You may want to go through the below post for your question regarding Cultus Brakes. Cheers.
https://www.pakwheels.com/forums/wheels-tyres-suspension-brakes-steering/165144-strange-issue#post2777154
Theoratically, what happens is, as the old contaminated oil starts eroding the pressure seals, after a certain point , as the pressure is applied, the pressure starts leaking back past the seal (back to that Yellow "I" shaped area shown below)
Thats the reason, no matter how hard you apply, the brakes will still be engaged upto a certain limit providing a certain vehicle stopping force. At times, you cant even purposefully jam the wheel.
Replace seals, bleed the system completely and then check. Not costly at all.
