how often do rear coil springs go bad?

[64stpside]

On my 64 c10 I got pretty bad bouncing in the rear when hitting bumps, especially when hitting a bump in a turn. I think my rear coil springs are shot. Things I have done so far to try to correct the issue:
new shocks, new trailing arm bushings (rubber) , new panhard bar w/rubber bushings.
I work at dealership and had the truck aligned this last saturday. I thought i found the issue as the thrust angle for the rear axle was +.37 deg (should be 0 deg). We got that right but I still got a bad bounce. Is there a common test as far as how many times the springs go up and down after pushing the rear bumper down by hand and letting it go? What do you guys think?