Compressor starts innitially,then when pressure gets low it trips breaker when it tries to start back up.20 amp breaker to light switch to compressor is about 2o ft on12-2 wire. tried 30 amp breaker,does same thing,just doesnt trip breaker as quick.compressor is 10 to 15 years old?

Most people recommend replacing the motor capacitor in this situation, since a faulty cap will reduce the starting torque. But in my case the check valve was the culprit - there was too much back pressure on the head.

start capacitor is NOT ok. Check the centrifugal switch in the motor to see of it's not stuck in the run position too.

THIS IS ALL I COULD FIND ON THE INTERNET NOT SURE IF ITS GOING TO BE OF ANY HELP TO YOU.
Do you have any other electrical appliance in the circuit that might cycle on when the compressor is starting? If there isn't enough current to go around, that might be an issue.
it could be a failing pressure switch.
Check your unloader valve. I think that it has failed, and as a result, the motor can't overcome the additional load on start up.
If the compressor is blowing the breaker, you know that it's either a short of some kind, or the electrical motor is pulling more amps than the circuit can handle..
If you are sure that it's not a short, and the motor actually starts (it's not the capacitor), and the motor shaft turns easily (meaning that the power train into the compressor head is not binding in some way and overloading the motor) then it's got to be the motor that's at fault.
Try this. Unplug the compressor, drain the tank all the way down to no PSI on the gauge, close the tank drain, and plug it back in / start the compressor.

Does it start?
I suspect either you are overheating your compressor to the point where it starts to seize (thus drawing more amps and popping the breaker) or you've already damaged the motor and it's cycle gets shorter and shorter before it's overheated to the point of seizing.
OR TRY THIS LINK.

air compressor starts then trips breaker Google Search

not likely as your pressure switch is pneumatic, What else operates on that same breaker, perhaps replace the breaker as they sometimes get "tired"

hope this helps

Let me know

What amp breaker do you have this hooked to. I would use no less than 15 amp ,preferrably a 20 amp. Next do you drain tank after use on a regular basis. If you don't moisture can build up inside tank and cause hard starting on automatic. If you need to drain tank there should be a drain valve on bottom of tank. try turning it counterclocwise with compressor off. If it dosen't turn that way try clockwise,air and moisture should drain. If you have alot of water in tank then shut valve and turn compressor on and bring to full pressure,then repeat draining procedure until no more water comes out. Hope this helps. Thank you

You gave a lot of information, but you didn't say if it worked when free wheeling. If not, I would suspect the inertia switch. If it is working, you should hear it click when the motor is winding down after being shut off, at least when free wheeling. A 20 amp breaker will certainly handle a 15 FLA motor, even on startup. They don't trip at 20 amps exactly, especially on a simple overload. Even at twice the rated amperage, it can take several minutes to trip it. They will trip quickly on a dead short, however, or a severe overload. I doubt the windings are shorted or it either wouldn't work (start windings) or would run at a slower speed with the inertia switch constantly tripping in and out with it coasting while tripped in (i.e., accelerate, coast, accelerate, coast, etc.)

If I did the math correctly, 5.5 HP is right at the boarderline of 20 amps, particularly for an older machine that has some deteriorating electronics.

What to do? Can you reconfig the motor to run on 220 Volts??? It would then run on 1/2 the amps.

If you have a dedicated 120 line for this compressor now ... all you have to do to change that to 220 is change the breaker (I presume you have a vacant slot to do this). Find the "neutral" for that wire (white) ane make it the "red" for your compressor line ... (and mark it as such for your future behefit) change the outlet from 110 V 20 A to 220 V 20 A. Both lines will run on #12 with G wire.

If you dont have a slot, you could change one of your standard slots to a "piggy back" remodeling style breaker (has two breakers in one slot). This will free up one slot in your panel for the new 220 (double pole) breaker.

Let me know what you think

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If it runs, turns off, then tries to restart and kicks breaker, look for bad start components. Check start relay, start capacitor. It may be trying to restart too quickly and the compressor over amps and breaker trips. The system needs to have at least 3 minutes of off time before trying to restart. This allows the pressures in the system to balance enough so the compressor can overcome the high side pressure and start. I've seen where people turn the control up and down very quickly causing the compressor to over amp and trip breakers and over loads.
Hope this gets it for you.

Air compressors tend to draw more power as they age, however, will not trip breaker unless some malfunction. Here are some simple tests. Unplug compressor and turn the pump by hand, you may have to remove a cover to do this. It should turn freely; any resistance could be failing bearing (would make noise) unloader/check valve leaking allowing back pressure to pump, or failed start capacitor/start winding. Easy test for checkvalve. With pressure in tank loosen discharge hose at tank and check for air leak. Check valve should hold pressure in tank. Replace as needed. Failed capacitor/s will cause high amp draw. Remove from circuit and test. UF reading should be within 95% of printed rating. Finally, most air compressor motors have two windings, start and run. Power to windings is controlled by flyball switch inside rear of motor. This switch has contacts that can stick in start position and cause high amp draw. Examine windings for signs of overheating. Copper windings generally have color of new penny. Very dark brown windings bad news. Gook luck

Check the reset button on the motor to see if it tripped. The motor may be overheating. Some motors have an internal breaker and they have to cool off before they restart. Also check the pressure switch. Move the switch back and forth ( on and off ) and listen for a click. If it doesn't click with low to no air pressure then the switch is bad.