Thus is the insidious nature of swollen brake rubber: the force applied to the brake pedal could be 200#, but the drag of swollen parts + the constriction of hoses kicks in and the various return springs cumulatively might only have 100# of force, so the hydraulic pressure releases slowly instead of almost instantaneously when brake pedal force is removed. Driving the car might duplicate the problem, but it may be time to inspect each brake part more closely. Even with new parts, damage may occur from brake fluid contamination. So starting with the MC sounds like a good approach, but physically inspecting each moving part might be in order. This is how I found out the depth of my brake problems on The QuadCab, by comparing how much effort was required to move caliper and MC pistons by hand with replacement parts. The caliper pistons varied in how hard it was to re-seat them, and none were as easy to re-seat as replacement calipers. The MC showed similar variation during bench bleeding compared to its replacement. At this point, my suspicions of brake fluid contamination were confirmed, and I went ahead and replaced rubber hoses, etc. flushed out hard lines with brake cleaner and compressed air, and power bled the system. It was tedious, messy, and kinda $$$ but the brake issue that I had fought for months was resolved...in that link I posted, I also touched on the importance af proper lubricant used on critical parts.
As I told a lady friend who could not wrap her head around why I was doing so much work on that old buggy, brakes are like being on a see saw: if one person is too hefty, then neither have any fun...the big kid can launch himself up a little, but the little kid doesn't balance him enough to teeter on the bar, so the big kid doesn't really go far (unless he tries to launch the little kid into orbit by cannonballing the see saw, slamming his end into the ground, then jumping off real quick if the runt is still on his end, just to be a jerk). This is kinda what's happening in car brakes, with the brake pedal applying force to energize hydraulic circuits, then the return springs de-energize the hydraulic circuits to complete the balancing act.