If your 87 is like my 88, there is a large cap on the end of the valve, take the cap off. The valve, with the larger pipe (low side). Hook up your gauge and valve to the can, let a little freon (very little) bleed through the hose to purge the air, hook up. Turn the shank (1/4" square), of the valve 1/4 turn to the right, with the compressor running. Open is clockwise, closed is counterclockwise as your facing the shaft. Or in other words, it opens in. Some compressors have a built in valve, looks like a tire valve, instead of the screw type valve, the end of the hose should have a nipple to push the valve stem open.
Don´t know how good your instructions are. So I´ll give you a few tips. With the compressor running, the low side is usually no more than say 25 PSI. When the compressor stops, it can go up to 60 PSI or more. The pressure in your can, is sometimes not much more than 60 and can even be less. So it´s a good idea, to add freon only when the clutch is engauged. Close the valve quickley when the compressor stops (clutch disengauges). On some systems, if there isn´t a minimum amount of freon in the system, the compressor won´t run. Then you have to feed it, with the compressor stopped, until it starts up, have to be quick with the valve though, as it will usually cycle (shutdown) quickly. Have to go through a few short cycles, until there is enough freon to keep it running for awhile.
Another tip, check the larger pipe with your finger, when it gets cold (sweats) stop filling. Keep an eye on the sight glass, when it´s near full stop. More freon doesn´t necessarily mean more cold and to much can mean component failure.
You can keep the compressor running longer, by running the A/C, with your doors open and with the A/C and fan on max.
Last tip, a close inspection of the condensor (in front of the radiator) and all the pipe and hoses (especially fittings) for oil stains, will save wasting a can of freon. The freon is mixed with oil and a leak often has an oil stain
(covered in oily dust).
