OK, we're in the process of a master bath remodel and I need to remove the mortar shower pan as we're expanding the size of the shower. What is the best way to remove the old base? Small sledge hammer and chisel? It appears to have a pretty standard construction (i.e. mortar bed, a plastic liner, then mortar on top, then tile). It was installed in about 1970.

Hello hoox, you could go at it with a sledge hammer and chisel, but it might take you more time than you want to put into removing it, or rent an electric jack hammer and get out quicker. Try hammer and chisel first , then decide whether or you need to rent a jack hammer. Either way don't forget to wear safety glasses, and ear plugs, leather gloves.
Good luck on your project.---Zeke

More great advice from Zeke,
Let me add a bit to it. If your just enlarging the shower then you won't need to move the drain. If you can try to save and reuse the flange type shower drain,(see image) The top part will unbolt from the bottom and the plastic shower pan can be removed. The drain can t6hen be reused for your new shower. Do you need instructions on how to bed, pan, mortar and tile the floor? If you wish we can walk you through the process. Good luck, tom

Here is a site that should compliment all of the above suggestions. It shows you how to build a custom shower CORRECTLY!

How to Build a Shower Pan (http://www.hoagy.org/house/HowToBuildShowerPan.html)

Let us know if need more info.

Mark