If your protein is close to 50K, you may have problems with fat heavy chain band that obscure your signal of interest. Jackson ImmunoResearch marketed a mouse Anti-Rabbit IgG, Light Chain Specific antibody. This Ab antibodies reacts strongly with Light chain of native primary antibodies(Rabbit only), but not bind to the reduced and denatured IgG heavy chain band (55 kD). I tried this antibody. It works extremely well at 1:10000 dilution. The price is also not vey expensive compare with other seconday Abs from other carriers. This Ab can only slightly reacts with reduced and denatured light chains. So the signal you got is mostly from the native light chain of your primary Ab in your western blot.

Now they have another Anti-Rat IgG, Light Chain Specific Antibody in their product line. I never got a chance to try this one and do not know how good it is.
http://www.jacksonimmuno.com/catalog/CatPages/rblc.asp