For most HP landing gear, the Schrader valve is mounted in a small fitting on or near the crossbar between the right and left upper strut tubes. I think that the tubing should be welded 4130 steel.

You may be able to get by with a regular tire valve core. However, according to the Aircraft Spruce and Specialty catalog, the standard aircraft tire valve core is good to about 400 psi, which is the fill pressure for many HP gear struts. But that pressure is just what’s in the strut at or near full extension. During the compression caused by a typical landing, however, the pressure will be much greater than that, leading me to believe that a high pressure valve core is a certainly advisable and perhaps a necessity.

So you probably ought to use a high-pressure valve core such as:

• The ACS high pressure valve core, part number 2300HV, which conforms to standard AN809-1, for $3.95
• The ACS high pressure valve core, part number 9914A, which does not conform to standard AN809-1 but is probably just as good, for $1.65

 Since you only buy one of these every fifteen years or so, and the price difference is pretty small, I’d be inclined to spring for the expensive one just for the warm fuzzy feeling of having gotten the AN approved part.

But let’s consider the maximum pressure in the gear strut. First, consider that the landing gear of most aircraft is designed to resolve around five times the gross weight of the aircraft. Given that an unballasted RS-15 or HP-18 has a gross weight of about 750 lbs, that means that the gear must resolve a maximum loading of about 3750 pounds. That 3750 lbs gets resolved by two pistons that have 1" diameter. Doing the math, the combined areas of the two pistons is 1.57 sq in. Dividing the landing force over the piston area (3750/1.57) gives a pressure of about 2388 psi. That’s a lot.