To prevent frosting of the evaporator, low side pressures have to be kept above 32F. The maximum low side pressure for cooling tends to be 45F.

So for r134a the range in low pressure is from 28 to 40 psi gauge. Ie. Don't exceed 40 psig in continuous running on the low side and presumably the high side won't exceed the pressure limits of the system.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

I am a little lost at where your reading is being taken. On the low side or high side?

You should have your gauge on the low side to see the evaporator temperature and consequent discharge temperature within about 10 degrees of actual suction pressure conversion.
Meaning the temperature to pressure relationship charts for your refrigerant used.

At ambient air temperatures on a system at rest or the charging bottle sitting in a shaded area, the pressure on that bottle @ 70 degrees will be about 70psi or 500 kPa.

Your statement of 70 kPa drop does not sound like a movement of a low side connection when running and is opposite of a high side reading that should move up!

If you are connected on the low side and filling the system then you are definitely over filled! On a start up the low side should drop 300 kPa and the high side go up that much or more.

Are you using a manifold gage set and a large bottle or one of those over the parts counter you fill 'em canisters! If you are using the latter then done for any attempt to recover the gas.

With a manifold gage set and connections to both sides (usually on the compressor) of the system you could pump gases back into your charging bottle. Watch both sides and know the balance!

I hate to say it, but those charging cartridges are a bane on the refrigeration trade because they can end up putting gases in to the atmosphere needlessly.

First of all most people do not fix their leaks, except a person like you. Then, other things like overcharging happens. "If a little works, then more must be better idea over rides the untrained mind!"

If grossly overfilled there is a safety valve that "hopefully" will lift and blow off the charge without harm to anyone standing around! If not, there can go a hose, compressor valves or its clutch.

So yes, you have a problem with pressures! Or eight pounds of crap in a five pound bag!

One other thing it could mean is the suction valves are not closing and the compressor is bad and you are not overfilled. IF, Overfilled means the gas on one side cannot be pushed to the other side.

Do you get what I mean now?
Phil