Is there a fluid you would recommend for cleaning out the hydraulic system? This machine has set idle for about 10-12 years disassembled.
The first thing to do is buy a 273A repair and parts manual if you do not already have one. Ebay has several copies for sale at $70.00
Heald 273A, Universal Grinding Machine, Instruct Service & Parts Manual 1969 | eBay
The hydraulic oil in your grinder had, at one time, additives that prevented corrosion. The additives have been consumed after 10 years. The sump needs to be emptied. The tank access plate needs to removed and the sump carefully cleaned out with lint free rags. There will be a large amount of "mud" at the bottom of the tank. The Heald may have a hydraulic oil return line filter. If so, it needs to be replaced. The old rubber sealing ring on the access plate will leak. Apply a bead of RTV sealant to the ring before reinstalling the access plate.
The manual will specify what viscosity oil will be needed. Do not use a NAPA or similar off brand hydraulic oil. The Mobile, Chevron, or Shell products are designed for long service life. You can substitute the equivalent Mobile Vaculine or Chevron products that are intended to be used as both the way oil and the recirculating hydraulic oil. They are slightly more expensive but are much more convenient than stocking two different oils.
The oil should be transferred from the 5 gallon bucket to the sump with a pump running the oil through a filter. The alternative is to slowly pour the oil into the sump and leave the last pint in the bucket. There is often mystery stuff in the bottom of the bucket.
The Heald grinder hydraulics are turned back on with the needle control valves fully open. The spool valves and hydraulic pistons are then run back and forth. If you are lucky every thing works. If not, the grinder hydraulics may have trapped air pockets, the pump pressure may be too low, or one or more spool valves are stuck. The Heald manual will have some suggestions on what to do next. The table piston o-ring seals may have gone flat after sitting for 10 years. Try removing the way covers and check if there is a flood of oil running into the table drains. If the table ways are in bad shape you will need to move over to the "Machine Rebuild Forum" for the expert opinions.
It may be worthwhile to pump the new oil out of the sump after it has run through the system for a while and run it through a filter into a bucket. The oil can then be returned it to the sump. Any grit remaining in the oil will eventually settle out in the sump and form a new "mud layer" at the bottom of the tank.
Robert