Brakes leaking within 200 miles-dot5 fluid

I've had Dot5 in my system since 1999.No problems, front is 70Duster KH4piston,and rear is rebuilt 69 Barracuda 10x 2s. M/C is a rebuilt,15/16 dual.
What did you use for a flushing agent?

The line to the rear should have a residual valve at the M/C, or an energizer spring in the wheel cylinder. Either system works to keep the W/C cups pressed tight to their bores