Warmed up my old F150 in the driveway today & got in for a test drive. Haven't driven it for months.
Tested the brake pedal before I put it into gear. The pedal sagged to the floor with very little resistance (but some resistance).
I looked at the master cylinder, plenty of fluid. I got back in and pumped the brake several times, same sagging to the floor. I also noticed a puff of smoke coming from underneath my side of the cab, pretty sure the fluid was spraying on part of my exhaust system.
I noticed a puddle under the cab almost directly under the driver's seat.
My ABS warning light has been on for a year - no noted braking problem during that time. My red brake warning light did NOT come on.
I'm done working on the truck for the day & will crawl out & inspect the damage later.
Question - this is only the 2nd time in my 1,000,000 or so miles of driving that I've lost brake pedal pressure. The first time was in my then-10-year-old F250 6.9L diesel -- the brake pedal suddenly sagged to the floor and I had no effective braking power whatsoever. I was only going 1 mph & was able to stop with the emergency brake. One of the lines to the left front brake had blown & got replaced for not a lot of money. The rest of the lines lasted over 20 years with no further problems.
This 2nd time I got the quick sag to the floor and an obvious acute brake line leak. Since I was parked in my driveway, I did not even put the F150 into gear. The pedal indicated to me that I had no effective hydraulic brakes.
I have heard statements like this many times before "since the front and rear brakes are on different lines, drivers should be able to stop even if one of the lines ruptures."
This doesn't seem to be true for me - X 2. The first time, only a brake line segment to the left front wheel had burst, and replacing it fully restored my hydraulic pressure. Since I haven't got underneath to look yet, I don't know what line is faulty. Does the complete lack of brakes suggest something besides a simple brake line has failed on my 14 year old truck

 

Probably one line has a leak and needs to be replaced. Looking at a couple hundred dollar repair to fix based on my experience in similar situations and paying to fix...

 

About 2 weeks ago the main line that is tucked in to the frame rail rusted in half. I just happened to notice a puddle under my truck when in the driveway. I lost all braking when that line rusted in half. Just by luck it happened when parked that it broke in half. It is the line that connects right above the fuel filter and runs all the way to the back and then tees off to both wheels. It was a Sunday, Ford sells that line for about $80.00, but I had to get it fixed. Turns out it was standard 3/16ths line, but the back fitting was a standard 3/8ths double flare, but the front fitting was a 5/16ths bubble flare.....thanks Ford. I bought a brass adapter that was a 5/16ths bobble flare to 3/8ths double flare, then bought 2 lines and a union and hooked them up. It sure would have been a lot easier to just buy the overpriced Ford line and be done with it.

 

thanks for the tip. I've been doing a lot of online reading about the type of repair you referred to, and those DIY brake line methods look to be very time-consuming. I'm not getting any younger!

 

artfd... back on the original question, I agree with you. I thought the whole idea of having a two section master brake was that one section could go dry, and you would still have brakes on the other end........ I thought that was a safety item designed into the truck ? If you fail the rear line and the front brakes don't work, how did the fluid get out of that section ? Or is the ABS valve doing something odd ?

 

My first experience with a ruptured brake line was on a 1983 F250 - which did not come with ABS. Same experience with that one - no hydraulic brake action at all.

 

Component specifics[edit]

(For typical light duty automotive braking systems)

In a four-wheel car, the FMVSS Standard 105, 1976; requires the master cylinder is divided internally into two sections, each of which pressurizes a separate hydraulic circuit. Each section supplies pressure to one circuit. The combination is known as a dual master cylinder. Passenger vehicles typically have either a front/rear split brake system or a diagonal split brake system (the master cylinder in a motorcycle or scooter may only pressurize a single unit, which will be the front brake).
A front/rear split system uses one master cylinder section to pressurize the front caliper pistons and the other section to pressurize the rear caliper pistons. A split circuit braking system is now required by law in most countries for safety reasons; if one circuit fails, the other circuit can stop the vehicle.

 

I disconnected the brake line to the master cylinder, the one that connects to the rear brakes - I could tell which was which without a diagram since the fluid leaking out of the ruptured rear line drained the rear compartment of the master cylinder, very easy to see after a pint of DOT3 leaked out onto my driveway.
The socket on a mystery device connected to the master cylinder mates with a 7/16"-24 inverted flare. I found an adapter with that threading on one end, that mates with a 3/8"-24 inverted flare plug nut that I already had. Snugged up the larger adapter and then snugged up the plug into the adapter, to completely seal off the rear part of the master cylinder.
Started the motor & tested the brake pedal -- with the rear port plugged it suddenly became much firmer. After I pumped the brake pedal about 10 times the pedal felt almost normal, and I again could use the to brake (the front 2 only) to stop my F150 at low speeds, enough to maneuver it in my driveway again.
Supposedly I have one of these "safe" dual master cylinder systems, but it sure didn't seem to give me any braking ability at the slowest speeds.
I was also pleased to note the master cylinder itself seems normal, and my remaining brake lines seemed to be working!

 

The brake line running to the rear on my 03 rotted out, right by the distribution block on the rear axle. I lock all braking in that case. The brake pedal would drop to the floor with little effort. I guess you have to either plug the master cylinder or the brake line fitting by the fuel filter, to get pressure back.

 

I disconnected the empty brake line to the rear, at the master cylinder, and plugged that odd looking adapter Ford had screwed into the cylinder with a both a 7/16"-24x3/8"-24 adapter and into that a standard 3/8"-24 inverted flare plug available in many auto parts store. With the rear system blocked off, I again had some usable braking action in the front tires. Mostly I wanted to be sure the master cylinder was working normally and the front brake system wasn't also defective.

 

There is a fitting under the drivers side and another at the rear axle. My line rusted in between them the day before I had to leave for some training. I replaced them in the Advance Auto parking lot in about 45 minutes, including bled (had my wife with me), and was still clean enough to go out for dinner. It required one adapter piece, as Ford uses some weird size on one end and then just a straight section of line. You can pull off the fittings at either end and bring with you, then just measure how long you need. The harder method is bulk line and to double flare it with new fittings, which I would've done had I been home.

But, your front and rear work on separate sections in your master cylinder. This was easily distinguishable on earlier cars as there was a separation that was visible. Today with the plastic reservoirs it's not so cut and dry. A lot of times there is no separation in the reservoir, but there will still be fluid in the master cylinder to be compressed. The pedal will sink far, but should still apply the brakes to either the front or rear. For example, that's exactly what happened with me as I still had to drive a couple miles to get to the parts store to replace my line.