I notice that when the bike is cold, the back brake is good
but when warm, there more play in the brake peddle
I had the back brake pads change last year and they change the fuild.

 

brake fluid is usually the first thing to go.double check the fluid is ok. checking the colour from original new fluid is a good indication. Its cheaper and easier to change fluid first before anything else. check the rubber hoses. see if they expand when they get hot?. check the bolts on the disk are tight. they can loosen up a little when hot and you loose feel.
If you change the brake fluid use a dot 3 or 4 ( its on the bottle) even a race fluid. they cope with heat better. check with the bike shop.