Hi.. Welcome to HEALTHCARE MAGIC.. As per your complain excessive mucous production in patients with cleft palate and lip is due to communications between the mouth and the nasal cavity leading to continuous irritation to the nasal passages and there can be irritation to sinuses too as they drain through node.. Irritation and inflammation leads to excessive mucous production and post nasal drip.. You should consult an Oral Surgeon and the most common treatment for cleft lip and palate is surgical correction of the clefts.. While if surgery is not found compatible or appropriate Prosthetic appliance like Palatal Obturator can be fabricated for the patient that he/she can wear to prevent communication between nose and mouth and then antibiotics, decongestant and mucolytics can be advised to reduce infection and post nasal drip.. Hope this information helps.. Thanks and regards.. Dr.Honey Nandwani Arora.