Craftsman compressor Ok I have a 3 gallon compressor that won't fill up it only goes to 40 lbs. any ideas

If it is running strong up to 40psi then it has to be your switch. Has the compressor worked prior to this or is this a used compressor that came with this problem?
Also does the compressor stop at 40psi or does it just continue to run?

If it shuts off at 40psi then this is something that can be adjusted at the switch.
To adjust the switch pull off the switch cover. Under you will find two adjustment screws. Under the plastic cap you should see the directions for adjusting the cut-off, cut-in psi. It's hard to say which adjustment to use because you don't say what the cut in pressure is.
But basically on screw will raise and lower both cut-in and cut-out psi at the same time. The other will raise and lower the cut-out psi only.


If however it just keeps running then you could have a leak or it could be just worn out. If this is the case no amount of adjustment is going to help and a new switch would be a waste.

If it is taking a long time to build even to 40psi then it's probably worn out.These types of compressors are basically throw away.

If it builds quickly then it may be a leak. It doesn't have to be big to impact the pump's performance. The best way to check for leaks is after it's built up to it's max psi what ever that is use some dish soap, water and a brush and paint all the joints, checking for bubbles.

These small compressors do not have a long life especialy if they are run for more than 15 min /hour , bet no body told you that when you bought it the ones with an iron barrel are better so to problem
1 piston rings worn or broken / replace
2 daft i know but air leaks/ when it is running you carn't hear a leak cos of the running noise,run it up as high as it will go switch off and listen /repair leak
3 sticking or passing valves , these are located in the cyl head /take filter unit off,run it when it comes up to 40 psi put your hand close to the inlet port if the inlet valve is passing you will feel a sort of chuff chuff on your hand ,switch it off ,if you can hear ,and feel air coming out of the inlet port the Exaust valve is leaking ,sadly these small comps are difficult and bloody expensive to get spares for"they want you to buy another one " so depending on the amount of use it has had it might not be woth while repairing hope this has been of some help
merlin v12