There are caliper brakes that are for 26" rims for sale $20.00 on pistonbikes.com here
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I feel compelled to inform you that the fenders you have on your bike have a tendency of rattling and breaking, Not the fenders but the mounts. there is a thread in this forum somewhere that explains both the danger and possible fixes of this problem.
Not trying to scare you just wanted to let you know before you get a nasty surprise. Another thing you want to keep an eye on is your cheap Chinese stock studs. These have a tendency of rattling loose or breaking off Sick Bike Parts sells replacement American steel parts for decent cost $9.95 here.....
Sick Bike Parts
No I'm not a salesman, just a dedicated member to MB's and safety.