Having a nightmare with my brakes... Failed ABS pump?

[Joemi]

I recently changed all 4 discs and pads on my 2016 transit connect but and found my front RH brake was dragging slight afterwards, turns out the bleed nipple was snapped so I decided to replace the caliper so I could bleed it properly.

After changing the caliper and bleeding it all up the front RH that I swapped completely stays seized on after pressing the brakes, the only way to release it is by slacking off the nipple where brake fluid then shoots out like a jet.

The fluid is basically not being allowed to return once entering the caliper, I've replaced the Flexi hose and the problem still persists.

This fault doesn't seem to affect any of the other 3 caliper so my next port of call is possible a faulty ABS pump but I'm not familiar how they work...

What component on a modern ABS braking system would allow fluid to pass through one way but not allow it to pass back? Abs pump? Master cylinder?

Any advice is really appreciated, many thanks.