Why won't my shop air compressor build air.?

I have an air compressor with a 15 gallon tank and a 3 HP motor. It will build about 5# of air then slow down to almost nothing on RPM. I have removed the belt and motor runs fine. I have also tore down compressor and it all looks good but still no air build up. Any suggestions would be great.

Answers:
1. Check your pressure switch.
2. Clean or replace the valves on your compressor.
3. Compressor rings may be worn.
4. Make sure your inter-cooler is clean.
5. Drain the water from the tank.
6. Are you using a non-detergent oil in the compressor? Regular motor oil will gum it up.
7. Motor may be unable to handle a load. Check the amperage draw.

Other answers:
You must have a leak in it somewhere. I was having a similar problem and found that the teflon tape came off of my hose connection and was leaking out faster than it could compress. If you are sure those are all ok, you may need to look for a crack in your tank. That could be a major disaster if it's not taken care of.
You must have a leak in it somewhere. I was having a similar problem and found that the teflon tape came off of my hose connection and was leaking out faster than it could compress. If you are sure those are all ok, you may need to look for a crack in your tank. That could be a major disaster if it's not taken care of.
Lack of electrical power. You're not using an extension cord are you?
Is it 3 phase, 220V, 110V?
do you have a 220V motor hooked to 110V?
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