Abstract: An improved back frame for supporting a tank of compressed breathing air in a life support breathing system wherein the back frame includes a harness for securing the frame to a user's back. The improvement resides in part in the provision of inclined lateral edge rails on the frame which are spaced outwardly from the user's back at the upper portion thereof and are oriented to incline or extend inwardly toward the user's back at the lower portion thereof. A retaining strap is slidably connected at each of its ends to the upper portion of a correspondingly associated edge rail. In this manner the strap may be slid along the rails to adjust the retention force imparted to a tank of compressed air mounted thereunderneath. In addition, the retaining strap includes at one end a stepped bracket connector for altering the nominal length of the strap while a toggle latch is provided at the opposite end thereof for loosening and tightening the strap with respect to a mounted air tank.