Heat cracking is normal to some extent on trailer brakes shoes.
Bet the wiring gauge is limiting the actual stopping potential of the installed brakes. That long of traielr needs pretty heavy wire gauge to allow the amps/volts to actually get to the magnets. OEm's cut corners on brake wiring to save a few $$.
If you install new loaded assemblies -get 7K rated 12x2 self adjusting Dexter. Eastern Marine or etrailer.com online usually have decent prices. Rights and lefts are different... All late model 5.2K, 6K & 7K 12x2 brakes interchange.
Al-Ko Axle Manual link:
http://al-kousa.com/download/ALKO_2kto7k_OwnerManual.pdfDexter Brake sub section manual -look at the wire gauge table! I have 10 gauge on my 30' Ragen TH fiver -this is what it took to have lesss than 3% voltage drop on the measured cable run wired in a star or home run scheme.
http://www.dexteraxle.com/i/u/6149609/f/600-8K_Service_Manual_3-13/Electric_Brakes_3-13.pdf
