Inside and outside calipers are used for this.
Caliper paint in a spray can is used to paint brake calipers when they are removed from the vehicle. There is a brush kit available to use when the calipers are still inside the vehicle.
to measure inside diameters of pipes
Calipers are used to measure the dimension of anarticle. In measuring a section of pipe you would use an inside caliper to give the internal diameter, and an outside caliper to give the external diameter. The difference being the thickness of the pipe wall.
No. If your calipers are leaking, they need to be replaced. Calipers leak because they're worn out, or because you didn't change the fluid and it corroded the inside of the piston bore.
the pistons inside your calipers are sticking
No, they were not yet invented.
Yes, they can.
There are many body fat calculators that can be used in place of calipers. These calculators however, do not give as accurate a result as calipers will.
Vernier Calipers are instruments that are used measure length. It is an instrument that provides a digit of accuracy.
A caliper measures distance. For example the distance between the insides of a pipe (diameter). There are inside calipers and outside calipers. Look at the 'related links' below. The links will take you to the wikipedia page on calipers. There are pictures etc that may help.
Bow Calipers is a device used to measure the distance between two opposite sides of an object.