low coolant light

about 3 weeks ago I started my truck up in the morning and the "low coolant" light came on...then went off after the diesel warmed up. it did
it again the next day. then on the third day it would stay on all the time. I know theres a seal leak, I replaced the gasket but that did not fix the leak so I am waiting from some special "goop" I ordered from a local dealer to come in...thats suppost to fix that problem. Anyway, I filled up the coolant tank, about 1/2-3/4 gallon worth and the light still comes on all the time, any ideas?

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