EK3 98 No ABS , Break problem

Hi Guys
I’m New to this and need your advice for my EK3 1998
Few weeks back I was driving at on a flat road and had to apply immediate break. That time my car was turned and it was skied. The speed was somewhere around 70 Km
I have noticed that the front right wheel was locked and none of other wheels.
Finally I checked all calliper status and noticed that pistons war badly corroded. So replaced them all with new O ring and dust cap...etc...Etc. Replaced with NISSIN brake Pads as well
Last week i did apply same break on a flat road and looks the issue still there but not that bad. Right front wheel will stop then left wheel.
Finally thought the Proportioning Control Valve is faulty and replace it today
Now the issue is 99% fixed.
My question is if the Master cylinder is bad will this issue can happen.
The primary line of the master cylinder goes to Front left wheel and rear right wheel via Proportioning Control Valve
The secondary line of the master cylinder is goes to front right break and rear left via next side of the Proportioning Control Valve
So once we apply break as per master cylinder it should apply break for front left wheel? So why it stop the front right wheel
I have not checked the master cylinder cost the paddle is not sinking and it with little press (1.5 inch distance) the break will apply. So i do not know how to test master cylinder
Please help me to rectify this issue
Thanks
Sira
http://www.nissinbrake.com/AVP/Catalog/?a=p&cid=1&pid=20