When you take out the old valve stem and there is enough weight on the tire it will go flat and possibly break the seal around the rim, making it difficult to re-inflate the tire. It is best to put a jack under the cart first. You don’t need to actually jack up the cart, but rather have the jack support the weight of the cart while you swap out the valve stem and re-inflate the tire. Also, I would recommend using an air compressor to blow out the threads on the inside of the tire valve before putting in the new valve stem just to be sure there is no grit in there that would prevent a good seal.

There are also 'Low Pressure Gauges' available, quite easy to find at any Auto store.
I use it for my ATV 'up north' and Golf Cart here, both run low pressure tires.
Seems to give more accurate calibration in .5 PSI increments.

Thought my cart tire PSI was low. On my cart, dealer installed decorative metal pieces which covered the rubber portion of the valve stem. When I put gauge on on the valve it showed low PSI and when I tried to put air in with a bike pump and at a gas station, the metal piece prevented me from putting air into tire and getting a correct PSI on my gauge. Took the metal pieces off and everything OK, but can now see the valve stems.

Correct, I have the same gauge for my ATV's. The tire pressure is a big part of the suspension system. For trail riding and snow between 3 and 3.5 PSI is good as the softer tires absorb many of the bumps and grip better in the snow. For riding on groomed trails or roads about 5 PSI is better. One pound of PSI in the tires makes a big difference on a quad.

The owners manual will state the correct tire pressure. Our Street Rod requires 25 PSI.