A method is provided for determining faults during the operation of a pump unit. Disposed on the machine frame are a tool holder, displaceable in a horizontal X1 direction, for a tool and another tool holder, displaceable in a horizontal X2 direction as well as in a vertical Z2 direction, for accommodation of a second tool. The amount of gas supplied to the airbags is preferably substantially proportional to the torque roll force at each of the airbags. A mixing element is disposed within the compartment of the collapsible bag. The terminal edge of the impingement surface extends to at least the outer edge of the second section.