It is my understanding, from reading my Haynes manual, that my '85 F250 has separate hydraulic lines for the front and rear brakes. Shouldn't that mean that if my front brakes fail, like if my left brake hose blows out and sprays brake fluid everywhere, that the rear brakes should still work?

 

Yes, that's exactly what it means, the separate front & rear circuits allow for that. Typically, rear-wheel-drive vehicles are split front/rear and those with front-wheel-drive use a diagonal system.

This technology has been around for quite a while (mid-60s?) and was mandated by the FMVSS in 1976.

Federal Motor Vehicle Safety Standards and Regulations

 

They still work but barely. I guess it's better than nothing but you definitely know you have a problem.

 

Thanks guys. When the front hose failed it felt like there was nothing when I pressed the brake. Maybe there would have been something from the back brakes but I didn't notice it. Luckily I was just parking in the driveway when it happened...
I am concerned that there are no working brakes in the back. I just got this truck and I'm an amateur at this. But I guess had there been no working brakes in the back, then I would have been able to tell when I was driving it? And I guess if the rear lines were disconnected there wouldn't be brake fluid in the rear reservoir of the master cylinder?

 

If the rear brakes didn't have fluid, or weren't working for any other reason, the warning light on the dashboard would come on - which should have also come on when you front brake hose failed. Despite that, yes, investigate your rear brakes; perhaps the self-adjusters no longer work and they're way out of adjustment, there are lots of things that could be going on.

The front brakes do the vast majority - something like 80% - of the stopping of the vehicle, it's a good thing this happened in your driveway and not on the public streets & highways.

Go to the auto parts store and get yourself a Haynes or Chilton's manual, and maybe pick up an Owner's Manual off of eBay; all of this information will get you going and will help you learn what you're getting yourself involved in.

 

BTW, the large reservoir in the master cylinder is for the front brakes, the smaller one for the rear.

 

If you push it all the way to the floor you usually have a little bit from the back brakes. It will slow you down and you can keep it under control, but it's definitely not going to stop like it normally does.

If you had problem with the rear brakes you would have had the same situation, a very low pedal and not very good braking until you are almost at the floor with the pedal.

P.S. The self adjusters on the rear brakes of these trucks usually don't work very well, they either work too good and people have to disconnect them, or they don't work at all because of corrosion in the adjuster. More than likely you just need to manually adjust the rear brakes to help them out.

 

1967: First year for dual master cylinders in Passenger Cars, Bronco's, Econolines and F100/350's.

 

All bow, The Great Dummy has spoken.

 

I want to say the government got involved and was the reason for this on all cars. Along with mandatory seat belts, little marker lights on the sides of the car, collapsible steering columns, and many other things I can't think of.

 

Oh but of course! Leave everything to our government and we'll be fine.

Just did some quick Googling, I remember a guy named Ralph Nader and something about seat belts and the Corvair... well, it seems he's a lawyer & consumer activist who wrote a book accusing the car manufacturers of ignoring safety, it was apparently a best-seller in the mid-60s:

https://en.wikipedia.org/wiki/Unsafe_at_Any_Speed

 

Our trucks have a very unusual quirk with the rear brakes that can essentially knock out both systems with only a single failure. I've got to wonder how this ever met the requirement for dual independent brake systems.

I'm a bit fuzzy on the details, but know this is covered in the factory service manual. Drum brakes actuate more slowly than disc brakes, I think because of how far they retract when released. To get all four brakes to come on at the same time, the front brakes (discs) are inhibited until pressure builds in the lines to the rear brakes (drums). Raise your hand if you can think of as glaring problem with that!

That's right, if pressure can't build up to the rear brakes for whatever reason (low fluid, leaking line, etc.), the front brakes won't do much of anything, if at all. The brake pedal becomes a foot rest, and little more. Your only option then is to proceed to the crash site.

I still wonder how such a design ever got approved.

Edit: Found the reference, from section 12-01-2

"The metering valve delays the effectiveness of the front disc brake until the rear brakes shoes are against the drums."

 

Read the rest of the sentence. "DURING HEAVY BRAKE APPLICATIONS" (READ PANIC STOPS) the proportioning valve reduces pressure to the rear brakes to reduce lock up.

The front brakes will still get full pressure even if you lose the rears.
The delay is there so if you stab the brakes the rears do not immediately lock up. Once the rear brakes show some pressure it can be proportioned correctly to try to prevent lock up..you STILL get full front braking regardless.
This is SOP in all disk drum brakes on trucks pre ABS or with out that stupid load sensing brake proportioner.

 

Sorry to change the subject, somewhat, but on to my next question...
I am replacing the brake lines in the front. The fitting connected to the differential valve is stripped, even though I am using a flare wrench. Now what the F am I supposed to do to get the line off?

 

Visegrips...

Don't laugh, I always us visegrips to break brake line fittings lose then use a wrench to take it off in 20 plus years I have been of doing this I have NEVER stripped a brake line fitting.

Be sure to have a newer pair with good sharp teeth in the jaws and you will be good to go.