brake servo wanted, 1989 hiace.

Hi, i own a 1988 Toyota hiace supercustom mini bus, i have done loads of work on it, great fun to drive but i havent seen another one like it, i have modded parts as Toyota main dealer parts are dam expensive, but now i think i need a brake servo as when i put my foot on the brake hard the pedal sinks and the warning light comes on, but im not loosing fluid at all, i just replaced the clutch master, anyone know of one breaking or where i can buy a repair kit i would be greatful, don't know how to put a photo on. my email is Roverp5b@hotmail.com