Inastalled Brake Shoes, front calipers, rear cylinders, master cylinder, fluid

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I had all New Parts installed on My brake system, and also had it completely bled 3 times. The Van brakes good, except that it still feels a little spongy, and the brakes has a squealing sound when applied, seems to come from the rear. They sound as if the Pads are rubbing dry. The Mechanics that did the work wont correct the problem.