Tires Squel When I Brake - Why?

[Gregski]

Got three dirt old Copper Trendsetters SE 235/75 R15 tires on my '74 1/2 ton GMC 350 manual and one brand new one of the same kind in the rear.

When I brake and it can be at very low speeds (10 to 15 MPH) or even slower as I come to a stop sign or stop light the tires squel it sounds as if I am peeling out.

I am gentle on the brakes as gentle as I can be.

Any idea what this could be? Old tires? Brakes locking up since they are not anti-lock? Driver not knowing how to brake properly? Etc.