Use pipe fittings designed for central vacuum.
For connecting a branch to the main line. Be sure the airflow is adequate. Do not install the tee backwards!
We suggest avoiding PVC sch40 fittings. They are thin and leave a ‘step’ where the pipe connects. Toothpicks or pins can get stuck creating clogs.
Also make sure fittings have a 3/4″ cuff, not 1/2″, to provide strength at joints.