There is no such thing as a weak caliper. Hydraulics are hydraulics. You didn't say where you live. If you live where they salt the roads and drive the car in winter you could have a sticking piston in one caliper. If you don't use the car much and don't bleed the brake system, water in the fluid could have rusted a piston. One of the brake hoses could be occluded. You said you had OE rotors installed. What does that mean? Do they have a star on them or are they original equipment manufacturers. They are not the same. MB spent a lot of money to the rights to use a British patented system to test the amount of trace elements in the pig iron used to make the rotors. MB rotors are matched in the amount of trace elements in the iron, because different amounts of these elements cause a different coefficient of friction in each rotor. In other words one rotor can have higher friction than the other leading to pulling during braking. Hope the info helps.
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