An implement with a ground working tool mounted on an arm pivotably movable between raised and lowered positions includes a pneumatic down pressure control system. The down pressure control system includes upper and lower mounts with an air spring located between them and having compressed and extended configurations with said arm raised and lowered respectively. A parallel plate mechanism maintains the upper and lower mounts in parallel relation and maintains a relatively consistent down pressure of the air spring. Multiple air springs can be provided on an implement and pneumatically interconnected by an air line for simultaneously charging to a constant air pressure.