Brake Problem

[Babar]

Dear Friend
I have encountered problem in brake in my Toyota Corolla 1971.
Brake Master Cylider have single line connected to all 4 wheels via a Tee. It is in geniune state.
Story: I found car brakes are not functioning in full capacity and when i press the brake padle, it occasionaly touch to the body.
I thought this need servicing.
Then I opened all relevant parts and serviced each wheel cylinder and removed rust and and then fit all. then i opened Master Cylinder and replced
its all rubber washers with new one. also cleaned all rust particles on its piston and everywhere.
Reinstall all, flushed and bleeded brake oil with Sieken DOT 3.
I enjoyed car brake working perfect for 4 days. After 4 days, when i started my car, and pressed brake pedal, it simply touched to the floor. but when i pump it again it showed some normal brake.
So i drove with this kind of problem. After some days, I observed I lost brake response at all and finally, i was unable to brake the car, because brake pedal was touching the floor even after many pumps.
I again bleeded but found no good response.
Then i opened master Cylinder and replaced it half cut with new one(half kit found inside master Cylinder sleeves that consist of
piston all rubber washers and complete assembly that resides inside master cylinder sleeves)
anyway i did this and bleeded again. Oh great i enjoyed 3 to 4 days again with excellent brakes.
Unfortunately after 3 to 4 days, got relatively different type of problem. Now when i push brake pedal gradually,
it simply touches to the floor. But when i presses brake pedal very fast it works normal.
Again pressing gradually leads to same result. again pressing very fast behaves normal.
That is : Sudden depressing brake pedal results normal braking but graduall depressing the pedal simply toches floor with no brake at all.

This type of problem i found very first time and unable to diagnose where the problem is? and how to rectify it?
Can anyone suggest me what to do so that trouble removes?
Thanks
Babar