OK - Still a leak with cap on? Check your sniffer. My own will pick up signals from any sprays used or if dirty in the area or windy goes bonkers.
R-12 tank would work - just vacuum it out if empty but there are easier choices.
This may hold 36 oz not the 32 oz I mentioned and changed if you look back. IMO that 4 ounces isn't going to make or break this truck's A/C it will or should work and your performance results are fine for that ambient temp.
Note on that: Ambient to the truck is the temp of the air coming in thru the grille NOT the day's forecast or a thermos on a wall thing. It's usually warmer than you think right in front of the grille of the truck is all it knows.
Check if you wish the charge to be super on target if it's 36 oz. that's 3 of the small cans but do weight them first new. OMG - I find them off new!
What to expect for pressures is highly dependent on air flow, RPM and how effective fan clutch is on this. Water on condenser will dramatically drop pressures just FYI to help or check max performance. Takes a lot of thermos placed around.
The high side pressure should reflect a multiple of the temp of air coming in X 2.5 or so for 134a at a raised RPM of about 1500 or a bit higher.
Here's a great link also above at this site for good basic info on charging...............http://aircondition.com/tech/questions/ ... Procedures
Notes: Vacuum at sea level is 29.92 or close - real close. How long a pump needs to run to get to that varies but once there you can shut the pump off and just let time pass while it holds. If anything leaking or more moisture either as liquid, in desiccant or as ice for any reason is will rise by itself and run again to see if it holds. No need to run the pump for an hour unless it's seriously too slow or defective. All connections are also always suspect too.
Back to the leak. Just get a high quality new cap is probably the solution for high side. It's getting old but might find one in stock new at dealer or even a great one used. Some cheap stuff will just fail right away and it does count on it.
BTW - know a tad more for this than others as I've owned a 96 Suburban and currently 97 C/K 2500 the difference is Vortec 5.7 engines not much else.
Chat: Going to be warm here soon and on my list (I'm done actively doing much of this) check all vehicles after Winter that they are working properly now old catching them early has saved them for decades on end by NOT allowing them to run while low and old enough can go a few years and need some not enough of a loss to require a repair on them yet as too slow to be credible,