[turbored]

run a T then on one leg put a 1 way check valve and on the other end put a open/close valve. re-tee and run to the boost gauge. When you close the valve, the 1 way valve will keep boost from returning, and will peak hold. Open the valve, and you have your normal boost gauge!

Gauge --<==valve on one side check valve on other=>--boost source

- is vacuum line
< is tee
= is two vacuum lines in parallel