1994 GS 300 Brake goes all the way down

Hi, I'll appreciate if somebody can help me with this,

While braking on my GS300 1994, the brake pedal goes all the way down. If while braking I pump it again it the brakes apply for a moment but the pedal then again goes all the way down, making it extremely hard to brake. I checked ground under the car several times for any leaking oil but did not find anything. I assumed that it might be the master cylinder and therefore changed it and bled the cylinder and all the four brakes looking for any sign for leakage, did'nt find any thing and the problem is still prevelant. Can there be another part involved, Is there something I'm missing out. Any help will be highly appreciated.

Did you make sure that the brake booster piston rod was adjusted for the new master cylinder? Per the Haynes manual:
Adjust the length of the brake booster push rod as follows:
Set the measuring tool (09737-00010 or equivalent) on the master cylinder with the gasket and lower the pin of the tool until it lightly contacts the piston.
Turn the measuring tool upside-down and position it onto the booster.
Measure the clearance between the booster push rod and the pin head of the tool. There must be zero clearance. To obtain zero clearance, adjust the push rod length until the push rod light contacts the head of the pin.
Before installing the master cylinder, make sure that the UP mark is in the correct position. Install the master cylinder over the mounting studs and tighten the three nuts to 9 ft. lbs. (13 Nm).A plunge gauge or micrometer can be used to measure the piston depth.

Another option might be a leak in your brake booster diaphragm.