A technique you can use on the client:
Client Suppine
After warming pec with effleurage, circular petrissage and cross fiber friction of muscle belly and all attachments, and including pec minor - gliding strokes and attachment frictioning......
Face the client, and with your outside hand firmly grasp the distal humerous (supporting the elbow) and with your inside hand, grasp the pec major with fingertips at clavicle and thumb (pointing to axilla) under the muscle tissue near humerous attachment.
With your outside hand, move the client's arm into horizontal adduction, while with your inside hand simultaneously finding a firmer grasp (starting as medially as possible) on pec tissues and allow them to melt in your hand. Then, slowly start to allow gravity to bring the client's arm (still supported with your outside hand) into horizontal abduction until you reach the end feel of range of motion. (For some this means their arm lowered all the way to the table, if they are close enough to the edge of the table some can go past the table.) As the pec muscle is stretching, you can unravel the pec muscle fibers by gently rolling it between thumb and fingers.
Do this several times, moving the grasp of your inside hand laterally each time. Move the grasp of your inside hand only AFTER you bring the arm into horizontal adduction again, when the pec muscle is slack. This should be done very slowly, and within the client's range of motion and comfort level. You can even teach them to perform this on themselves as self care. It does feel a little different with passive vs. active ROM, but still an effective self massage technique.