A low-boy toilet is more complex than most, but parts are available to repair the low-boy toilet. The Briggs low-boy toilet repair with a Case ballcock is easy enough. There are at least four plumbing suppliers in the Detroit area that stock Case parts including Gilbert’s Hardware.
First we need to determine which part is the cause of your leak. Remove the tank lid and observe the water level against the overflow. In some models it is a one-inch pipe near the center of the tank and in others it is built into the china against the side of the tank.
If the water is below the overflow it indicates the leakage is from the flush valve. The flush valve is either a larger than normal tank ball or a flapper, in either case replace this part first.
If the water level is right up to the edge of the overflow we know the washer in the ballcock is worn out. By moving the float gently up and down you will see the plunger that controls the flow of water into the tank. This is what you need to remove. The Case model 50 plunger is retained by a thumb screw and a pivot arm. Remove the thumb screw, slide out the plunger and take it to the store for correct parts. The Case model 62 has a plunger retained by a plate with three screws.
Before you run off to the store with your parts, check the plastic seat the plunger pushes against in the ballcock. If it has a flaw, you will have to replace it. Usually an Allen wrench will remove the seat so you can take it with you. Take all the parts with you to the store; having more parts with you can save you a trip.