Technical Hydraulic Throttle

Anyone have experience refreshing these old hydraulic actuators? I using it as a throttle on my front engine dragster. Seems like it's down on fluid a bit and I don't know the proper way to service it.

Thanks
Bruce

 

I found this but I do not know if it applies:
"The slave cyl must be higher than the pedal to bleed. If it isn't then it must be removed and elevated and held horizontally to bleed. Right before the slave cyl there is a small alm cyl with a petcock at the bottom of it. Remove the small screw off cap on the top of the cyl, be sure the vessel is full of trans fluid, and open petcock. Air will exit as fluid enters. Once all air is free from the line shut off pet cock and replace screw on cap."