Does the pyloric sphincter contract after ingesting only liquid?
NO IT DOESNT CONTRACT AFTER INGESTING LIQUID

1)The pyloric sphincter always remain  tonically contracted even in an empty stomach.However this tone of the pyloric sphincter is insufficient to prevent the passage of liquids to the duodenum.Thus almost all the liquid intaken in an empty stomach passes immediately to the small intestine.

2)Filling of the stomach triggers an enteric reflex that increases the tonical contraction of the pyloric sphincter.When fluids are taken then,their rate of passage to the duodenum is considerably reduced.(Although little is slowly absorbed into blood from the stomach).

REFERENCE:TEXTBOOK OF MEDICAL PHYSIOLOGY(GUYTON AND HALL)

3)As the small intestine is the seat of absorption,delaying passage to it delays large amount of alcohol absorption into blood.This not only delays its effect in the body but also fascilitates the persons capability to intake more alcohol.(This varies considerably in different persons but the central physiology is this)

REFERENCE:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1705129/