I am in the compressor business and see this all the time. Generally it is caused by the use of an extension cord or generator to power the compressor. IF you are using a cord try it without one. If you are using a generator you will need one 3 times the size you think you need. Since you know your check valve is good the other thing could be a valve problem in the head. But I'll bet you it is electrical.

The unit is plugged in directly to a dedicated line. That was the first thing I did when moving in.
Roger

Is it wired for 240VAC? Normal household 120VAC results in a horrendous current draw on startup. If possible, install an unloader by changing the check valve and pressure switch. It is not difficult. This will release the pressure in the head when the unit shuts off.

It is 110 v. What is an unloader? Will it work on an old machine?

An unloader is a valve that lets the pressure off the pump head and discharge line. Most are on the pressure switch with a 1/4" tube running back to the check valve. When the pressure switch shuts off the unloader opens and you hear a hiss as the air on the discharge line and head bleeds off. If you hear this hiss now your unit is unloading. If not your unloader is either not hooked up or not working. There are also self unloading check valves that bleed all the time. The check valve holds the tank pressure in and the weep hole bleeds the discharge line down.

thanks I will give it a try.

Well i'm back. Changed the check valve to an underloader, took forever to get the right fittings. It started up and ran through the first cycle fine. When the pressure dropped it started up and did the same thing labored for a few seconds and pop there goes the breaker again. It blew off pressure just like the underloader was suppose to. I'm at a loss??New switch new valve oil is fine. What else?
Roger

Just an idea,but is it possible the centrifugal switch in the motor that switches from "start" to "run" windings is stuck in the run position?
Mike

I don't know. How would I check that?

Thsi sure sounds like low voltage due to wire size, generator or extension cord. But you have said that is not the case but it is a classic symptom and sometimes caused by loose wiring.
The start switch is a possible problem but usually they will do it from a cold start as well.
The start switch is just inside the back cover of the motor. It is a circuit card with a couple of points on it. File them and it shold be fine if burnt.
I would get to checking electric connections and wire size back to the breaker.

I ran the line with 12/3 direct from the 20amp. breaker.I think it maybe voltage also, when I first put the new switch on it worked great but I don't use it that often and now it's driving me nuts.

I have a couple of questions.
1. What horsepower is the motor? Look for the amperage rating if it says "spec" instead of listing the hp. Make sure you note at what voltage the rating is spec'd at. IE: 12a/220v
2. What type of compressor is it? Scroll, piston, screw?
(doubtful it's screw)
3. Have you checked to see what voltage the motor is wired for?
4. Make?

It has a 1Hp motor, Twin Pistons, and is wired for 110v. Its made by Brown. It has run well in the past. I'll have to check the spec's when I get home.

http://www.kilowattgenerators.com/contents/sizing.html

So the compressor is made by Brown. It's 1 HP, 115/230 volts rated at 15 0/7.5 amps. Set up for 110v.
Any ideas?