1. rusty cap + no cylinder wrench = gas leak
Do NOT use
wrench, hammer, crowbar, etc. to force a valve open or remove a cylinder cap that is stuck (rust, worn threads)
WD-40 or other lubricant
Safe way to remove a Cylinder Cap
Use a cylinder wrench as shown below.
Tap very gently on either side of cap to loosen the threads.
If you still can’t get the cap off and you don’t have a cylinder wrench, ask John Jordan to come to your lab. 
If necessary, contact the supplier to replace the cylinder. Airgas Customer Service   provide the PO# that is on the tag and/or the barcode # on the cylinder (not the tag)

2. To order a cylinder wrench from Airgas or Matheson
“A specially constructed Cylinder Hand Wheel Wrench enables easy opening of cylinder valves that have been fitted with a hand wheel. The design is also suitable for removing difficult cylinder caps.”
Airgas part# SGD90003   $27.52
Matheson model TW-5 $32.62

4. corroded valve
If a corroded valve is forced open, it may not close properly afterwards and may leak. You may break the valve causing the entire contents of the cylinder to leak.
Contact the gas supplier to replace the cylinder if a valve is difficult to operate.