I am attempting to change to pex pipe in a bathroom remodel. When I solder the Brass pex fitting to the copper pipe the joint feels loose. I pulled one apart and the solder had completely filled the joint. If I wait a minute then the joint feels tight but now I am nervous about how solid these joints are. Does the brass cool differently than copper so that you have to wait until the brass cools? I need to solder fittings onto a new brass diverter valve and am having doubts about my ability.