Re: Rear brakes
OK, let's back-up a bit on this. Many shops do not have the equipment to do this task correctly, -and there in itself can be things that will exacerbate the problem just as you are describing. When you describing pulsating, I am willing to bet the problem originated in the hub(s).
To begin with, before the new drums were ever installed, the hub itself needed to be checked. Often times, the hub is not (re)usable due to the hub race inside the drum being worn. I have a machined slug that we use to slip into the race area. You can visually see if the hub race is out of round. Our test is to put our finger over the small end of the hub and remove the slug. It it pops due to the vacuum, it is a good hub. If it won't pop, then the cost to restore exceeds the cost of a new hub.
Now even if the plug pops, this only checks concentricity of the race surface however the hub bearing race also needs to be parallel with the centerline of the hub. If this is not repaired, the drum will not be riveted on exactly perpendicular to the centerline axis of the hub. This will cause pulsation.
Next, if the shop used a cone on the mandrel to center the small end, they could have missed it there as often times the ends are damaged from the axle shaft nut. I use the centerline of the hub to reference off of, and when I machined my mandrel, I have a live center pilot hole on the small end of the mandrel. I mount the hub on my mandrel which centerlines off of the machined taper where I can true the flange so that it is parallel to the centerline. Then, when the drum is riveted to the flange, I know the shoe surface of the drum is parallel with the centerline. The machining of that shoe surface then is only needed to ensure concentricity of the centerline.
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