More handbrake issues: 1995 Sierra

When the rear brakes get really wet from heavy rain or when launching my boat, the parking brake freezes when applied and won't let loose. I have to
gun the engine and then it will let go with a loud crack. Only happens when the brakes are wet and the truck sits for awhile. Any ideas for a fix (other than "don't use the parking brake when wet')? Suggestions? TIA. BTY; front disc and rear drum brakes.
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