Has the MASTER CYLINDER got any adjustment on the pushrod. I had similar problems on a Long Wheelbase Land rover.
Traced the problem to the push rod on the Master Cylinder being to far into the Cylinder. This has the effect of reducing the amount of fluid compressed at first stroke.Stripped Master Cylinder down, Cleaned all moving parts and re assembled making sure that the Piston was as close to the Rubber boot at the back of the Cylinder as possible.
Brake actuation improved greatly having a full charge of the fluid pushed through the whole system at first push.
Also if the Piston is to far
forward at the start of its operation there is the danger of it not being fully retracted when you remove your foot from the brake pedal.The Piston covers the return port. The fluid in the system cannot return to the Cylinder Resevoir properley and you can end up with brakes binding and overheating.
Does the Master Cylinder top / filler have a vent hole to let Air into the Reservoir to replace Fluid that has been used.Try brake actuation with the cap OFF and then On see if it makes any difference.
good luck and let us know how you go with this problem.
