Exactly! All that money you spent to get you to this point can go down the drain in a minute second. You could tell the captain sure lets go, but as soon as your airborne the valve could fail again. Yeah you might have planned for a fuel emergency, and can divert to an emergency landing field, but if something "unforseen" should happen the NTSB and the FAA will come a calling. Luckily I find that my maintenance background helps me out when a maintence issue arises and a go/no go decision needs to be made.
This is maintenace controls' foul up. Even if it's a 1.5 hour drive, there was probably sufficient time make the drive and fix the discrepency. The flight might have been delayed, but the repair would have been done properly by an A&P. Who's to say thay there might not have been something else wrong with the valve, or it might have been FUBAR, but only a mech would know upon inspection of the discrepency.