Just fitted new disc's and pads to wifes Ka , applied copper slip to back faces and contact points . Has been OK for a week or so , now it squealing on the first couple of applications of brakes , there after is fine. Any ideas?

were the discs cleaned to remove the anti rust protection before fitting,if not the face of the pads could have glazed up,a quick rub on some emery paper will deglaze the pads.

i would remove the pads & use a disk cleaner to remove any residue.

you mayhave put too much copper slip &
contaminated them.
remove all the copper slip then only use it on the threads of the bolts & no where else
make sure all is clean prior to refitting.
also check pads are not contaminated