80 series -Fuel Tank Fault

Last 2 trips have seen the Fuel Tank warning light glow on the petrol GXL.
first was in the Vic high Country at Xmas and then again last weekend at Yadboro.
Both times I was under 1/4 tank full in main tank but low fuel light had not come on.
Problem is that when this Fuel Tank warning light is on you cannot pump from secondary tank to main.

I know there is a float to prevent overfilling main tank and assume this is sticking when vehicle is at steep angle for extended periods.

I can get rid if the warning light by removing and replacing fuse but the first time couldnt get pump to work until I had completely refilled main tank then ran it down again.

last weekend I put in 20 litres drove around for 1/2 hour and light just reset after stopping on a slight uphill and then reversing.
what the hell?

Vehicle has 375K plus and I thought maybe there is wear on float pivot pins or something. However as I have never delved inside the fuel tank would appreciati if someone out there who has can tell me how it all is supposed to work.

need to start somewhere.
cheers
Howard