Sworn Brothers

Sworn Brothers is a 1987 Hong Kong action crime drama film directed by David Lai and starring Andy Lau and Cheung Kwokkeung as step brothers on the opposite sides of the law as a triad and cop respectively.

When Kwok came back to Hong Kong, he and his father and Ting had dinner and after wards they went out a competed on who can pee the longest in an alley. There, they see a police officer Shek, nicknamed Portuguese, harass a call girl and Ting fights with him and Kwok helps him and cuffs Shek on a cart and pushes him down the alley.The next day Kwok reports to the police and is given a case of investigating Yeungs gang and Shek, who has been following this case, was assigned as his subordinate to assist him. In the files, Kwok also sees a file of Ting. ........

Source: Wikipedia