Somewhere between essential and nice-to-have -- I recommend it. Without it, your power-assisted brakes go away immediately if the engine dies.... with a functioning check valve in place, you'll get a couple good shots of assisted braking before the pedal gets unhappy. The valve itself hardly ever faills - I recommend recycling your existing valve by splicing it into the new hose. You can test your valve by sucking/blowing on each end. The vacuum booster side should blow freely, but not permit you to suck, and the engine side should suck and blow freely. There's an easy joke in there about how hard I suck. <img src="/forums/images/graemlins/pfft.gif" alt="" />

If you do not have that valve, you should be able to snag one from any u-pull-it junkyard.

Randii