I can't stand that, you think you got it an Bam, nope not it.
Have you cracked open one of the bleeders, when the brakes start to drag?
Just to see if it is in-fact something up line causing your problem?
How do you know it's not the residual valve?
I'd even crack open the fitting at the M/C to narrow things down.
Like, if you did and found too much or pressure at X then you'd be sure.
Know what I'm saying...