Check the proportioning valve... Follow the brake lines from the master cylinder...it should take you to this valve.... I believe it is on the fire wall...
Another place to check is the rear brakes... You he Sephia has front discs and rear drums...
I wish that JesZek was still around as he was the specialist on Sephias...
Dave

 

... I wish that JesZek was still around as he was the specialist on Sephias...

I'm still here, somehow. (without motivation, only.)

Regarding the vanishing brake fluid, I kindly suggest to the one who asked, that he / she must check carefully the rear of each wheel, in order to locate probable leakages, either from the flexible rubber hose or from the pistons on the drums / calipers.

If there's nothing, then the fluid might be going inside the brake's Booster, which might lead to a sudden fail of the braking system; if you don't know what you're doing, please take your Sephia to a certified mechanic and let they fix it for you.

Other point to consider: Sephias with rear drums, have a design flaw that leads to make the rear drums to be poorly braking, due to misadjustment on the self adjusters known as "Struts" inside the Drums; either that or very wear out brake shoes, could make the pistons inside the drum brakes, to Deploy beyond they should, leading to an inner brake fluid leakage...

I suggest to start by inspecting the state of the rear drums, and the Proportioning valves; because the original ones are prone to Leak; Kia offers improved ones that are a direct replacement, cheap.

Kind Regards.