What Pete said. Plus finding a leak in the bellows is tough. You not only cannot see all around them, but worse, you cannot get soapy stuff all around them either. You can use my stethoscope and ultrasonic leak detector in OKC. (If you can stand the abuse from the multitude, not that a POG member would break your chops.)

Can we assume that your aux air pressure remains up while the belt tensioners lose air? If not, the problem may not be the tensioners, but a leak in the aux system.