Lately my rear brakes have been locking up, even after a "rebuild" on the rear calipers. Only after city driving (lots of braking) the pressure seems to build in the rear portion without releasing.

I, on a hunch, decided to try cracking the rear brake line loose up at the master cylinder and..behold, she rolled again.

What could this be? New master cylinder time?

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Vehicles:
2002 SLK 230
Gone but not forgotten:
1983 300D
1981 El Camino 'OILBRNR' - 6.2L diesel
OM617 powered '86 F150
1984 BMW 524td
2001 VW Beetle TDI
1994 Sunurban 4x4 6.5L diesel