Trying to maintain a 50 year old cracked concrete slab is the same as flogging a dead horse, very frustrating.
The biggest enemy of concrete is water and freezing weather. Water gets into the cracks, freezes, expands, and your left with a hole full small rubble.
Cure. Rip it all out.
Get rid of all top soil, put in a sub-base of 150mm of compacted mill waste, lay a minimum 100mm (150mm ideal) with some reinforcing. Put a small gradient in the concrete towards a drain to stop the puddles. If it is a large area your concreting, put in a movement joint because over the years the sub-base WILL move.
If can't be arsed to do all that get someone else to do it. Failing that carry on repairing like rjd1 suggests but be aware larger repairs last better than smaller ones (that's why the council dig up half the road for a small hole) and never use fairy liquid.