Client terminated its union contract because it was no longer performing covered work. However, it may have continued covered work through the use of subcontractors without making trust contributions. Now, it is asking whether it can avoid liability by rejoining the union. I assume the employer would still owe contributions for covered work when it was not a party to the CBA, but I'm having trouble finding the right statutory/regulatory source to explain this and show how the calculation would work, and my search terms are not giving me good case law results. Does anyone have any thoughts for where I should be looking?