ho long has it been doing this?
how many miles is on the blazer?
what have you tried to fix the problem?
tell me all you can
this sounds as if you may have a collapsed rubber brake hoses....sometimes they will do this and work like a check valve...sense it is both rear brakes..it may be the hose in the center of the rear end where it comes down from the body
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4. Connect the brake pipe fitting to brake flex hose. TightenTighten the brake pipe fittings to 17 Nm (13 ft. lbs.). 5. Bleed the brake system.
6. Remove the safety stands and lower the vehicle.