I own a 1986 GMC C20 pickup Im trying to save. It was dads' 1st new truck. It wont drive pass 30mph seems like its struggling. I checked transmission oil and it had none so I put 2 1/2 quarts but still same issue. Any advise out there I know nothing about mechanics.
You have given very little information about how the truck is behaving. And I say this not meaning to offend you but it appears that you may not have a lot of experience fixing cars, which is fine we can walk you though a lot of stuff, but we got to know what's happening. Could the transmission be having a problem, I agree it's possible but I can't say based on low fluid that, that is what has happened. He could be starving for fuel, and I agree with the others that have said brakes sticking are possible.
Now some questions:
About the transmission, did you check the fluid with the engine running and the shift lever in park? or did you check it with the engine shut off? These transmissions should be checked with the engine running, if checked with the engine turned off you'd now have it over filled.
Is it in overdrive transmission? Or three speed transmission?
Can you take a video of the truck driving and what happens when it starts to struggle? If you have a tach in the truck point the camera at that so we can see the RPM's and hear the sounds it's making. Even better if the tach and speedo are close enough to capture them both in the video.