Wonderful tractors. Almost bought one myself.
If the hydraulics brakes on the 5310 run on the same fluid that the rest of that open center tractor hydraulic system uses, it could be heat or a suction air leak in the line from the sump to the hydraulic pump. Those and a clogged cleanable sump filter are the two or three most common causes of hydraulic problems in open center hydraulics. If the loader is working at full cyce speed it's not the sump filter.
You can check for heat or entrained air by pulling a sample of the hot oil for smell and then look to see if it is cloudy. If it is too hot, the oil may be ruined. In that case, clean the hydraulic fluid radiator fins and generally check it with a heat gun. Replace the oil with new.
If what you see is cloudy oil and you suspect suction air is being entrained into the fluid, put a few ounces of hot cloudy oil into a jar and let it set overnight to see if it clears up. Replace the suction hose just for drill.
If the brakes controller fluid is independent of the tractor hydraulics and has its own pump and reservoir just for the brakes.... well, them I don't have any ideas except that the friction plates/shoes may be set up too tight.
good luck,
rScotty