Water in the brake fluid will cause this also, as the boiling point isn't as high as brake fluid and it expands when hot, brake fluid being hygroscopic absorbs water over time so needs to be changed at times to keep rust out of the system and things working correctly.

70 cuda 440 6bbl Auto with a dana 60
68 road runner QQ1 383 auto
1970 6 barrel 4 speed Superbird FY1 number matching
15 Challenger 6.4 SRT pearl White 6 Speed
72 Cuda 440 5 speed
71 Road Runner 440 4 speed