These images show partial disassembly views of an O'Connor Model 30 fluid head. It was purchased used on Ebay in 2007 and is of uncertain age; possibly from the 1970s or earlier. The tilt shaft is supported on one side by the fluid tilt bearing, and on the opposite side by the counterbalance spring assembly. There are two coaxial pairs of counterbalance springs, mounted between two plates. The top spring plate moves down and compresses both pairs of springs as the head tilts off level. The top plate has an eccentric mount to the tilt shaft, so that spring compression and tilt restoring force are sinusoidal. The bottom spring plate moves up as the spring adjust bolt at right is turned clockwise. This increases the restoring force for a given tilt angle, allowing the head to balance a heavier camera. The fluid-drag action is provided by the fluid shear between a rotating steel cylinder and a surrounding adjustable-diameter plastic sleeve, shown in this photo. In operation the pan and tilt bearing areas are filled with a very thick silicone fluid.

When I got it, the head was filled with a very thick sticky grey material. I have heard that the original O'Connor fluid was clear, so I have no idea of the history of this particular head, or what the grey goo I found in it really was. The total fluid volume is approximately 4 fluid oz. (120 ml). I cleaned out the grey goop and replaced it with a clear polydimethylsiloxane called Dow Corning 200 fluid in the 100,000 centistoke (cSt) viscosity, which is really quite viscous but not as thick as what came in the head. I believe the minimum order from Dow Corning is 50 pounds (!) but companies can get a free 500 ml sample, which is more than enough for this purpose.

I am satisfied with the resulting pan and tilt action of my re-filled head. The drag adjustments go from loose to "moderate" drag. I would guess some might want a more viscous fluid to achieve greater drag, such as the 300k cst viscosity. If the Dow Corning fluid is not easily available I don't know of good alternatives; Cargille Type OVH immersion oil is sold in small quantities but is lower viscosity, at 46000 cSt.  By the way, O'Connor will sell you a #30 head fluid refill directly for about $90, and they can refurbish your used Model 30 head for $1100. (Prices as of April 2007)

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1) There's no service manual for this head I know of.  I asked O'Connor about it on the phone, and they said there never had been one.
2) I spent about two months looking for replacement fluid. 
3) If you clean out & replace the fluid, plan on spending a full weekend at least on it.  Cleaning the gunk off all the parts was very time consuming, and there's no solvent I could find that had much effect (I tried at different points water, soapy water, isopropyl alcohol, mineral spirits, and acetone). Gojo orange pumice hand cleaner did help get it off my hands.
4) My #30 head had no gaskets on the access plates, at least no recognizable ones. During reassembly I used a thin bead of "shoe goo" to seal all around the edges of the plate, another somewhat fiddly process.  If you don't have the o-ring around the rotating bottom part pressed in and fully seated all the way around, it will slowly leak fluid out from the base. Even some factory-original models of this head were known to leak fluid over time.