The problem with the U channel is you have to comepletely clean the dirt out of it or the seals lip will not seat into the bottom of the channel...I use a soap spray and a few Q-Tips..you then just WD40 the lip and pop it into the channel, but you have to start at the end and work it in as you go, just like you would do with a a zip-lock bag seal.

Clean the seal too.........

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A Dalton