well the master cylinder in my car has gone bad. so my question is, how hard is it to replace one of these. is it just unbolting the old one and then bolting the new one on, or is it a little more complicated?

 

are you sure you're not talking about the clutch master cylinder? I've never known a brake one going out..Anyways, all you do is unbolt the old one, and put the new one on, and then bleed your system until it builds up pressure

 

They are dead simple to replace. Most master brake cyclinders will come with the parts that will enable you to bench bleed the cylinder. This is much better than having to bleed the entire system. If it doesn't come with it, get a bolt that fits into the chamber. There are 2 chambers but one you can just plug with your finger as the pressure is low. Manually press in the piston until fluid comes out. Then switch sides. Its almost like bleeding the main brake lines. Then install the unit and you are done. Make the fluid is always topped up though.