A mouse to mouse kit is one of my best investments. 
There are several on the market, Scytek is mine, very good but there
are others of equal quality  Zymed, Innogenex, etc. also have kits
specifically designed for this staining, with the blocking reagents
in the kit.  We also found we had to dilute the primary out a bit more,
and you may have to tweak the secondary also.  The mouse to mouse kit
stopped the flow of tears!

GMA is not the best embedding media for IHC, the immunoglobulins are 
prevented from reaching antigenic sites due plastic, which cannot
be removed.  Polymethylmethacrylate is a better plastic since it is
removable, but there are some strict antigen retrieval considerations.

J of Histotechology, Neil Hand et al Superheating using pressure cooking:
its use and application in unmasking antigens embedded in methyl
methacrylate.  21:231-236, sept 1998.  

Neil has many publications on this plastic protocol, and he had tried
GMA and PMMA, cuts with glass knives, and has incredible results
for plastic embedding immunostaining.  I highly recommend his
procedures.

Gayle Callis