It's a good bet that your '77 LeSabre likely doesn't have antilock brakes (it was extremely rare in the '70s). Changing the master cylinder is fairly easy, but the entire system will have to be bled afterward. Loosening brake bleeder screws on a 34 year old car will require patience, good penetrating oil, the right tools, and some luck. Once the bleeders have been freed and can be operated, the system can be bled.
You will need a six-point bleeder wrench with probably 5-16" and 3/8" ends. You will need a line fitting wrench (flare nut wrench) with probably 7/16" and 1/2" ends. You will also need some threaded fittings and hose barbs with tubing to bench bleed the master cylinder before installing it.
A remanufactured MC for that should probably cost about $30. A master cylinder, a quart of brake fluid, and an assistant to bleed the brakes should be all that is necessary. Having a little bit of lithium grease to dab on the tip of the plunger for the master cylinder will also be a good idea.