I have had terifying experiances with vehicles. This is a true story and I hope no one ever has to go through. Thank God I was not injured. Apparently the old 70s and 80s Dodge Tradesman vans I have sheet metal weakness around the now it's points for the steering box. I'm not sure if mine was failing or not however one day they decided to rip out of the frame. Mostly lost steering but the tires shimmied violently causing the van to rock back and forth very very quickly. There was a recall against the Tradesman van so I sent it into an automotive body shop to reinforce the frame. They park the van out back and I went to start it. I could not start the van so I thought well maybe the starter relay was not working properly. The vehicle was in park and I went to jumper the starter relay. The engine started the engine RPMs accelerated and the van took off in reverse. I absolutely freaked out ran alongside and went to put my hand in the window to jam the shifter in the park. It did make a difference. It didn't it rammed into my car causing damage to the back of my car and continue to push my vehicle and it ruined it another vehicle and the van died. I was so upset I used about every four letter word in the profanity book. It was also pushing me against the fence. I think my adrenaline was flowing a thousand miles. All the body shop mechanics came racing outside to see what was happening. I hate cars I hate pickles when I step on a SkyTrain in Vancouver and so at peaceIt's not really unlike any other braking system that is 'boosted' (Whether Vacuum or Hydraulic). When you lose 'boost/assist' your braking ability is compromised.
With our system (Toyota) when/if you have booster pump or accumulator failure....you still have some residual pressure to work with. BUT.....it requires the driver to pull over right away.
When boost/assist is lost...it happens quickly, meaning you don't gradually lose brake pedal pressure which would be a more tactile way of knowing something was wrong.
Instead....when the pressure is gone, its gone! The pedal will drop dramatically toward the floor. It is this action that prompts folks to describe it as 'losing all brakes' which is not technically true.
**Provided your master cylinder is not leaking**.
IF your master cylinder is leaking AND you lose boost....you are definitely in a bad situation.
Losing boost alone is not a great thing to happen either since any braking you have will be purely manual and at that point the vehicle has the mechanical advantage.
You have to really stand on the brake pedal to get any results and frankly....for some folks it just wouldn't be possible to bring the vehicle to a quick stop (if that were needed).
Bottom line here is: DO NOT ignore any warning lights/tones associated with the brake system.