Hi there, I have a 93 F150 5spd with hydraulic clutch, and the master
cylinder reservoir does have a rubber cap on it, but if it is full, I
would think that the rubber's only function is a seal to keep dust out. I
do know mine does travel with the clutch fluid level. Do you see fluid
coming out of the slave anywhere? I know mine has a cylindrical slave
cylinder that sems to be integral with the release bearing, but I have
never actually taken it apart. Hope that helps even a little.