Rear Master Rebuild? 2004 SM450R

Hello, I recently picked up a 450 SMR and have encountered some problems with the brakes locking up. I was able to find a rebuild kit for the front master cylinder, but I am having trouble finding one for the rear. It's a Brembo unit, part number 800076338. This seems to have been used on the TE's from the same vintage.
I'm confident the master cylinder is not allowing fluid return, because if I relieve the pressure in the line, the caliper returns.
Any help or suggestions is appreciated.

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