2002 Ford Ranger clutch fluid leaking

I have a nice truck but every few months I have no pedal or clutch in it. I have to bleed the system at the bleeder valve at the manual transmission. I can get it to work but this time I get nothing. There is no evidence of fluid under the vehicle, so where is the fluid going? Could it be the master cylinder under the pedal inside the truck? I have bled it a couple of times and can not get any pressure on the pedal. I need some help here! Thanks, Tom