I'm pretty new to trucks so i've mostly just been using a haynes manual to check the condition of the truck i got. I was following the directions to check the coolant level and according to the manual I don't have any coolant at all, but the truck runs fine. Maybe its just really low? i'm not sure. There is no coolant in the reservoir next to the washer fluid reservoir. And there are no indications of where the reservoir is full or needs coolant to be added. When I look down the radiator cap I don't see any coolant either, but there is some residue that looks discolored. Help please

 

This is basically one of those use your best judgement things. When cold, the coolant will be a little below the filler neck and the overflow tank should be on or just above the "cold" mark. If you can't see any coolant, you could just top it off with water to check for any leaks. If no leaks, then you can drain it, flush and replace with new coolant. OR just top it off with fresh mix and go from there.

I would check all hoses for cracks and such first though. If your coolant is discolored, it may be time for a flush and new fluid.

 

If the coolant has not been changed regularly, or recently, or you don't know the last time it was done, you should consider a good flush and fill.
Here's how I do it. I open the petcock and drain the old fluid out, then close it. Take the upper hose off of the radiator and leave it kind of hanging. Fill the radiator with fresh water. Crank the engine and bring up it to operating temperature. When the thermostat opens, the old fluid in the engine will come out of the upper radiator hose where it is disconnected. When this happens you will need to run just enough water from the hose into the radiator to keep the water level just below the overflowing level. Once the water temperature in the engine cools enough the thermostat will close again. You will have to repeat this procedure until the water runs completely clear. Once that happens, I turn off the engine, turn off the water, and open the petcock to drain the radiator, then close it. Put the upper radiator hose back on the radiator outlet. I then take two gallons of undiluted anti-freeze and empty one gallon into the radiator. I take the second gallon and pour half of it in the empty gallon container, and fill both with water. Then I crank the engine and use those to fill the radiator level to full as the engine comes to operating temperature and the thermostat opens. Any leftover mixed fluid I use to top off as necessary until the next time I need to flush. I have found this to be the best, most efficient way of doing a complete coolant flush, and have been doing it this way for some thirty odd years.