A Mechanic reports he serviced and re-installed a composite type crew oxygen bottle in a B737-700. Three days later; a flight crew noticed there was a pressure drop greater than 100 psi during their O2 mask check.

Narrative

It was my 5th night on the job after new hire class. There were three mechanics (two new hires and one with approx 1.5 years with the company) working four aircraft. While performing a Service Check on a B737-700; I noted that the crew oxygen required servicing. Upon installation of the bottle I stopped to question my Regional Director; Mr. 'X'; about the use of .032 safety wire instead of breakaway wire. After installation of the bottle I went into the cockpit and verified the pressure on the gage matched the pressure on the bottle and performed an operational check to verify that when the test button on the mask boxes were pushed; that the yellow indicator appeared. Three days after I installed the bottle it was discovered by a flight crew that there was a pressure drop greater than 100 psi.I was unaware of the previous problems with the O2 bottles and that there is more resistance on the valve of the composite bottles when opening it [valve] as compared to the steel bottles that I have installed in the past. [I suggest] more training on the differences of the bottle types and their shut-off valve operation; and more clarification of the note that states that damage to the valve can occur if too much force is used when opening and closing the valve. (The valve on the composite bottle opens easy; then after approximately two turns it gets stiff to turn; the valve on the steel bottle turns easy from open to close.)