I have a 1988 volvo 240 gl and my main fuel isnt getting power

Check you in tank low pressure fuel pump and over all fuel pressure.

what do you mean by main belts? alternator belts? see brickboard.com

the valve is only to test fuel pressure not for air. when you try to start the car the air will come out by itself just keep cranking untill the fuel makes it there

buy a replacement owners manual on eBay. there is a complete listing in it, and you will have it on hand for your next issue. keep it in the glovebox

You can test anything electrical with a voltmeter, which you can purchase at a place like Harbor Freight for $3.
But you can also buy a test light for $1.
Either one has 2 contacts. When you test for current at the fuel pump plug, there will be one that is power and one ground. You attach your test device across them, and see what you get. If they are recessed and you have clips, then you may need to use small nails to complete the circuit. If you need to ensure your test device is working, try it right on the battery.

The most likely problem is not the pump, but the fuel pump relay. It not only requires ignition, but feedback from either the oil switch or alternator, to ensure the engine is running. When you find the relay in the fuse block, it will have a diagram on its side that will show you what sockets to check. You can then also make a jumper to by pass the relay for testing purposes only.

DOE IT BURN OIL ALOT?

Check out your batt ,to see if it is change up,next is the alt,check batt cables for loose ends.

Have you ever replaced the engine wiring harnesses.Check your main your main fuse its an 25amp fuse next to your batt.Also check your fuses 4 and 6.

check the actual connection at the rear window take it out and clean then/ check for power to that end/ if there is power then its the connection if there is no power going to the unit its the switch/ replaceing it shouldnt be to bad it should just pop out check where the clips are they are push in clips get a small flat screw driver/ please rate my response thank you very much

Check for vacuum leaks,pull off your intake hose and look for small cracks,also go to the rear of car and listen for you intank fuel pump,needs to making an humming sound,no sound or intermediate this can be your problem.