Stuck service brakes

[N1VSM]

Seeking the hive-mind wisdom on this one.

My brakes: 4 wheel disk with dual chamber m/c. (I highly recommend the mod!) Installed abt. 12 years ago.

Problem: after applying the brakes a few times, the pressure in the system does not drop back down. Brake pedal is rock hard. All 4 brakes are locked up.
When I bleed the brake cylinders (one front, one back), brakes are happy & back to normal ... until I hit the brake pedal a few times, then locked up again.

Recent work: I recently bled the entire brake system - replaced all of the fluid.

My best guess is that there's crud in the master cylinder blocking the intake/return ports on both front & back circuits, but only on the return of the fluid.

My thought is to drain & flush everything (lines, wheel cylinders, m/c), then fill & bleed entire system. (I'm guessing the main problem is in the m/c, but if there's crud there, I'll bet theres crud everywhere.) Any ideas, comments, words of wisdom before I go through a few gallons of brake fluid on this one?

[j mccormick]

When the brakes are stuck, loosen the master cylinder mounting bolts some and see if the brakes release, if they do, may be needing to adjust the pushrod length so there's a little more clearance. Joe

[N1VSM]

Good thinking - so the idea is that if there isn't enough free play in the return stroke of the brake pedal, then the ports will be blocked by the piston.
I'll try that first - less messy than flushing the whole system.

[just me]

Bypass port plugged in mc or pushrod out of adjustment.

[rickf]

I would think if they worked for 12 years then the rod is probably adjusted right, My guess would be crap in the master cylinder blocking the port. You didn't mix fluids did you?