A visitor asks "My air compressor is leaking air pressure? When I start my compressor and it builds the pressure fine, but by the end of the day all air leaks out of compressor. Please advise. Thank you."
I think it is a good idea to leave the compressor tank empty and with the tank drain open to allow tank water to void after every use. Having said that, if you want to leave air in the compressor tank, it is entirely up to you.
So, it seems from your question that your air compressor has a leak somewhere. Finding it is the challenge, yes?
The first thing I would do with your air compressor if I was experiencing air leaking out of the compressor is to let the compressor fill to cut off. When it shuts off, assuming no noisy appliances nearby, the compressor is quiet. Listen carefully for an audible air leak.
Next, feel around the unloader valve if your compressor has one on or inside of the pressure switch. If you find that air is bleeding out of the pressure switch or the unloader valve, that suggests that your tank check valve is not seating fully or properly. The tank will be the source of the air that you feel. You will need to clean or replace your tank check valve to stop this as the source of the leak.
If you are not sure what the tank check valve is, or the unloader valve is for that matter, see the page that describes what they are and how they work, linked from the sitemap page, for details.
If air is not bleeding out of the unloader valve, next, unplug your air line from the compressor discharge coupler after the compressor tank has filled to cut out pressure and stops. This will isolate the compressor from the air hose, air tools and air fittings which, in themselves, might be the source of an air leak. Monitor the tank gauge to see if the pressure drop continues. If so, now you know the leak is at the compressor for sure.
Your next check when air leaks out of the compressor is to soap all the fittings on the compressor. Mix liquid dish detergent (10 parts water, one part soap) and use a paint brush to swipe the soap solution over all the fittings on the compressor discharge plumbing from the tank to the discharge coupler, to see if any of them leak. If so, gently tighten the offending fitting with a wrench.
If you do not find a leak in the discharge plumbing, and it is not the tank check valve leaking air out the unloader valve, then double check your tank drain valve to make sure it is shut tight. Or, as I have already recommended, when you are finished with the air compressor open that tank drain fully to blow out all the water. Now you have no air in the tank to leak!