Front brake inconsistent resistance

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( can't help myself ) If there is loss of fluid ( the reservoir level is dropping ) it's anything but the master cylinder ( at the handle bars ).
And if there are no leaks it IS the master which is letting fluid getting past the first sealing rubber cup ( there are two, one for pressure and the second one which you can see in the reservoir opening to stop fluid leaking out of the cylinder ).
The fact that it restores itself with a few pumps suggests to me that it does point to the master once it resets itself.

Could be the brake is not used frequently or a myriad of other causes, one of which is corrosion in the cylinder,worth a look, easy to replace the parts on the handlebars and should quickly prove the point, not a usually mentioned fault if i recall correctly.
Here's how it works;



All the best with the bike Chief.

Excellent illustration

Excellent illustration
Hi Rocky, good to see you