The brakes can bind for mechanical or hydraulic reasons. Mechanical ones include sticking pistons or calipers. Hydraulic reasons can only be if pressure builds in the system. Pressure can only build if fluid is not free to return to the reservoir when it expands because of heat. This can be caused by the piston not being able to return far enough to uncover the return orifice. This in turn could be caused by the vacuum assist valve still applying vacuum to the system. Try to make it happen again and open one brake bleeder to release pressure. If it goes away you are on the right track.
Another reason on old cars can be that the hoses are so swollen internally that the fluid can't return once applied. The hoses in this case act like one way valves.