Low brake pedal GK7aa stock brakes

[tomx611]

Low brake pedal in front brake system.

I try to bleed all the air bubble, changed new plastic part on the master cylinder. but the front brake also very low.

i want to change the dust seal and O ring of calipers, but they cost about $40 USD for 1 set. any other brand of o ring set compatible with it?

Thanks!