Flush it if you suspect that you have salt buildup.
Coco pH should be right around 6- lower than soil- and you need to use a nute designed for coco, or add calmag to your feeding regime, or pre-prep the coco to deal with its bizarre interaction with calcium. I don't really know how to do that, and if I ever grew in coco again I think I'd go with the canna line of nutes for it. Calcium lockout is a real pain to try to treat.
Oh, and check your fert solution pH values AFTER you mix it if you aren't yet.