Brake has very limited component to doubt on. First and foremost get the brake fluid changed 100% (not just top up). Do bleed the brake while flushing the brake fluid out. Google that "How to bleed brakes" and you will understand. That's a cheaper fix.
Second one to doubt is brake pad, which i assume you are saying its new pad. Moderate expensive.
Third and last one to doubt is the brake caliper or pistons, and if any of 4 caliper is weak then brake applying power will also reduce tremendously. You can ask mechanic to identify which one is weak and get that one changed and if mechanic is in don't then change the both front ones first as any car uses front brake first and then rear one, that's why front brake used 70% of time and wears out faster.