1997 F150 w/ 130K miles, 15K on brakes. Recently, it started feeling like it was in 4 wheel drive when it wasn't. I pulled over and could smell that the front brakes were hot. I found the front left caliper had a problem and replaced both front calipers and front brake lines hoses. Improved, but did not fix. I found that the pressure was not being fully released from the front brake lines...left and right seem equally affected. Replaced the proportioning valve...no difference. Replaced the master cylinder...no difference. Pedal is a little soft, but not bad. Cannot remove back right bleeder so I could have a little air still in the lines. Pedal rod distance seems fine.

Any ideas?

 

We had a similar problem in the last shop I was at on a Jeep Cherokee. The tech did everything he could possibly do to find out that at some point someone added brake fluid and got a piece of that protective film stuck in the bottom of the master cylinder. I doubt its this just thought it was funny.

Do you have ABS on the truck? I would soak that R/R bleeder in PB blaster until it breaks loose. Then I would bleed the whole truck including the master cylinder. Did you install the new master cylinder or did a shop? Well you might want to try and adjust the arm going from the brake pedal to the master cylinder. This might be causing it to not fully disengage the brakes. Since there are a lot of new parts in I would double check that arm (forgot the name for it) since it’s a likely cause for this.

 

Check the brake booster, this is from the Ford manual:

Brake Booster

1.Check the hydraulic brake system for leaks or insufficient fluid.
With the transmission (7003) in NEUTRAL, stop the engine (6007) and apply the parking brake control (2780). Apply the brake pedal several times to exhaust all vacuum in the system.

2.Apply the brake pedal (2455) and hold it in the applied position. Start the engine. If the vacuum system is operating, the brake pedal will tend to move downward under constant foot pressure. If no motion is felt, the power brake booster (2005) system is not functioning. Continue the test with the following steps.

3.Remove the vacuum booster hose (2A047) from check valve connection. Manifold vacuum must be available at the check valve end of the vacuum booster hose with the engine at idle speed and the transmission in NEUTRAL. If the manifold vacuum is available to the power brake booster, connect the vacuum booster hose to the power brake booster check valve and repeat Steps 2 and 3 above.

4.Check and if no downward movement of the brake pedal is felt, install a new power brake booster.

5.Operate the engine a minimum of 10 seconds at fast idle. Stop the engine, and let the vehicle stand for 10 minutes. Then apply the brake pedal with approximately 89 N (20 lbs) of force. The brake pedal feel should be the same as that noted with the engine operating. If the brake pedal feels hard (no power assist), install a new check valve and retest. If the brake pedal feels spongy, bleed the hydraulic system to remove air. For additional information, refer to Bleeding—System in this section.

 

To answer a few questions at least... and provide some hopefully useful input...

This makes sense - trucks use a front/rear split on the master cylinder. The primary circuit feeds the front wheels and the secondary circuit feeds the rears.

no surprise here either... the proportioning valve is used to reduce pressure to the rear brakes so it would not be causing trapped pressure at the front brakes.

This is what concerns me most... trapped pressure makes the brake pedal feel very stiff. To say your pedal is a little soft (and the fact that replacing the MC did not solve the issue) makes me think that the issue is neither the booster nor the master cylinder. If the booster output rod (the thing that bxstang2000 was referring to) were too long, then you would be trapping pressure in both circuits and the pedal would feel very stiff. If it were a Master cylinder issue then replacing the MC would have fixed it.

SO - my best guess is there is something in one of your brake lines that is preventing the pressure from fully releasing. Since you replaced your flex lines (and since its happening at both front wheels) I'm going to guess that its somewhere in the main primary line (the closest line to the booster) between the MC and where the line splits off to each front wheel. If it were in one of the sections after the split, only one wheel would trap pressure.

Take a look at the line first to see if you can see any dents or crimps in the line (like maybe something flew up and hit it?) If not, I'd pull the line off and blow some air through it (or just replace it... once its off its worth it to buy new ones) I'm thinking there might be some big piece of contamination that got in there and is working like its own proportioning valve... letting the majority of the pressure run back to the MC but then holding a small amount of pressure so the brakes drag.

good luck!

PS - I'm in Paris for work this week so I might not get on here much to reply to any further questions... feel free to post them, but it might take a couple days for me to see them.

 

Great information on how to diagnose a booster issue, but not applicable to his problem. If he had a vacuum leak or a bad check valve it would only make the pedal harder... and even reduce the chance of trapping pressure. When you apply vacuum to these boosters it actually draws the front shell in by a small amount (like 0.05 mm) which extends the output rod out by that same amount (with reference to the front shell... the MC is bolted to the front shell).

So if it were a vacuum leak the pedal would not feel soft.

 

I'm not a mechanic by trade, and only offer this because it happened to me, My brake line to one of my calipers was twisted, and caused a dragging pad. Is it possible that both flex brake lines are twisted from their replacement?

 

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I too have the front brake problem. My brakes will work fine for several months then start dragging (front only). I have replaced the pads and pins and again after several months same thing. Dealer said everything was ok when they checked it out. Just did the same thing today again. I have owned the truck since new (1999 f150 2wd) and it has very low mileage. I am beginning to think there is a problem blocking atmospheric pressure to the backside of the vaccuum assist diaphragm. Is that even possible? I've never heard anyone even consider that so perhaps it is a stupid idea? My thinking is if the valve wasn't closing to block that when the pedal was released you would still have atmospheric pressure applying the brakes. The reason I suspect this is everytime I have this problem it solves itself by letting the truck sit for a while. It is really frustrating because no mechanic can ever duplicate it. Any ideas?

 

I just had the front left caliper replaced about 2 weeks ago (somewhere around that time frame anyway) and I've noticed it sticking again. I'm taking it in tonight to be looked at. I'll report back with the mechanics results.

 

We replaced a brake hose and the line running to it for the left front caliper and everything is fully operational now. My brand new caliper got so hot though it looks like it's about 10 years old. I've gotta do the rest of my brake lines too, they're rusted all to hell.

Old hose and line
http://i177.photobucket.com/albums/w...rock12/007.jpg

New hose and line
http://i177.photobucket.com/albums/w...g?t=1276995707