Agreed, Dad n I converted from Dot 3 to Dot 5 his factory disc brake(non pwr) fronts, and drum rear 67 Cuda. We had a leaking MC that stripped paint. Leak was not caused by fluid, but the bubbled paint was. While I had MC out and fixed the leaked, we bled the brakes and put in Dot 5. We did as basic a bleed job as can be. Simply bled them all 4 in proper sequence until the Purple Dot 5 was there. NO issues, NO leaks.
I believe something else is the problem. Good luck.