old time 4X4

I have an 1980 pickup 4X4 wih power steering. Bought the truck new. So been a lot of good miles together. I have 31X10.50R 15LT tires on it. I put those size tires on about 6 months after buying it. My be a year or so when I turned I got a noise like when you turn the wheel all the way to the stop and get that bypass noise or like air in lines, intermititly. Then over the years it got worse. Now I have a almost no power steering gettin arms like popeye turning it now and wife wont drive it anymore. My question is, on the pump there is a small philips screw head just behind the pully in front of pump in line with the out line in the back. Is that an adjustable spring loaded relief valve? May be with the bigger tires and all the back pressure that valve has now stuck open/closed. Clean pump, replace pump, steering box? Fluid lvl fine. unemployed atm so dont have a lot of money to throw around replacing pump and or steering box. Cant find any diagram showing what that little screw does. Your help would be greatly appreciated.