Total hydraulic braking failure possible?


#1

Two years ago, I bought a '94 F150. The day I bought it, I stopped at a garage to inquire about renting a tow dolly (they were also a U-Haul franchise.) Up to that point, the truck drove and braked great.

They didn’t have any dollies, so I left. Upon stepping on the brake prior to engine start, the pedal went to the floor with NO resistance. Upon getting out, I noticed a large and spreading leak along the left frame rail, near the firewall (around where the proportioning valve is). I went in, said, “Oh, by the way…” and they fixed it properly to this day.

My question: are any “single point failures” possible in the hydraulic system beyond the master cylinder? I realize a bad master, or the pedal decoupling from the master…but I thought once you got past that point, you had two fully independent hydraulic circuits.

Did I encounter a “perfect storm” failure of two systems simultaneously, was one circuit previously inop, or what?


#2

There are too many possibilities to sort through, and a truck of that age is prone to a rust caused problem in the brake system, that allows the fluid to expel unexpectedly. Don.t be too surprised by other rust related issues, such as the oil pan rusting, or an exhaust manifold leak because the heads of the bolts have rusted off, been there with my 93 f150!


#3

All points that have connections, all places that have seals, all metal lines (subject to corrosion), and all flex lines (subject to age-related deterioration of the elastomer) are potential sudden failure points. That pretty much covers the entire brake system except the rubber pad on the pedal.

Question: what was the cause of your sudden leak? If it was corrosion, you may want to have the entire system gone over. If one fitting corroded, there may be others on their way out. That leak happening in a parking lot may have been a fortuitous warning.


#4

The rear brakes are likely drums and if they were in need of adjustment the loss of pressure on the front might leave no braking response from the rear.


#5

Thank you.

Yes, I had them “go over” the system when they fixed the lines to/from the proportioning valve. The rest of the system looks good, not least because I have begun a regemin of waste-oil chassis rustproofing.

Nearly 40,000 miles since and no leaks. Both front and rear brake circuits operate.

I ask because I HAVE lost the front circuit on a truck once when the flex line to the LF caliper blew. High pucker factor, to be sure, but I retained the rear brakes. I thought brakes were engineered to make a total failure “impossible,” right up until it happened to me!


#6

20 years ago, a united jet had an exploded engine in flight. It was a relatively minor incident except the debris cut all independent hydraulics. The probability of all hydraulics failing in flight was thought to be one in a billion, but freak accidents do happen. Investigators found that all hydraulic lines were routed through a 10 inch space in some places and one piece of debris could have cut all of them at once.

If you look under your car, I’d bet that you won’t see brake lines on either side of the car-it’s cheaper to secure them close together with only one fastener. Straddle road debris at your own risk.

There is no perfectly fail safe in many engineering design. But we often accept the compromise between economy and safety. Under the right circumstances, total hydraulic brake failure is survivable. You still have a mechanical parking brake. Those united pilots had no back up system at all and yet they put it down with almost 2/3 survived.


#7

I too am puzzled by the OP’s post. Since 1967, all vehicles sold in the US have a backup (where most or all implement it with a split master cylinder/dual braking system).

So if the fluid leak the OP discovered caused a complete loss of the braking system, then some part of his system had to already be defective.


#8

Chunkyazian
1:48AM edited 4:13AM

Yeah, and it still saved a lot of the passengers when it crashed in Sioux City. (??) The transcripts of the cockpit were breath-taking. Pilot asked to be connected to support, and asked them what they had for total loss of hydraulics. A very long stunned silence!

They tried many times on the simulators to control the plane like the crew did, and virtually no one could do it.


#9

"They tried many times on the simulators to control the plane like the crew did, and virtually no one could do it. "

It’s amazing what a massive shot of adrenaline can do.


#10

“I thought brakes were engineered to make a total failure “impossible,” right up until it happened to me!”

No amount of engineering can completely eliminate the potential for failure. In the design process they go through a hazard and risk assessment. A score is given to each risk factor based on probability of occurance and severity of the failure. Mitigations are put in place to reduce the risk level to an acceptable level. When they had single circuit brake systems, the risk factor was deemed unacceptable and therefore they designed the later systems with two “independent” circuits for front and back. It is far better but doesn’t eliminate all risk. There are still single point failures that can cause total lack of braking as you already pointed out. But the later design was still better then what preceeded it and is a balance between acceptable risk and economics.

BTW, the two circuits come together physically at the proportioning valve. The hydraulics are separated and serve to move a pintle to one side or the other based on which side has more pressure applied. If that block corrodes, you could lose all braking there. Highly unlikely as they are fairly robust (they used to be brass) construction but possible.

However, it could have been a two point failure like Rod Knox mentioned.


#11

Surely most of you have bled your brakes. When I have been the one in the car, the brake pedal seems to go to the floor when the guy on the caliper opens the bleed valve. I remember thinking, “Why am I not feeling the other brake system come on?” I can only hope that if one system fails, at least pumping the pedal will bring the other system into action.


#12

The dual circuit design is not intended to provide prolonged braking in the event of a catastrophic failure on one side. If you notice, the pedal goes down to the floor albeit slowly. During this time, there should continue to be braking effect on the undisturbed side, enough to slow the car and hopefully prevent a crash.


#13

Having had a flex brake line blow on me, I’d describe losing the front circuit as suddenly VERY spongy brakes, with reduced effect. Still stops the car, though.

Also, the brake master reservoir is set up like an overfilled ice cube tray: it leaks down to the level where the partition is, then the unaffected circuit has a small fluid reserve that won’t leak out of the failed circuit. One reason I don’t think my newly-acquired truck was down one circuit was because the reservoir was full.


#14

I think @TwinTurbo has it right. A failed prop valve would cause a single point of failure if it was the type that included a switch to tell you one circuit has failed. That pintle shuttles front or back to the side that lost pressure to hit a switch. If the fitting that holds it in fails, you lose the whole system.

Another ugly single-pont failure mode. Front wheel drive vehicles are split RF-LR and LF-RR instead of front-back because the rear brakes won’t provide enough braking meet the federal spec FMVSS 135. If you forget to release your parking brake completely,or it is frozen or rusted, the dragging rear brakes will boil the brake fluid and you will lose BOTH circuits and have NO brakes. Even the parking brake will not stop the car if it acts on the regular shoes or pads. Clean underwear won’t matter, first you’ll SAY it, then you’ll DO it!


#15

One reason I don’t think my newly-acquired truck was down one circuit was because the reservoir was full.

Right, but the condition mentioned (I think it was Rod Knox but can’t see now due to being on page 2) of poorly adjusted rear brakes would mean they would go the full travel of the pedal but not make significant contact with the drum would explain the effect of no brakes even though the MC was full…


#16

Clean underwear won’t matter, first you’ll SAY it, then you’ll DO it!

Consider that line stolen!! That’s funny right there…

Excellent point about the FWD set ups!


#17

I believe the pilots in the Sioux City flight controlled the plane using engine if I remember right. Quite a feat just like Sully landing in the river. I like my pilots gray and prefer ones that have been shot at before. When you have to improvise and the book doesn’t cover it, experience in the Air Force counts.

Same as a car, anything can happen and you simply have to keep your cool and quickly run through the options for the least damaging. No brakes? Steer, lower gear, brush on the side of the road, pick a hill, choose which side of the car you want to crash, and so on. The options are endless. Not the time to cover your eyes and panic like in the movies. In my old Buick wagon, I managed to drive through Minneapolis traffic and 50 miles home with a bum master cylinder, which was quickly repaired.


#18

On a car with a dual master cylinder, if you blow one side the brake pedal can hit the floor. You are than supposed to let the pedal up and step on it again and you should have pressure in the opposite diagonal side.
It used to be in the owners manuals, I don’t know if it still is.