Cruzaleks has a great point there. If there is a problem with the brakes it's just dangerous period. I'm sure you know that a car that doesn't stop is no good at all. And to add to what I said earlier, if YOU do attempt to change out the master cyliner yourself (if it is in fact your problem) you WILL need a flare-nut wrench, Basically a regular wrench with a few extra teeth, in order to get the brake lines free from the master cylinder. I tried my best with a regular wrench, didn't work, hit it with the flare-nut wrench and the lines broke free right away. Also, I don't know if your mechanically inclined at all but if it makes you uncomfortable messing with such an important system as brakes are then pay some one to do it. If you can handle it go for it and save yourself some money. After getting the lines free, and removing faulty master cylinder, bench bleeding the new master cylinder and then bleeding the lines is the hard part and not that bad really. Bench bleeding the master cylinder isn't that hard you just need to hook up some tubing from the outlets, add fluid and feed them back into the MC reservoir making sure not to let them suck in any air and pump the piston (where the pushrod contacts) untill you can no longer see any bubles being formed in the reservoir. Essentially you are bleeding the air from the master cylinder itself. You MUST do this before bolting it back up into the engine bay. Reconnect the brake lines.
Once you have the MC mounted and are ready to bleed the lines bleed left front, right rear, right front, left rear in that order. You do this with a small bolt that is on the caliper. Just open it up and have someone pump the brakes untill you see a steady, air pocket free, flow of fluid coming out, tighten it back up as soon as you see that its air pocket free. Also you will be dumping some fluid out in this proccess so make sure not to run out of fluid in the reservoir while bleeding. When your done with that top the reservoir off of neccessary. Again good luck, brakes are your friend, and if it's not your MC then sorry for making you read all that. I hope that we have been a help to you.