What I have:
Coleman 5hp, 25gal horizontal tank, 90 on-120 off, belt drive, single stage, 120 volt, portable
Model MP0502513

What it does:
When I got this one, it did nothing. I checked the capacitors and they both tested defective so I replaced them.
Now it will run.
At initial power, it starts and builds up pressure and turns off as it is supposed to but the unloader did not work (no hiss on shut off).
I put my amp meter on it and it starts at about 8 amps and gradually increases to a little over 16 at shut off.
The problem occurs at restart. When it restarts, it is drawing 80-90 amps for a few seconds until it trips the breaker.
I tried cleaning the unloader in ultrasound but it still was sticking. Since it was cheaper to replace the entire pressure switch with an unloader rather than just the unloader, I replaced the entire unit.
The check valve. I know this problem usually occurs with a bad check =valve. What I don’t get is why does it work ok when starting from no pressure but fail when starting at restart which is 95psi (my new pressure switch is set at 95-125). I made an assembly to hold the check valve and applied air to itb from another source. It works like it should – passes air in one direction but not the other. This proves it can work but maybe something is happening when it is working at a higher speed.
I have so much time and energy and money in this thing that I hate to give up but I’m running out of things to try.

Sorry to be so slow in getting back. I did the test of opening the tube to the head when the tank is full and the compressor is off. There is no leak at that connection. I have also replaced the check valve just to be sure and still get the same results. I put an amp meter on the power cord (I have a plug that sandwiches into the power cord) and find that on start up, it is drawing 75 - 85 amps for about 1.5 seconds and switches to about 11 amps. The 11 amps slowly increases until it reaches cut out pressure. At cut out, it is drawing about 17 amps. I have compared this to another similar (5hp direct drive) compressor and get approximately the same readings readings. On restart, it is drawing about 80 amps until the breaker trips.

Now I'm thinking motor. Do these motors have a centrifugal contact where after a certain speed the start circuit is disabled. On start up the current draw drops off quickly but on restart, it does not. Maybe the motor is not reaching a high enough rpm to force the start circuit contacts to open the start circuit.

Thanks, Mike

this compressor has:
new capacitors
new pressure switch with unloader
new check valve
new line from unloader to pump head
new line from check valve pump head

I don't have much experience with this stuff so I am not sure about the pump. With the belt off, I can turn it but it requires some effort to overcome the compression in the pump. When I had the belt off, I did not open the line to the head and try to turn it without compression. I would say it feels similar to turning the blade on a small lawnmower engine. Belt is good.
With everything new, I'm fairly certain that everything external to the pump is clear. I do not know how to tell about the pump internally.
Given the way it is acting, I'm leaning toward the motor. When it starts from empty tank, it draws high (75-80) amps current for about 1.5 seconds and then switches to about 11 amps. On cutin, it draw 75-80 amps but never switches so after about 3-4 seconds, the breaker trips.
The breaker is a 20 amp and I made sure it is good by swapping it.
I tried different tank pressures on restart. Up to 40 psi, it will restart. At 50 and up, it trips the breaker which makes me think it can't get up enough speed to release the start mechanism.