Mechanical temp gauge is pretty tough. If it leaks, however it is toast. Common fail points are the firewall(chafing). Back of gauge in the cluster(bend and pinch). And crud in the head preventing water contact. Bend fractures and kinks will defeat even a new one. Good to run your fingers the length of the "pipe" from "bulb"(the bit in the head) to the back of the gauge. Should be smooth and not kinked. Look at the joint at the back of the gauge that joint has a sleeve to support the tube/pipe and might be where it is kinked or leaking. I have seen them fail because they are too close or touch the exhaust manifold. Common error if you don't know how they work. I hope that helps some.

Steve H