You say that the manual says 3.89 is the capacity. 3.89 what? Ounces? Pounds? Cans?
OK, so I will assume it means pounds. 16 ounces in a pound times 3.89 is 62.24 ounces. Divide that by 12 ounces in a can and you get roughly 5.18 cans for a full charge.
Your statement that the high side pressure jumped up to 300 would indicate that your compressor is working OK. Might not be up to 100% efficiency, but it should be working enough to get cooling.
Usually, when the low side pressure is too high, it points to the expansion valve. But, if your numbers look like it should be cooling, it might indicate other problems. Too much moisture or air in the system are deadly to an A/C system. Air is simply a non-condensable gas at A/C system temperature and pressures, and will not allow any cooling - even if the pressures look OK. Another possibility is oil. If there is too much oil in the system, cooling performance will be poor.
Vacuum - on a standard day - (70 degrees at 29.92 atmospheric pressure) your vacuum pump should pull it down to an indicated 30 inches of vacuum. The system should hold that vacuum for at least 20 minutes or you have a leak big enough to cause problems. For leak checking, I like to use a combination of a leak detector and dye with black light. (I know, a lot of guys do not like dye checking. Heard it all). Look at connections, fittings, and the compressor shaft. If no leaks are found, REPLACE THE HOSES!!! Been there, done that! 134 refrigerant has a tendency to migrate through hoses as if they were porous. Those leaks are nearly impossible to find.
For as simple as a refrigeration system is, they can really give you fits trying to get them to work as they are supposed to. Good luck.
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