I've had this happen twice, I thought it might have been the small amounts of axis play in the pulley shaft (it moves in and out slightly). Fitted a new belt and the same thing happened again. The only thing I can think of is when you tighten the securing nut the pump moves towards the engine slightly which reduces the tension on the belt. The next time I used a bar between the engine and pump to hold it in position whist I tightened the nut. I also tightened the nut that the pump pivots on from under the car,which I left loose the first time. If it had an adjusting bracket like the Alternator it would make things a lot easer. So far I've done 20 k miles without a problem.
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