Not surprising. The original hoses were made of nitrile. It was determined back in the early 90's, when systems were being studied to determine what steps would be necessary to retrofit an R-12 system to R-134a, that nitrile hoses which had seen at least a season of use would have absorbed enough mineral oil in their interior surface to act as a barrier against R-134a leakage. The same goes for the nitrile o-rings commonly used in R-12 systems back then. It was recommended that used hoses could be re-used if they were otherwise in good condition. It was also recommended that, if a hose fitting were disconnected, to replace the o-ring with HNBR (or neoprene). Any hose fittings that were not disconnected and had used but leak-free nitrile o-rings did not need to have the o-rings replaced. The same was true for used nitrile o-rings used in compressor body and shaft seals. Also, although nitrile o-rings are supposedly a bit more permeable to R-134a vs HNBR or neoprene, it was said to be acceptable to use nitrile o-rings during a compressor rebuild or shaft seal replacement if HNBR or neoprene were not available.
As a side note, it was during this period that many R-12 "retrofit" refrigerant blends were developed. They were intended to be near-drop-in replacements for R-12, with the main feature that they would work with the original mineral oil. A couple of these were HFC based, with the addition of small amounts of propane and/or isobutane to help carry the mineral oil, and those refrigerants carried the same hose and seal recommendations as R-134a. Most of the retrofit blends, however, were based on R-22. While they worked extremely well, R-22 severely swells nitrile and HNBR. For applications using such blends, the hoses had to be replaced with 134a-type barrier hose which has a neoprene-coated nylon inner lining. All o-rings had to be replaced with neoprene o-rings, including those in the compressor. Some people tried to use the original hose and nitrile or HNBR o-rings and quickly found that their system leaked like the proverbial sieve. Because of the requirements to change hoses and all o-rings, the R-22 based blends ended up not being used much, even though most worked better than 134a. Particularly on systems with stock tube-and-fin condensers. Some of the blends even gave the system more capacity than the original R-12 did, although they did run at head pressures similar to 134a rather than the lower pressures of R-12.
You are going by absolute pressures, as you would with the EPR valve. Expansion valves work on differential pressures. In the temp range where evaporators work for AC and medium temp refrigeration, the delta P for a given delta T is nearly identical for both refrigerants. For example, at an evap temp of 30° (your daughter's system is probably running around 25°-28° saturated in order to give a 32° duct temp), the pressure of R-134a is 26.1psi. At 40°, it is 35psi, which is a difference of 8.9psi. With R-12, 30° is 28.4psi and 40° is 36.9psi, for a difference of 8.5psi. The difference between them over that span is only 0.4psi. The difference in temperature per psi is only about 0.05°F, so over a 10psi range the superheat would only change by about a half a degree.
All that said, you should still check the superheat to make sure you have enough. In refrigeration, 10° is typical. However, the specs I have seen for automotive valves is usually around 4°-6°. IMO, 4° is a bit close, and can risk a bit of flood-back if the valve hunts or evap pressure suddenly changes (i.e. with engine RPM). I'd like to see at least 8°, and ideally 10°, even if it causes vent temps to go up a degree or two. If the superheat is at least 8° under steady-state operation, I would leave it alone. Even if it is 12° or 15°, I'd leave it alone. You're getting excellent vent temps, so you're better off to protect the compressor even if it means warming up the discharge air slightly.
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