Start here and download a manual so that you have something to refer to because chances are, you're going to have to do some light disassembly to fix the issue no matter what....
https://threewheelermanuals.com
I would pull off the brake cables from both ends and check for free movement. They should also be lubed as well.
Then I would pull the front wheel and remove the drum backing plate which has the shoes in it, pull the arm and shaft out of the backing plate, clean the shaft, lube with grease, and reinstall...I'll almost guarantee the shaft is rusty. Those seals are only moderately effective keeping the water out especially over long periods of time.
Then you're going to need to pull the rear drum to get at those shoes and the same shaft assembly that will require cleaning and lubing. The rear drum should get a few days of penetrating oil applied at the axle nut and the splines on the axle that the hub slides on. They are notoriously difficult to remove when they've been on since new and the machine saw a lot of creeks or mud in it's lifetime. Make sure to put Anti-sieze on the axle nuts and splines, and drum splines when you put it back together
Unfortunately, it's not something that spraying penetrating oil from outside the assembly will easily fix in my experiences. I've tried but I always end up taking them apart to fix right or else I had sticky levers and pedals no matter what i tried to get around taking it apart
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