Pulls to the right when he steps hard on them can be a few things. Either the left front brake shoes are not adjusted out enough so the front shoes on both sides dont grab at the same time. The left front wheel cylinder could be sticking and not moving.
It may be something as simple as that, or the left front fluid line could have some rust in it restricting it. Disconnect the line at both ends, and put compressed air through it to see if it's clear. Sometimes the old rubber flex hoses swell shut. I would replace them anyways.
