Safety question
If auto motor stalls then power brakes are not functional because there is no vacuum.
Question. Is there available, as an add on, an auxiliary battery operated vacuum pump that can made to kick in if the engine stalls?
There is an even better way. Install a large cannister to hold the vacuum (boy does that sound strange). It can even be installed in the trunk. Remember that it has to have a check valve. Some vehicles have these cans. I know pumps are available for hot rods that don’t want to use engine vacuum.
You could always use the e-brake. 
The vacuum booster is a large reservoir. But, you’ll have enough power to apply the brakes once, and maybe twice. If not, you have a bad check valve.
Good thought. The real problem is when the power steering pump quits. A lot of people are not prepared for the large amount of resistance in the steering wheel when the power steering quits. But for brakes, there’s always the e-brake.
Some time when you are going 25 mph try stopping your car in a hurry with just the parking brake. Then think about what good it would be at 65 mph.
Some cars have a vacuum tank that will work for a short time.