rear brake problem on 1990 GMC Suburban 1500

i have a 87 suburban. the rear wheel cyclinder went out. we replaced it 3 times. still no brakes so we replaced the master cyclinder and rebleed the brakes. still no breaks. the fluid is getting to the rear wheels but it will not make the cyclinder expand. does anyone have any ideas of what can be wrong an how to fix it ? i really need my ride thanks.

Asked by for the 1990 GMC Suburban 1500
If you are concerned the problem is in the rear wheel cylinders or rear brakes, locate the brake flexible line going to the rear axle, put a rubber hose over both jaws of a pair of needle hose vise grips and gently press the jaws closed on the rear flex line. You have now eliminated the rear brakes the pedal should be "hard" if the problem is indeed in the rear brakes. If the pedal is still soft the problem is not i the rear brakes.
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thanks for the tip i will try it tomorrow. thanks again