The British offered freedom to individual slaves of Patriots who left their masters and fought for the Loyalists. Several Northern colonies/states also offered to free slaves who fought for their side, as did many individual slave owners on both sides (though this promise was often broken). Nonetheless, Great Britain didn't abolish slavery in the Empire as a whole until 1833, more than half a century after the American Revolution ended. The British did not intend to end slavery as a whole in the American colonies.
The point is that, even though the antislavery movement did already exist by the 1770s, slavery was still legal and widely socially acceptable. As time went on, and ideas about race relations and human rights became more progressive, slavery became less and less acceptable and was slowly banned by more and more nations. The Founding Fathers were generally more conservative than modern society, as can be seen in the example of their support for or tolerance of slavery. However, judging political philosophies requires looking at the society in which they existed. Conservatives in the late 1700s were monarchists, while radicals supported constitutions and republics. So, in the reality of their time, most of those who fought for independence from Britain were really radical leftists who were committed to challenging the old order and authoritarianism.
I'm not trying to say that Washington and Jefferson would be Democrats if they were alive today. We can't make that judgement, because too much has changed in the past two and a half centuries. I just take issue with the right-wing assertion that the Founding Fathers were all conservatives.