If you got oil on the new shoes, the first few stops will take longer until it burns off. One example of oil contamination would be if you had the drums turned. The residual cutting oil left on the drums would contaminate the shoes. Also, if you got a lot of greasy fingerprints on the braking surface of the drums while you had them off, it would contaminate the shoes. If there's no oil to explain it, then you might need to do some more investigating.
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