Originally Posted by
Retro Grouch
People spend a lot of time worrying about shifters but, in my experience, brakes are more troublesome. Assuming reliable bikes are a part of your business plan, I'd buy 7 pairs of Shimano Acera V-brake calipers and solve the problem permanently.
Better brakes are always a nice thing,
but won't solve the problem if it's the cables or posts. The first objective is to identify and resolve the actual problem. Then the OP can consider new brakes to improve overall performance.