Answer #1, Yes, you can use more modern master cylinder and booster. Several vendors sell different kits and brackets to do this. The important thing is using a MC with the same bore size to the original. So the MC sold for the 1/2 ton guys is likely not suitable for a 1 ton. So you could go to your local auto parts store and source MC with the correct bore, and suitable booster too.
Some of those brackets look better designed than others. I saw one from CPP a couple months ago and it looked pretty floppy.
Answer #2, yes it does hold some residual pressure in the lines, likely about 10psi however it is not specified exactly how much.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0504.htm
