First of all if it's a Daybrooke it's at least 35-40 years old as they went out of business about 1970 or so. But your likely OK on the power unit parts as it is "likely" a Monarch brand made in Grand Rapids, MI who are still very much in business. What you've described is a very common 12V manual valve power unit that has a motor/valve body/reservoir built into a sandwich design. The center section contains a small gear pump and there should be a manual directional valve that has a rod and lever attached to it. When you pull on the lever the dog where it attaches to the rod pushes on a starter button that starts the motor. A solenoid is on top of the motor also. The 3 position directional valve does not need a globe valve to operate the gate. It is a simple raise/hold/lower plunger type valve. If someone has installed a shutoff valve in the line that tells me that the directional valve is not holding the gate in position so they used a ball valve to block the line. Take the unit off, remove the oil reservoir, drain it, clean the tank and valve assembly, replace the O ring tank seal and re-assemble. Probably has a bunch of crud in the valve as there is little that could happen to it short of someone tampering with it. Most of the time the operating lever got bent and out of whack (a technical term
)from backing into freight docks and that caused the directional valve to be out of adjustment. Replace the rods, crossshaft, add or replace turnbuckle and lock nuts and put it back together. Make sure it operates free and easy but also be sure the handle weight doesn't cause the button to start the motor by itself. The solenoids are subject to corrosion and while they look like F*^d style they are a continuous duty solenoid that you can get at most truck equipment shops. The Monarch units are simple, dependable and remain the industry standard for power packs. Also replace the old push button starter switch while your at it and maybe the battery cables too. Use 2ga cable at a minimum. What I've described here is obviously a single acting unit but double acting is also a possibility albeit a somewhat more remote likelehood as SA was the rule back in the day yours was built. Same deal if manual valve only a 4 way valve in lieu of 3 way. Other brands less likely but quite similar in concept and design. Even less likely is that it is an electrically operated valve and then the solenoid cartridge valves get gummed up. OH, and yes you can suction the old oil out the fill cap hole if you wish. That about does it unless you want a model number and I'd guess it's an M500xx something or other, built in both vertical and horizontal styles. Good Luck.
One last thing and it's a biggy: be darn sure the safety catches on this thing will hold the weight of the gate safely in place when in it's stored position. If one of these things falls on you then all anyone will find is a greasy spot where it happened. Most critical if your gate is a conventional style and stores in the upright position. VERY important.
