Once the keg empties, it will simply spit pressure and a little foam. If you are experiencing slow foam, could be a over carbonation problem. Your system may require balancing. Before attempting this, check for pressure leaks. Spray Windex on all of your fittings and look for bubbling. Repair as necessary.
The high pressure gauge should read constant until all of the liquid gas is gone. Then the gauge begins to drop and your cylinder is close to being empty.
As to balancing, what brand of beer do you have, temperature of the beer, etc. Please advise.
Scott Zuhse, Instructor
Micro Matic Dispense Institute