Look at this a completely different way, possibly this helps, possibly not.

Suppose you only had oil on the bottom side of the piston (thinking of piston rod pointing up towards the sky).

At the bottom outlet port, you have the needle and parallel check mentioned above, then connected to a 36" piece of 3/4" black pipe, pointed vertical towards the sky, and open at the top.

That could potentially be the reservoir, and would not need any connections to the top port of the cylinder.

However, you probably would like to modify that to use the top side of the piston as the reservoir...packages better.

Irontoys made a great observation there..you probably can't expect full extension of the piston rod as the volumes are slightly different top and bottom.

Possibly one way to get the vent and reservoir worked into the design is this. Buy (or modify) a cylinder's top endcap that has 2 ports. One top port is connected to the needle and check that goes to the bottom port. The other top port could be connected to an elbow, and a vented pipe cap, which is where you'd add fluid, and would account for any "extra" fluid that could not fit in the top side of the piston. The reservoir doesn't need to be very big, again, think of a pipe street elbow and possibly a close nipple screwed into that, vented at the top.

Hope this was useful information.

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Lets see if I can give a little help.
1. The model number on most Rotary lifts that I have seen in the shops I worked at were attached to the lift head on a metal tag, riveted in place. You may be able to contact Rotary with some photos and they can probably guide you a bit on what you have there.

2. The pipe plug/cap in the center of the piston should be the fill point. It will normally have a dipstick attached to it, but depending on the model and age, may not. You do not have any other above ground hydraulic tank or another cap in the floor do you? That would indicate a separate hydraulic tank buried under the floor as a reservoir. Two post lifts that I have dealt with/used/repaired have those. I think the single post may not and uses the hollow piston as the reservoir, but don't quote me.

Make sure the air is off and the control handle has bled all pressure down before removing that cap or you will make a mess and maybe even seriously hurt yourself when it rockets off.

3. The cap should be a standard pipe plug with regular tapered threads. Use tfe paste or tape of your choice when you put it back on or it will leak.

4. Treat the cap like any other rusted in place plumbing piece. Heat, vibration, penetrants of your choice, appropriate wrenches, etc. Worst case, you drill it out, cut it off, clean out the threads and make a new one from a plumbing store or the manufacturer. Just don't damage the threads on the piston.

Now without the top of the lift in place, when you start to bear down on that cap with the wrench, that piston will start to rotate. You may have to fab up a reaction bar to bolt to the top of the piston and the safety bar next to it so it won't spin. Mark the position of the piston now to the bearing/seal ring around it and to the floor as well. You will want to get those back in the same position to avoid possible sticking problems from years of wearing in together in that position. I had one that got rotated to avoid a gouge/leak in the piston and it caused lots of problems.

Also, some of those pistons can be rotated in a full circle with no issue (hence the original Rotary Lift design for easy access to lubricate all points of the car by rotating it in a circle) but some you turn about a quarter turn with the upper bearing off and you can then remove the piston completely from the floor.

5. The safety post next to the piston should be loose in the pipe in the floor and move up and down freely. The fact that it is so frozen might be a big part of why it lifts and drops so slowly. In ground lifts rust horribly under the floor even if they were installed with the proper anodes long ago. The steel pipes, external tanks (if you have one) and the safety bars and tubes will rust away if given enough time. I have personally dug out and replumbed a couple two post units and put in new above ground tanks and had to decommission one permanently that was beyond practical repair.

If yours is that stiff, put the top back on and just disconnect the safety bar, then run it up and down a bit. If it moves pretty well, there is your problem. Start by moving it up and down about 6 to 12 inches at first in case it gets stuck. At least then you can try and get something heavy on it to push it back down. Full up and stuck and you will be in big trouble.

Once you have that prove out and before you lift another vehicle, you need to run the lift all the way up and back down to make sure it will work properly. Do it in stages and take your time, until you are sure it will work properly.

If it still won't move with the safety bar disconnected then you have to think on some other possibilities. The piston could be seizing in the bearing. You could have no oil (though those units usually exhibit a violent pumping/hopping up and down issue with low or no oil, enough to shake a car right off of it). You could have a major air leak underground.

I would put my money on the safety bar being the cause myself.

Best not to lift the lift without the top on, it needs the weight for the gravity return. It can take lots of effort to get the piston back down without it.

Also for future diagnosis, do you have a two lever control - one to select up or down and one to activate the lift - or do you have the single lever rocker control?

Sorry if this is a little disjointed in response, I am adding info as I am remembering it from past experiences.

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