Chain Knocking sound when applying rear break.
 

My chain clanks or knocks, when i apply the rear brake. is that due to it being loss or to tight. i just adjusted my chain and i gave it 1 1/4 inch of free play. Is that free play from the top of the chain when is pressed down to the bottom of the chain when its pulled up. or the middle of the chain when pressed down to top of chain when its pushed up.

i always check on the bottom of the chain in the middle of its span between front and rear sprockets

it's total movement up and down that you're looking at

also, you should check it with the rear end on the ground, not on a stand preferrably with a rider of your weight astride the bike