Sharon,

This explains why your front disc brakes have residual pressure valves:

https://techtalk.mpbrakes.com/how-to-series/four-things-to-know-about-brake-valves

Obviously your overall system is correct for your application or it wouldn't have worked perfectly for 3 years. Now something is keeping hydraulic pressure on your calipers when you take your foot off the brake pedal. I would use a process of elimination releasing pressure at the MC flare nut first, then at the residual pressure valve and finally at the caliper and see where in the system the blockage is.

Good Luck!

Ray