Sinking brake pedal due to ABS

[Mun33b]

The car in discussion here is civic 2006 VTI Oriel. The purpose of this thread is to share the findings and resolution and help others stuck in such situation.
Issue:
The brake pedal sank on brake application. It was happening whether the car is stationary or moving. It did sink little when the car was shut off (no booster assist). This was dangerous as the car was not stopping properly, and lunged forward even on full brake application even on lower speeds. The abs kicked in randomly on braking, everytime on hard braking (even on slower speeds with medium braking force while the wheels werent even close to locking).
Checked brake oil level, it was full. Cheked for leaks, none found in lines, calipers, master cylinder.
Flushed and bled the system to no avail. The most obvious cause in this case is Master cylinder itself (having an internal leak and bypassing fluid via cup seals). Without giving it much thought, replaced it with new Honda genuine part.
New parts was bench bled and then bled the system again and lo and behold, the pedal is still sinking. Bled again and again, actuating ABS aswell, wasting couple litres of brake fluid (must have cleaned the lines good lol).
Doubted that the new master cylinder is faulty aswell. Tested it by blocking the two outlet ports with cutup line ends with ends shut (desi blockers). The pedal was rock solid. Well, put it all back together and back to sinking.
Disassembled all 4 calipers and verified for working, found nothing wrong. The brake pads were nearly new from the last brake job.
Searched around on internet and people were having similar issues, a video pointed that it could be the ABS.
Will be updating the proceedings in below posts.

[Hassan1991]

[Mun33b]

The searches pointed that there is a way to check ABS for this behaviour. There are two ports on the ABS hydraulic system block itself, a stick is put in these ports and then the brake is applied on static car. If even one of the stick is pushed out, there is the fault. However, i looked hard at the abs module of civic was unable to find such ports.
Otherway is to isolate the system, take out all 4 wheel lines from ABS module and block the ports, leave the 2 lines coming from Master cylinder, if the pedal is still sinking, its the ABS.
As I had already ruled out remaining components, decided to procure one and slap it on. Bought one from junk yard for 4k. The catch is it has to be the exact same part. The shopkeepers want to frankenstine it by swapping hydraulic body to existing solenoid acutators (peddling out oddball parts).
For PKDM 7th gen civic VTI oriel, the ABS module part code is BA, some other origin has J. Here is how the parts look:

[Mun33b]

Replaced the ABS Unit, and voila, no more sinking pedal.
Here is the explanation of the issue:
ABS unit regulates (open/shut) the brake line pressure as per the wheel sensor input during braking. If the wheel locks up, ABS unit actuates respective wheel solenoid and reroutes the brake fluid pressure to free up the wheel momentarily.
While doing so, the pressurised brake fluid has to go somewhere, thats where fluid reservoir comes into play. These reservoirs (2 of them for civic) are built into the hydraulic body of ABS unit and have a spring loaded piston present in them.
(picture from youtube video):

When ABS is actuated for a wheel, a return solenoid actuates and releases the pressure to this reservoir. In my case, the solenoid was sticking open and dumping the fluid in reservoir all the time, hence causing sinking pedal.
Later on I opened up the old ABS module to find the reservoirs:

Notice the two circular holes above actuator valves, these are the reservoirs, opened up one of them, opened one of them up, the piston is stuck inside still but you get the idea:

[Xulfiqar]

And this is why you flush your brake fluid yearly on an abs vehicle

[waqar747]

Considering almost all relatively new cars/SUV have ABS now, this may be an important maintenance most people are missing. I never knew this had to be done yearly.

I think 2 out of my 3 current vehicles have had 'brake service' from 3S within the last 6 months (Honda, Kia), but not sure if they flushed / replaced the brake fluid during this service. Just checked receipts on my log, they haven't charged me the fluid. Now what does that mean and what am I supposed to do now?

[Mun33b]

Get it replaced. Also when caliper pistons are pushed back without loosening bleeder, the gunk and rust present in calipers is pushed back to Proportioning valves/ABS module and clogs up the ports.

[Xulfiqar]

Ah yes

I find it very odd that a lot of mechanics simply push back the caliper pistons forcing all trash upwards

Opening the bleeder and connecting a tube takes all of 1 minute

[Mun33b]

The local myth with ustads over here is that it sends air in the system, and they are too lazy to bleed out the system. It also makes them money in the long run when proportioning valves get choked.

[Xulfiqar]

Ustaads have yet to discover that one has to connect a hose to the bleeder and loop it upwards.