Bleeding Front-Disc Brake using a one-man pump kit, but the vacuum level dropped after stop pumping, why?

What is your question?

I tested my newly purchased one-man disc brake bleeding kit with hand pump. I used a tight fit rubber adapter to the brake valve, after pumping to get a moderate vacuum level, the vacuum level dropped after stopped pumping. Is this okay to do brake fluid bleeding, keep pumping the vacuum? or there is a leak? thanks