will it tell you if all the valves are properly opening and closing if all cylinders have the same compression?

 

Compression test tells you how much pressure is being built up inside the cylinders. All the cylinders should be within a few percent of each other. If all are equally low then the engine is wearing evenly. How much pressure is made depends on a lot of factors including valve grind.

If a problem is suspected then a leakdown test should be performed to determine the cause of the lower pressure.

Excess air escaping into the crankcase normally means worn rings but could also indicate a damaged piston, cylinder or head gasket. Air escaping out the intake indicates worn intake valves. Air escaping out the exhaust indicates worn exhaust valves. Bubbles in the coolant normally means a bad head gasket but could also be a cracked head or block.

To know if the valves are opening and closing properly, the valve covers need to be removed and the rockers visually inspected while the engine is being cranked over. A rocker barely moving will indicate a worn cam lobe.