Robert McPhee and his son-in-law John Miller were convicted for the crime of servitude - holding men against their will and forcing them to work for little or no money.
James McPhee was found guilty of assault and abduction.
Another man Steven McPhee pleaded guilty to assault earlier in the trial.
The men were found guilty of more than 20 charges between them after five-week trial heard that they prey on vulnerable men from troubled backgrounds.
The victims were promised work doing jobs such as mono-blocking driveways but were held captive and made to do mammoth shifts for little or no pay.
They were brutally punished if they complained or tried to escape.
The charges spanned dates from 1992 to 2016.
They included a charge that Robert McPhee, known as Bobby and nicknamed the Tank Commander, held Graeme Lamb in “servitude” against his will for three years at a piggery in Shotts, North Lanarkshire.
Robert McPhee and John Miller were also convicted of holding Kieran Williams in “servitude” under their control for two months in 2016 at the same site.
Robert McPhee, whose age was given as either 69 or 66, was also found guilty of repeatedly punching Brian McEwen and forcing him to eat a jar of coffee granules.
He was also convicted of abducting James Keith and detaining him against his will.
His son Stephen McPhee, 37, pleaded guilty earlier in the trial to a charge of assaulting Mr Keith.
Another of Robert McPhee’s sons, James, was also found guilty of attacks on Mr Keith.
The 45-year-old was also convicted of a eight-year campaign of violence against John Anderson, including punching and kicking him and hitting him with a pick axe handle. He was also found guilty of making Mr Anderson fight with other men.
In 1999, Mr Anderson escaped to Paisley but James McPhee tracked him down and forced him to return to the traveller site at Bathgate where they were living at the time.
On another occasion Mr Anderson escaped to Cambridge but James McPhee once again tracked him down and hauled him back to Scotland.
Another man lived in a caravan with no water or toilet, before being told her was “owned” by the family.
One petrified worker was left “cowering like a dog” due to the abuse.
Another who fled was told he had a £5,000 bounty on his head and would be “skinned” with a razor unless he returned.
Robert McPhee and his son James were convicted of numerous other charges of assault to severe injury, abduction and holding people against their will, taking place over two decades.
The gang had denied the charges, claiming they were picked on by “overzealous” opolice and had shown kindness towards workers.