Interesting thing happened today. While stopped at a few lights today, I noticed the pedal of my truck slowly migrated to the floor as I maintained pressure on it. The truck never moved, but the pedal definitely went down. Braking seemed a bit sluggish but the truck stopped fine overall. Fluid checked out fine, no leaks noted on any brake lines, no fluid on rims or under truck. I'm mechanically inclined, so these were the first things i checked as I knew where to look.

I'm thinking the brake hoses were swelling somehow. Didn't feel like air in the lines, as it cleared up after letting the truck sit for a while. It's been real hot lately and I did a lot of urban driving today, so I'm thinking this may have contributed to it. Anybody have this happen, or thought about steel braided brake lines for these things?

 

i think this would require a trip to the dealer. I have read a couple of post on here where ppl had that happen when they went to apply the brakes and they didn't have any would really recomend that you a least let them check it out

 

i think this would require a trip to the dealer. I have read a couple of post on here where ppl had that happen when they went to apply the brakes and they didn't have any would really recomend that you a least let them check it out

 

This is a classic symptom of the master cylinder going bad. Will get worse till you can not stop, if you do not rebuild or replace it in the near future.

 

One or both of your master cylinder's primary cups has sprung a leak (internally). Take it in.

Personally I don't like the brakes on these trucks. The brakes have poor intial bite and the pedal has a wooden feel. Braided lines will improve feel and should offer better initial bite—or so braided lines do on motorcycles.

 

My brother had the brake fade condition transam396 mentioned. But as you mention, the brakes returned to normal after it sat. Dealer couldn't find anything so they turned the rotors (???). They made a complaint with the NHTSA. Havent had a problem since. I would definately take it to the dealer.

 

The same thing happend to me and I took it back to the dealer. Dealer said they checked the brakes and could not find anything wrong. The truck only had 800 miles on it when I took it in . Brakes are still the same , VERY SOFT and the pedal goes down when sitting still and applying pressure to the pedal. Dealer is supposed to be making an appointment with the local factory rep about the chrome on the front bumper . Will talk to the factory rep when I take it in for the bumper problem . Been two weeks and I still haven't heard from the dealer about that appointment - guess I had better call them back .

 

It was fine today but i'm going to bring it in. and i'm not going to accept any shizza that it's the rotors. Dealer's need to understand that some of us know alot about cars and that a simple unnecessary fix like turning rotors doesn't solve all the problems.

One thing, if it's the master cylinder, would i see any fluid leaks or is it more a true internal leak, where fluid would go back and forth between the cups. This would account for poor pressure and no external fluid leaks, or fluid level decrease. Thanks for the replies.

 

Fluid would leak past one or both of the piston cups and into the transfer port that goes to the reservoir. You wouldn't be able to see a leak.

 

Awesome diagram eric. That's exactly what i pictured when typing my last post. Would this still fail completely in the near future, being that no fluid is actually being lost? I can see a drastically diminished pressure in the lines, but you should technically still be able to stop. I hope. Would having too much fluid in the master cylinder cause this? it looks like it may be slightly overfilled.

Also, how could I prove this to Ford? I doubt they'll simply take my shade-tree mechanic word for it, and I can't rightfully demand that they replace the master cylinder without a diagnosis, can I? thanks again.

 

Normally the only reasons master cylinders fail the way we've been talking is either the cup on the front of the pistons has become hard and brittle from age, the wrong brake fluid was added that goofed up the cups, machining problems left scaring that tore the cups causing one or both cups to leak, schmeggy (its a technical term) built up in the master cylinder's bore and the cups cannot seal, or alternatively, the cups have worn the master cylinder's bore so the cups cannot seal.

Those are the only things that can go wrong in the master that would cause the complaint we're talking about and not leak brake fliud.

How you test it is to recreate the conditions that were present when your brake pedal started moving.

Personally, from what I've gathered from this thread, i suspect a manufacturing defeat or somebody put in the wrong kind of brake fluid.

You can eliminate the later by flushing the brake system but if the wrong type of brake fliud was installed at some point that damaged some rubber parts, new brake fluid will not fix the problem.

Any competant ASE tech will quickly get to the bottom of this problem as long as it is repeatable.

 

Just returned a 2005 rental with the same problem, seems common. Our 2005 shows no signs of it... might be a recall in the future???

 

VERY INTERESTING THING TODAY!!!

I was installing my bug deflector today and decided to poke around the engine afterwards. I noticed brake fluid on the outside of the master cylinder, from the cap to the midline seam. *good time to mention how flimsy this part is, what ever happened to the old metal master cylinders with the metal caps and that metal latching clip that you had to force over the cap?*.

Anyway, at first i thought it was a seam leak, but it seemed to be coming from the cap. wiped her down, took it for a spin with hard braking, including pressing the pedal as far as possible while at a dead stop. Checked again, and the fluid was definitely coming from the cap. I drained quite a bit of fluid out to bring it below the max line (paper towel absorbing trick- used 2 full towels) and recapped it. It seemed okay for now, but i will be driving it in Boston tomorrow, that should stir it up.

Would a leaky cap be a problem here, rather than the internals? or was it just too full?

 

The reservior is supposed to have ambiant air pressure applied to the brake fluid within. If there was a vacuum, it is possible (although I don't think likely) that this would compromise braking quality. And if there was pressure in the reservior I don't see how this would cause your problem outside of causing the brakes to drag and heat the fluid up further because this can happen only under the most extreme conditions. That said, it is possible to get the brakes so hot that the brake fluid boils inside the lines which will cause bad to no brakes and a low pedal. I've experienced this problem once on my race bike when I started with too much brake fluid on my master cylinder's resverior and as the fluid expanded from heat, the brakes started dragging compounding the problem.

 

It has so seemed I've ran across the same issue this past sat. (july 23-05), it started upon leaving my parents house and made a stop back to my work (about 15miles away) and made a visual inspection in the parking lot ...I did not notice any fluid leakage around the callipers or master cyl ...and rsv'oir.

...But did stumble across a theory.. In my line of work..I deal with a lot of different kind of valves anywhere between pnuematic, hydraulic to aquatic... from 1- 45,000 psi. The valve that powers our abs units ....could be referred to as a "proportional dump-valve" allowing the fluid to travel in a particular direction (or hose in this case) to help allow the abs to properly respond to a variable within the overall stopping procedure. It seems as tho since the abs we have is electronically controlled...it could have been that the valve was simply "stuck-open". When it is stuck open I have the feeling that it is truley making the truck stop as normal it is just all the lines are probably all getting the pretty close to the same amount of fluid; thus probably making most of all the fluid pass out of the cyl. giving us the spongy feel. I came across this b/c when I started the truck back up...it must have restored the abs valve back to the original condition as a result... left with the tighter feeling pedal for the rest of my trip.