Playing with a bad reservoir is playing with fire. An old, bad reservoir might be your only problem.

Try running the car with the cap off - have the coolant an inch or so below normal. This is because you do not want to take the cap off when hot.

Run the engine until it reaches normal operating temperature.

Wear a full face shield.

Have someone rev the engine while you look in the reservoir.

If the pressure is caused by a bad head gasket, you may see bubbles or hear a noise. This would reveal a pretty bad head gasket.

Our '95 854 base blew a head gasket at 98K.

My first car, a '61 SAAB went gurk, gurk, gurk just before it blew a head gasket and would not start.

I think the noise was the pressure cap vibrating as it relieved pressure.

There are many other tests that require a bit more effort, such as a compression test, checking the torque of the head bolts, etc

You should act quickly because filling your engine with coolant can get Very expensive.

Engines can just get old. I had a Suburban that blew a head gasket and when I took it apart, the first few bolts on the ends of the heads were only finger tight