Was backing through my mother-in-law's garage door entirely my fault?

Dear Car Talk

Dear Car Talk | Oct 01, 1997

Dear Tom and Ray:

Recently, I drove my mother-in-law's car over to my house to take a look at her
muffler. I backed her car into my driveway that slopes a bit downhill to the
garage. When I got out, I could see I needed to back it up another foot or so
to get clear of the front walk. I got back in the car and figured I would not
bother to dig my keys out of my pocket again, and just let it roll backwards
using the brakes.

The car is an '84 VW Quantum. I've studied Quantum mechanics, have an official
factory repair manual for our '85 Quantum, and remembered that the owner's
guide mentions that even with the engine off, the power brakes will still
function, but more pedal pressure is required. I put my foot on the brake,
shifted into Neutral, released the foot brake, put my right hand on the
steering wheel and my left hand on the slightly opened door -- looking
backwards to judge my position. The foot brake held the car in place with no

I eased just a bit off the foot brake to permit the car to roll a few feet,
then I stepped on the brake to stop and found nothing. I pressed extremely
hard, but the car continued to accelerate. I pumped the brake, fearing the
master cylinder had leaked down ... .nothing. By this time, I was still barely
rolling, but I had started with the rear bumper only six feet from the garage
door. Now I was closing in on it fast!

I rammed the shift lever into Reverse, then grabbed at the emergency-brake
handle, still holding the door with my left hand on the window sill as the car
crashed through the garage door and finally halted when the driver's door hit
the garage-door framing. The car got totaled out by the insurer. My
mother-in-law bought a new VW Jetta. I repaired and repainted our garage, and
yesterday, a new roll-up door was installed. But I'm wondering, what happened?
-- John

TOM: Oh, John, I could tell by the first sentence of your letter that things
weren't going to turn out well.

RAY: Normally, you CAN stop a car with the foot brake, even if it has power
brakes and the engine is off. That's because there's a reserve of vacuum that
the power-brake system stores just for this purpose.

TOM: So my guess is that something was indeed wrong with your mother-in-law's
car. And it was probably a leaking power-brake booster. The power-brake booster
has a diaphragm in it that's supposed to hold a "vacuum reserve." That way,
even with the engine off, there's still enough vacuum in the booster to "power"
the brakes for another stop or two.

RAY: But if that diaphragm had a small hole in it, it wouldn't hold the vacuum
reserve for very long. You wouldn't notice this during normal driving, because
the engine is constantly creating new vacuum. But once you stop, air would leak
in, and you'd lose that vacuum. Then you WOULDN'T have that cushion that's
mentioned in the owner's manual, and it would take superhuman strength to stop
the car. It can be done, but it's much harder than you might imagine.

TOM: So I'm sure you've learned a very valuable lesson here, John: Don't ever
work on your mother-in-law's car.

* * *

TOM: Hey, do you think you're taking good care of your car? Are you sure?

RAY: If you're like many of our customers, you may be ruining your car without
even knowing it. Yes, even you! Find out how. Send for your copy of our
informative pamphlet, "Ten Ways You May Be Ruining Your Car Without Even
Knowing It!"

TOM: Send $3 and a stamped (55 cents), self-addressed No.10 envelope to Ruin
No.1, PO Box 6420, Riverton, NJ 08077-6420.

Get the Car Talk Newsletter