There's a good chance it's the Check Valve, as it's the most common problem there is with compressors when this problem occurs.
It's easy for debris from the Head (due to a dirty Air Filter) or condensation from the tank (too much moisture) to gunk up the Check Valve. You may want to take it out of the tank and try cleaning it up and re-installing it before you replace it.

Otherwise, you could have defective flapper Valves in the head, which is the second most common problem with this unit. Make sure the Valves "spring" up and down, although it may be difficult to tell because there are valve Retainers on top of the actual Valves.
If the Check Valve doesn't fix the problem, you'll probably need to replace the Valve Plate Assy, which is the only way the Valves come: www.mastertoolrepair.com/sanborn-130-and...-0222-pump-p-35.html .

Also, and just for the heck of it, make sure the Belt isn't too loose on the Pulley and the Flywheel.

Hope this helps!
- MTR