What you doing using the brakes!!!!!
Strip em down and clean em (that's what you would've said to me!!!!).
Try riding slower???
Coat them with grease (the non slip stuff, chip fat will do!!)
swop the back on for one off the front, it'll be too big but you can always dremmell it to fit. Don't worry if it's not round because wavey discs are all the rage.
Go to Halfrauds and buy a cable operated bicycle disc and caliper set. Fit using superglue and fishing line.
Hope this helps pops!!!!