Bearing in mind that this may not be your problem at all, but it's worth knowing if you work on your brakes...When the actuator (in this case the wheel cylinder) is at the top, the leading shoe is the one closer to the front of the vehicle. Because it's firmly anchored at the bottom, as the top edge of the lining is pushed out onto the drum, the rotation of the drum will tend to pull the shoe out and apply the brake harder- this is the 'self servo' effect (disc brakes don't do this, so they have a 'vacuum servo', otherwise known as a 'booster'). To lessen this effect somewhat and avoid the brakes tending to grab, the top part of that shoe will often have the lining material either chamfered, or starting further down the shoe, so the top part of the shoe has an area with no lining. A new set of four brake shoes will have two with lining all the way to the top, and two with a gap at the top- the latter is the leading shoe and has to be fitted in that position. Note that this principle isn't used on all cars, may not have been used on the 500 (I haven't dismantled my brakes yet, I pulled the assemblies off whole and put them aside for later attention) and may not be used in aftermarket replacements even if it was used originally...HOWEVER if this feature IS present on your brake shoes, they MUST be fitted in the right place.
This is getting longwinded but it's hard to explain briefly, again it may be irrelevant to your problem but anyone working on drum brakes needs to know it. The picture in this post shows it well- look at the difference in the two brake shoes. They are oriented correctly.
