches are required), it can be nearly impossible to open the valves. It is also important to pump the pedal between strokes; in order to more effectively draw trapped air into the master cylinder.
If you are not mechanically inclined, I would not recommend doing this yourself. Find a shop with pressure bleeding equipment, and have them do it.