An old style mechanical gauge will work, but needs care. Clearly, you have to avoid misplacing the connector and gettng a faceful of petrol spray. The ideal way is a dial gauge with a flexi connector that screws onto the Schrader valve. There's a thread showing this somewhere in these annals. However it's done, you need to see 50psi or more when your assistant turns on the ignition.
To manipulate the rear seat cushion, I made a special tool that makes it easy (easier anyway). See (scroll down to Post 3):
https://www.the75andztclub.co.uk/for....php?p=2531165
TC