hard to find info... but, found(according to the internet...) that:
part number 2779923 = 15/16 bore (~.69 sq.in. piston area)
part number 2812896 = 3/4 bore (~.44 sq.in. piston area)

both numbers were found as replacements for a GPX truck

so *if* its got the bigger one, changing to the 3/4 will give ~56% increased PSI at same pedal effort...sounds likely the issue think i'll order one :)