On a typical road car something like 60-80% of the braking force comes from the front brakes. So the fact that you say "I can feel the front brakes doing all the work" isn't necessarily an indication of a problem.
This bias prevents you from locking up the rear axle under braking as most of the weight shifts to the front. Therefore the rear brakes will wear a lot slower than the fronts and the rust may take a lot longer to rub off
That said, you may still have a problem if the rust doesn't go away completely.
1. Seized rear calipers.
It's not uncommon. Pull the rear calipers off and see if you can move the pins freely. If they're stuck you'll need to disassemble them, clean them up, and use quality brake (moly) lube.
2. Seized rear pistons.
Less common, but still plausible if the rust is bad enough.
3. Bad master cylinder.
Our MCs have 2 internal seals that create 2 internal chambers - one that creates the pressure for the front and one for the rear brakes. If there is an internal leak causing the pressure to bleed off it could lead to weak rear brakes.
4. Pinched brake lines.
Less likely, but worth checking.
Basically you need to inspect the rear brakes to make sure everything moves freely. Repair/replace/lubricate anything you don't like the look of - it's not rocket science.
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