Have a craftsman 105 hp 20 gal comp it build air to shut off point then leaks air out of the little valve that has a alum tube hooked to it.

This sounds like the compression release valve is leaking. This valve bleeds the cylinder pressure to allow the motor to start without having the load on it. On this size compressor the valve is often built into the pressure switch. It is a little "pintle" type valve and you will hear hissing while unit is running but the opressure will not build up very much. When the pressure switch calls for the unit to run, the little valve closes with the switch. COntamination or other stiction keeping the valve form closing will cause the problem. You will find this near a tube that goes to the pressure switch. The switch itself hooks into the tank, but this little tube will connect to the cylinder head or the main tube from the compressor. Yours may look similar to ones at this site:
http://www.generalairproducts.com/Fire-Protection-Products/accessories-pressure-switch-a-switches-check-drain-a-pressure-relief-valves.html
The valve is also commonly called the "unloader" valve.

The problem with compressors not building pressure is usually a bad pressure switch or bad regulator. Does the motor shut off at 20psi or does it continue running and not build pressure? If the motor continues to run with no increase in pressure, the you should go over the machine looking for air leaks from your regulator back all the way to the motor. Any air leak while the compressor is running can cause the pressure to "peak" way before the intended shutoff that it is regulated to.

Your problem lies with the valve plate and/or piston & cylinder. If one or more reed valves on the valve plate are cracked, bent or broken; pressure will only build to a certain point. If either the ring on the piston is damaged or the cylinder is badly scored it will not be able to compress air beyond a certain point. Unless the tank pressure reaches 120psi to 130psi the pressure switch will never turn the motor off.

It is possible the reeds in the intake are loose. This requires removing the top of the cylinder, not for amateurs as the bolts need proper torque and the crack easily. Also check the cold start valve, it is a small brass valve with a needle in the center it bleeds air for cold starts, should be on the tank if so equipped. Check the unloader too. Run the unit then shut off the motor at 20 lbs and listen for air leaking. Not hearing air leaking would indicate the first suggestion of reeds or if it is old, worn rings.

Oil free pumps do tend to blow the piston ring (not too expensive) available at searsparts.com. Remove covers to pump and manually turn crank until piston is at top and look at the condition of the cylinder neer top, If it looks scratched, order cylinder kit with piston ring. If it looks good and shiney, could be the valve plate in the head. There are reed valves that tend to break. Run pump however with covers removed and check with soapy water for air leaks and repair as needed (air leaks under the cover will prevent unit from filling). Good luck

Easy to check. Follow discharge air tube from compressor to the tank. The brass valve that the tube attaches to is the check valve. You will need some air in the tank for the test. Remove the discharge air tube at tank and spray soapy water. If leaking replace valve or remove and attempt to clean debris. Does not sound like your problem is the valve however. If no leak at valve, check reed valves or head gasket. If your pump is dry lube type, check cylinder for wear / scratches close to top of cylinder. Good luck

The compressor will not auto shut off until it reaches the preset pressure which is usually around 100-130 psi - your problem is leakage of air from the oil filler cap suggesting that you have a failed internal valve.

Check to see if air is leaking past 'pressure relief' valve. It's probably leaking and that's where all the air is going.

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The compressor valve may have a faulty gasket or seal, and maybe leaking air internally. Or the compressor might be going downhill. You can try to replace the valve seal and see if that makes a difference, otherwise you need to replace the compressor.

sounds like the regulator is fualty.