I'm having strange brake issue with my 2003 F150 SuperCrew. I was stopped at a light with my foot pressing the brake pedal as usual. Suddenly, the truck started to move slowly forward. So pressed the pedal little harder, then the truck stopped. Then again it moved slowly forward, and I kept pressing the brake harder. This continued until the pedal was near the floor, and the truck eventually stopped moving. I can pump the brake pedal and the pressure comes back, but then slowly the pedal becomes softer towards the floor again.

I took the truck to the nearest tires and brake shop, and the mechanic showed me that the brake pads and rotors are fine, although one of the rotor I can see and feel some lightly grooved rings. He also said the fluid was ok too, and perhaps it could be the master cylinder but he didn't wanted start replacing parts not knowing for sure. So he recommended me to another mechanic shop which I will stop by tomorrow.

What should I suspect? Master cylinder? Air in line?

 

one or more of your brakes is sticking. I replaced the slides on my truck, but I will be changing out my calipers because it's still there. The truck stops fine, but the pedal still drifts to the floor. I put two new master cylinders in it and the pads and slides.
I talked to a guy that works at a local brake shop, he told me he's 99% I have a sticking caliper.
Good luck

 

I've googled and found 2 symptoms for sticking caliper:

"vehicle pulls more to one side when braking"
"brakes don't seem to release all the way after you let go of the pedal"

My truck does seem to have the 2nd symptom. But it does release all the way after little while. I have appointment today to have ball joints replaced. I'll mention that to him.

Greatly appreciate your help.

 

Brakes could need bleeding, another thing to look at is front brake hose deterioration. I just had to replace my front hoses. Have someone step on the brakes and watch the hoses for bulging.

 

Good point glc

I chased this problem by bleeding and re being the brakes, ran two quarts or more through the system so I know it's all fresh fluid. I also had a buddy run his snap on scanner on it to do the abs bleed process. When I finally buy new calipers for it I'll post a thread on the results.

 

Hoses or master cylinder if the system hasn't been opened up and air allowed to get inside. Or the level in the MC got low enough for air to get in the system. I admit I don't know much about ABS and don't know if that could cause the symptoms.
I don't see how sticking calipers could cause this issue.

 

Roadie, it's a real issue. The guy I spoke with had been in the business for 30 years and he's seen it allot on F150's. As he described it to me the caliper will stick and then move causing the pedal to drift slowly to the floor. It doesn't make sense to me either, but at least with my truck, I can hear the rear brakes dragging when I pull out of my drive in the morning or after its sat a few hours. I still have braking but after I come to a stop the pedal will slowly go to the floor. If I jam on them they're firm and grab until the abs kicks on. No fluid loss either which was the part that had me stumped.

 

After my experience with dragging brakes, I decided that from now on, I'm going to rebuild or replace the calipers every time I replace pads. When you take your foot off the brake pedal, the spring in the master cylinder pulls the piston back creating a low pressure in the brake system. Atmospheric pressure pushes the calipers in until the pressure equalizes. If you have a sticking caliper, the other caliper pistons will be pushed in more to make up for the one that won't move. I still can't visualize how a sticking caliper could cause the pedal to go to the floor. That is the symptom of air in the brake lines.

 

Agree with the last part, But also air in the system would just make the pedal spongy, it won't cause it to gradually sink. What OP has is a leaking master cylinder. Under pressure, brake fluid is slowly leaking past the pistons in the MC and that's why the pedal sinks and also why his brakes aren't holding. Fluid probably isn't leaking OUT of the MC so he's probably not losing any fluid and doesn't see any fluid on the outside but it is leaking past the MC pistons internally and back to the reservoir side of the MC.

 

Joe51, exactly what I thought with my truck.
Guess what, master cylinder number 3, still exactly the same problem!
I would buy one being the "bad out of the box" thing, but not three in a row. I bench bled the master. Then bled the lines at the ABS, by cracking them open with someone on the pedal. Followed by bleeding from the furthest to closest brake. I ran over a quart of fluid through the system with each new master cylinder.
My pedal is firm on pressing it and the brakes do slow me down as normal, but when I've come to a stop the pedal sinks, sometimes to the floor sometimes not. And FYI the ABS does function as normal, verified with a scanner (Snap-on) and by getting up speed on a gravel road and slamming on the brakes.

Now maybe he'll get lucky and it is a bad master, so I guess we'll have to wait and see if that is his cure.

I'm only able to speak from what I know is true to my situation and what I was told by an experienced brake specialist. Yeah it goes against everything I learned in school about braking systems, but at that time not many cars had 4 wheel discs and even fewer, if any, had ABS.

 

After reading about master cylinder, I've googled it, and one of the symptom is:

"Turn engine off and push brake pedal, it should be hard. If you keep pushing the pedal lightly and the pedal keeps sinking to the floor, then the master cylinder may be leaking internally."

I've tried it, and sure enough, the pedal slowly sinks to the floor.

I dropped my truck off to the mechanic this afternoon. I wrote down all that you guys mentioned and gave to that mechanic. Hopefully I'll get a good result tomorrow.

Thanks all.

 

Please let us know how it turns out.

 

Make sure he checks the front brake hoses. In fact, have them replaced on general principle. They are at the age where they deteriorate.

 

Sometimes it is really really hard to get all the air out especially when the MC is replaced. I bench bled mine until I was exhausted pushing in the piston against the spring before I got all the air out of the MC. I agree with replacing the hoses regardless with an old truck. I replaced all 5 of the hoses on my truck.

 

Got my truck back on Saturday. Attached the receipt.

Had the Master Cylinder replaced, and the brake pedal still slowly sinking toward the floor.

I noticed the brake pedal has gotten harder, it used to be softer, so now I have to be more aware of it to apply less pressure at the beginning. The pedal, though, seems to be sinking at slower pace than before.

Just got off the phone with the mechanic who worked on it. He said, he didn't noticed anything unusual with the caliper and the hose.

Since Master cylinder has been replaced and bled, now I may as well just go ahead and replace the caliper and the hose.

Also, I did noticed that driver side rotor wasn't as smooth as left side one. It did have some grooves of rings. Not sure how deep of the grooves are bad enough that needs to be replaced, but by feeling it, seems about 1/16" or less. Although the mechanic said it's ok, do you think the rotor should be replaced as well?