What you need to know to survive catastrophic brake failure

I’ve nearly made a career out of touting “The Big Seven” reasons a vintage car is likely to leave you waiting for a tow on the side of the road—fuel (specifically, the lack of it), ignition, cooling system, charging system, belts, ball joints, and clutch hydraulics. I’ve often had folks ask me if clutch hydraulics and ball joints are on the list, why isn’t brake failure?

It’s a fair question. After all, similar to ball joint failure, brake failure can make you lose control of the car. And, if clutch hydraulics can sideline you, brake hydraulics must surely be more important, right?

I usually give a two-part answer. First, brake performance tends to degrade over time rather than fail suddenly. Second, if brake failure happens, nearly every car built since 1968 has a tandem master cylinder with separate internal pistons feeding brake lines for the front and rear brakes, so you shouldn’t lose all of your braking. Thus, catastrophic brake failure (no brakes—zero, zip, nada) is pretty rare.

Standard tandem master cylinder arrangement in a post-1968 car, with each half independently fed by one side of a divided brake fluid reservoir.
Standard tandem master cylinder arrangement in a post-1968 car, with each half independently fed by one side of a divided brake fluid reservoir. Rob Siegel

So you can imagine how surprised I was when the brakes essentially failed on my RV. After my heart rate finally dropped back near normal, I contemplated revising the list.

Let’s talk for a moment about the mechanics of brake failure on a street car (I’ll ignore the issues of high-temperature-related failure that can occur on the track). Unlike normal wear-and-tear brake maintenance, brake failure nearly always traces back to three items—metal lines, the master cylinder, or the booster.

Booster failure is easiest to diagnose. If the brake pedal becomes difficult to press, the power assist booster probably isn’t working. Nearly all cars use a booster with a vacuum-actuated diaphragm located between the master cylinder and the firewall. With age, the diaphragm can perforate or tear. This can be diagnosed by blowing into or sucking on its vacuum hose. Some cars instead use a canister to accumulate hydraulic pressure generated by the power steering pump. In either case, if the booster fails, your car still has all its braking, but your leg has to provide the assist, pushing the pedal much harder than it normally does. On a small light car, this may be doable (and, indeed, on Lotus Europas owners routinely remove malfunctioning brake boosters), but it’s usually not safe to drive larger heavier cars without the power assist working.

The opposite and far more dangerous situation is where the pedal rapidly becomes soft and then sinks to the floor. There are two distinct situations—master cylinder failure and loss of fluid. To know which you have, you need to stop and check the level in the brake fluid reservoir. If the reservoir is full and the pedal feels pretty good during normal braking but sinks slowly to the floor when you stand on it, the odds are strong that the master cylinder is failing and one or more of the seals inside it is going bad. You may be able to pump the pedal and restore pressure, but don’t be fooled. If it happened once, it can and probably will happen again. The master cylinder should be replaced as soon as possible and certainly before driving again.

If the fluid level is below its specified level, then you have a leak. It is possible for wheel cylinders on drum brake systems to leak and make the bottoms of the drums damp, and for the master cylinder to leak out the seal in the back and into the brake booster. Both of these can cause asymptomatic slow fluid loss with no fluid reaching the ground. They are, however, unlikely to be the cause of a suddenly soft pedal and loss of braking.

If the pedal suddenly goes soft, or to the floor, and if the fluid level is dramatically down—or worse, empty—you have a serious leak that must be repaired immediately. Crouching down and looking under the car will usually reveal the source of the leak. You’ll likely see either a plainly-obvious drip or a spreading puddle under one of the tires. You may need to refill the reservoir with brake fluid and pump the pedal to catch it in the act and pinpoint the source.

Forensically, what will usually happen is that a rusty brake line will start to weep fluid, then perforate and actively leak, and finally, when the pedal is mashed hard, erupt into a gusher. This creates a four-stage effect. In the first, it’s asymptomatic; there may be virtually no fluid loss, but if you inspect the brake lines you may find one of them is damp. In the second, the pedal feels soft due to fluid being pushed out the leak, but the brakes are still functional because fluid is also reaching the wheels. The pedal won’t “pump up” and get firmer, but as long as there’s fluid in the reservoir, the brakes will still slow the car. In the third, the line ruptures, and the pedal becomes much softer due to the decreased fluid pressure. Shortly after, the reservoir runs out of fluid and the brake pedal goes right to the floor, with only residual braking left at the bottom of the pedal travel due to fluid in the half of the master cylinder unaffected by the leak.

This brings us to the “if you pop a brake line in a car with a tandem master cylinder, you still have half the braking” fallacy. It’s simply not true. You should have some braking, and some is way better than none, but it sure as hell won’t be half, and the bigger and heavier the vehicle, the less the braking will be. I’m here to tell you, in the two vehicles I’ve ruptured brake lines while driving—a 4500-pound Suburban and a 7000-pound RV—the result is that it renders the vehicle clearly unsafe to drive at normal speeds on public roads.

With that technical backdrop, here’s what happened. My wife and I have a 1996 Winnebago Rialta. For an RV, it’s tiny, a 21-foot Volkswagen Eurovan with a Winnebago camper body on the back, but it is a 7000-pound vehicle. It has discs in front and drums in back, so the braking under the best of circumstances is far from stellar. As we were driving down Route 3 on the way to Cape Cod over Labor Day weekend (our anniversary), the braking transitioned from mediocre to poor, with pumping of the pedal required to safely manage stopping distance. Then, when I tried to slow down to take the next exit, the pedal went to the floor, with only a bit of braking left at the bottom of its travel.

Brake line failure
The Rialta is a small RV, but 7000 pounds is 7000 pounds. Rob Siegel

I assumed that I’d popped a brake line. We pulled carefully into the parking lot of a grocery store. I checked the fluid reservoir and found it empty, but when I crouched to look under the vehicle, I didn’t see any obvious dripping or puddling of fluid. The Rialta is low enough that you can’t easily skooch beneath it, and the 95-degree temperature and black asphalt weren’t exactly inviting me to try.

So, what was the play here? I thought if I had a bottle of brake fluid, I could catch it in the act—fill the reservoir, pound on the pedal, see where it streamed out—and judge whether I could do a field repair. Except I hadn’t brought any fluid with me. I checked the grocery store and the nearby CVS and came up empty. My phone showed a Midas Muffler shop was around the corner, but when we crept there in the rig, we found it closed. The nearest auto parts store was about six miles away on highly-trafficked roads.

These are the moments when you rely on your gut. My wife and I badly wanted to get to the campground. We were only about five miles away, and there was even a way there that didn’t require getting back on highly-trafficked Route 3. But with the size and weight of the Rialta, it simply wasn’t safe to drive it any faster than a careful crawl or any further than absolutely necessary. So I decided not to.

I was about to call for a tow when a most unlikely chain of events led to our salvation. I saw a NAPA delivery vehicle turn into the next driveway. I carefully followed it and asked the driver if he had any brake fluid. He curtly said no and walked away. I then went into the shop at which NAPA was making a delivery. A man inside said that he had no brake fluid either, but told he me that there was a repair shop practically across the street.

And thus we found our way to Rebel Motor Works in Plymouth, Massachusetts. I explained the situation to the shop manager, saying that I’d be grateful if he’d sell me a bottle of brake fluid so I could suss things out, and that, depending on what I found, I might want them to fix the rig. He provided the brake fluid, but explained apologetically that due to the extreme heat, they were closing early. He said that if need be, they could look at the vehicle in the morning.

The Rialta in front of Rebel Motor Works in Plymouth, Massachusetts. That ’59 AMC Ambassador wagon was pretty cool too.
The Rialta in front of Rebel Motor Works in Plymouth, Massachusetts. That ’59 AMC Ambassador wagon was pretty cool too. Rob Siegel

I filled up the reservoir and pumped the pedal repeatedly. To my surprise, the level didn’t drop, and pressure in the pedal didn’t return. This was starting to feel more like a failed master cylinder than a popped brake line. But eventually, the reservoir swallowed the fluid, I began to feel pressure at the pedal, and I saw fluid streaming out from near the right rear wheel. I drove the Rialta around the parking lot, and it had a working, although soft, pedal again.

brake line failure puddle on ground
Gotcha Rob Siegel

My wife and I talked about whether this changed anything. Should we make a run for the campground? If not, we needed to stay somewhere for the evening. Get a hotel? Call one of our kids up in Boston and ask them to pick us up? Bit of a pain, right?

There are times when you need to remind yourself that logistics are merely that, and that they pale in significance to potential loss of life, limb, or property. Not only were we and the RV somewhere safe, the vehicle was directly in front of a shop that could likely fix it in the morning. If we left, we’d have to get the RV back there the next day. Besides, isn’t knowingly driving a vehicle with barely-functional brakes almost the legal definition of negligence? I find that imagining someone cutting me off and me crashing into them rapidly disabuses me of risky behavior.

My wife remembered she had a friend who lived just one town away. Two texts and a phone call later, we had a place to stay for the night. We left the rig in front of the shop and took an Uber to our evening’s accommodations. First thing the following morning, the folks at Rebel Motor Works looked at the Rialta. The problem was, as I strongly suspected, a popped brake line. One day and $233 later, we were safely on our way with full braking. Including two Uber rides, total cost was around $300. Relatively cheap.

Still, I did a post-mortem about what I might have done differently. Years ago when the brake line popped on my 2000 Suburban (a vehicle so renowned for bursting brake lines that there was an NHTSA investigation into the matter), we were on Nantucket, which has fewer cars, smaller roads, and lower speeds than where the Rialta gave us trouble. I creeped the Suburban about a mile to our rental house, measured the length of the line I needed, rode my bike to a nearby NAPA, bought a brake line, and installed it. But I had no way to jack up the truck, pull off the wheels, and bleed the brakes. I wound up driving the ’Burb very slowly, under cover of darkness, several miles to a shop to finish the repair. Although I was convinced at the time I wasn’t doing anything unsafe, I doubt I’d do it again.

If the goal is to be able to replace a popped brake line on the road, you’d need a roll of copper-nickel tubing, a tubing cutter, the correct end fittings, a flaring tool, a brake bleeder, plus a floor jack and jack stands. I’ve only traveled with this assortment once, when I bought my decade-dead BMW 2002tii—1000 miles from home—and was prepared to resurrect it where it sat.

I don’t routinely travel with all this. But I have. brake failure safety kit
I don’t routinely travel with all this. But I have. Rob Siegel

There is an old-school technique of pinching off a leaking brake line with a vise grip. In retrospect, I might have tried this on the Rialta, but, as I said, it was beastly hot out, and I couldn’t easily crawl under it. Having wound up, by sheer chance, in front of a repair shop, any choice other than paying to get it fixed seemed silly, messy, hot, or dangerous—or all four.

I’m not revising “The Big Seven” list just yet, but the take-away is this: Inspect your metal brake lines. If there’s a damp rusty spot that looks like coffee cake, you might be seeing a leak in its early stages. And, above all, remember that tows are cheap compared with accidents. Stay safe.


Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel magazine for 30 years. His new book, Just Needs a Recharge: The Hack MechanicGuide to Vintage Air Conditioning, is now available on Amazon. You can also order a personally inscribed copy here.

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