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You've a fuel bleed back from injectors and pump because there's a leak somewhere that is not noticeable. This leak is letting air in while cranking it and there must be no air in the system. It may not be a large enough opening for fuel to come out to be noticed, but still just large enough for air to be sucked in. Put Light air pressure to the fuel tank with the cap on to make it show you the leak position. Soon as you see the leak, take the fuel cap off and shut down the air pressure. Fix the leakage problem, re-pressurize the tank to get fuel back up, then fire it up.
No matter whether you have enough air pressure of not that is a low pressure warning, you must have a air pressure system sensor for the air brakes that is reading incorrectly (reading low air pressure), it should be on the main pressure storage tank.
The light remains illuminated while the air pump is given power to provide pressure to the lines. If the light is not going OFF, the system is either not developing enough pressure, or the air pressure sensor has failed and the air compressor motor just keeps running (do you hear it?).
It determines the fuel pressure in the line from your fuel pump to the engine. If the pressure is high enough, the pump can cycle off temporarily or at least slow down until the pressure gets low enough again to need to turn it back on. If that sensor is not working, your fuel pump may stay turned on when it is not needed or may stay turned off when it is needed causing either too much pressure or not enough pressure to keep the engine supplied with the proper amount of fuel. You need just the right amount of fuel pressure or your engine will run rough due to the air to fuel mixture being wrong. There is also a mass air flow sensor so that the computer can calculate how much air and how much fuel are entering the engine at any given time. This is all computer controlled, but it does rely on the sensors to tell it what's going on. I hope this helps.
check your fuel pressure if you dont have enough pressure this will happen when the fuel pressure is good the system will just push the air out through the injectors or the fuel return line
Check your pressures again. Jump the pressure switch and see if this makes the compressor come on. If it does then jump it long enough to get some more freon into the system. This should give switch enough pressure.
Make sure your vacuum is good on the system before you fill.
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