Tools, Sharpening and Power Machinery - Compressor Not Pumping Up

I have a 3.5HP 60gal. 130 psi. U.S. General compressor that is about 5 years old. It will pump up to 68psi and will go no higher. I have sprayed soap on the pop off valve and it's not leaking. I removed the check valve and it seams to be working correctly. When it gets to 68 lbs of pressure it just keeps running but doesn't increase. It used to go all the way to 130 lbs. Does anyone have any idea what could be wrong? I'm thinking maybe an exhaust valve is bad and when it tries to compress it blows back out the intake but I can't tell because of the air from the fan. Below is a link to the compressor I have. Thanks for any advice.

- Harbor Freight Tools - Quality Tools at the Lowest Prices (http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=55222)

I am sold on regular oil type compressors and I was surprised to read you were having problems with one that age. It could have leaky valves,worn rings,blown head gasket. I would disconnect any hoses and take the belt off and turn the compressor pump over by hand slowly. If there are leaks its much easier to see,hear them when the motor is not running and turning it slowly. Also you will see how much compression the piston has by the resistance when it starts compressing he air.

good luck

Not too sure how it is built, but all the above idmason60 says, plus if it is equipped with reed valves, they could be rusted, trashed up or just leaking. The hand turning will let you know about compression, but maybe not the loss of compression, which I think may necessitate removal of the head to get at the valves.