MG90 servo (with external 6v supply)seems to slip when pulling a lever

Hi - I have a MG90 servo and am using it to control the tension of a bike brake lever on a bicycle handlebar. I have a small plastic arm attached to the servo head and have a piece of cable looped around the servo arm and the brake lever. I can tighten/release the lever in increments without any issue (0-40 degrees) but when it gets to the part where I need the most friction or tightness on the lever (40-60) the servo seems to slip and readjust and seems to go into a loop of loosening and tightening the lever. If I manually close the brake lever myself and then change the servo degree level to 60 the servo can keep the lever closed without any problem. The problem is getting the servo to get to that stage of applying 'maximum brake' tension on its own.

I had been using the 5v from the arduino board and thought that might have been the issue but I've added an external servo power supply using an adjustable wall socket adapter (for guitar pedals) - its set to 6v. Any ideas ?

Sounds like your servo just doesn't have enough "grunt" (torque), can you attach the cable closer to the servo hub for a shorter lever arm? You might need a pulley of the correct diameter for the cable to wrap around.

Hi Outsider - can you explain this part in more detail as I don't fully understand what you mean: You might need a pulley of the correct diameter for the cable to wrap around.

The smaller the pulley diameter, the higher torque but less available travel distance, same for cable attachment distance to brake lever pivot point, the farther from the hub the higher the torque, you have to compromise between how hard you can pull and how far you can pull.
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ok - I get you - thanks for explaining