Take the tire off completely and get an old pair of nylons and ball up one leg, put them inside your tire and slide it around to see if a fine wire or thorn is stuck in the casing.
Get a Thorn proof tube for the rear.
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72 Geoffery Butler, 72 Gugificatizion Witcomb, 72 Raleigh Gran Sport SS, 74 Raleigh GrandPrix dingle speed, 74 Raleigh international, 81 Centurion Pro-Tour, 85 Gazelle Primeur, 29rBMX, Surley Steamroller 650b