This usally happens on the first or second brake useage in the morning or when I leave work. It is random as it doesn't do it all the time.

If I just touch the brake pedal the truck feels like I shoved the pedal through the floor board it stops so hard. What could cause this? It doesn't do it all the time just maybe once a week or so. If I just ever so slightly touch the brake the front tires will lock while only going 15 MPH or so. Most of the time the pedal is mushy. What could be causing this?

 

Maybe a master cylinder or brake booster is the culprit. I had this problem with my back brakes, it ended up being the RABS pump.

 

I had a problem similar to what you describe and for almost over a year, I still haven't been able to figure it out. Although I will say that mine usually happens when it's been raining heavily throughout the night. There have been many instances where they lock up so hard when I just tap the brakes, the steering wheel could have been my breakfast. I sure hope someone here has been able to figure it out.

 

Got a 1992 F-150 w/175K and mine do the same thing practically every morning. Seems they do it when it rains or when there is dew and condensation. About 3-5 times a month it sounds to the people at the gas station near my house like I've narrowly avoided a Ford-crunching devastating accident when in actuality I'm just trying to stop at the exit.

Your post made me look for an answer and here's what I came up with:

http://www.cartalk.com/content/colum.../April/01.html

These guys have a great column and I trust their answers from having read them for years.

I'm sure I'm not leaking any fluid and I have the rear ABS, so from now on, I'm just going to ride the brakes while backing out of the driveway in order to dry them off.

Hope this helps shed some light on it and thanks for reminding me.

-Johnny-

 

My rear brakes are fine. I don't think I've ever locked them up. Its just the front brakes which are disc do it and it doesn't always happen after it rains or is moist outside. I thought it was the cold causing this about a month ago when it first started but this warm snap we have killed that theory.

 

Interesting...my truck does this too. First couple of brake applications in the morning will nearly, or sometimes, lock up the right front wheel. The rears never lock up and the ABS works great.

 

My 96 F150 does the same ... In the morning when I barely touch the pedal , it actually applies more pressure itself and the pedal leaves my foot .... you can WATCH it go down . Of course the truck halts , then I have to get my foot under it to pull the pedal back and release it . By afternoon when its a 100 degrees here in Texas , it works fine . Im stumped .......

 

I've pretty much concluded that it has to do with the moisture and rust on the rotors.

However I have a new but random problem. Some times when I hit the brakes I here a pop and there is al ittle bit of resistance before I here that pop.

What could be going wrong?

 

If i can i would like to bring this thread back from the dead. I recently got (another) F150, this time a beautiful 1988 F150 300 I6 4x4 rabs. Same thing happens with this truck. touch the pedal actually when the brakes are cold, and whammo, the back brakes will lock up. Ever since i fixed the back e brake cables. I'm starting to think now that i read that post above that there's probably grease or something on the brake shoe's. Can i clean this off with brake clean? Just blast it on the drum and directly on the shoes? or should i just do the drum, i dont want to wreck the pads. Or just not worry about it for a while, or drive around a little bit with the ebrake on?

 

I used to have a 1989 f250 with ABS and had a brake job done. The shoes had life left in them, or so it appeared, but they had been soaked with brake fluid so the rear wheels locked up whenever hitting the brakes, especially during in-town stop and go driving. Very dangerous, especially on ice/snow. Had the shoes replaced and the problem went away.

Also, a few years prior to this had the same problem but it turned out to be a abs sensor on the rear axle housing, if I remember correctly.

 

The most common cause I have ever faound for locking up front brakes is the front brake lines. In fact mine have gone bad probably three times that I can remember. The last time it was one of the fronts causing a lockup and a couple of near miss accidents and I just replaced all three hoses. It cured it real quick.

Brake hoses almost never get diagnosed right or the first go around at the dealers. In watching everyone else at the ones I've worked at, they just throw master cylinders and full brake jobs at them first. Lockups almost always end up being the hoses though.

I'm one of the few that actually change my fluid once a year but I do have a lot of miles on my truck. Still brake fluid is really hard on rubber hoses. It's only a matter of time before they collapse inside.

Try cracking one bleeder at a time and having someone push the pedal. Engine off. If nothing or very ittle comes out then it's the hose. SOmetimes but not often the hose will let a good flow going into the caliper but not back the other way. I've really pulled my hair out on a couple that did that.

 

Now my ABS light is turning on. When i start the truck it will turn on, and stay on, then after i drive the truck maybe 100 yards or so it will turn off. It usually does this after i have my ebrake on. After i drive it for a while the brakes will start working right. The brakes locking up and the light dont seem to be related though. The light will go off first then a couple miles later the brakes will work, i think when they get warm. I should just replace the rear brakes. New drums/shoes/wheel cylinders/hardware kit. I noticed that it looked like the wheel cylinders were leaking too when i fixed my ebrake cables. I would have normally replaced them but i was in the middle of a parking lot fixing my cable.

 

Wow, i just figured out my problem and i cant believe it was a no brainer. In the above post i mentioned the wheel cylinders looked like they were leaking. Well, now i drove the truck and the abs light never went out. so i popped the hood and the master cyl was almost empty. The wheel cylinders are leaking on the brake drum/shoe causing my brake lockup, and the low fluid levels are causing the Abs light. Just have to replace them wheel cylinders now!

 

I've been getting front wheel lockups on my '92 since about 40K. I just changed out the factory pads last month, with 137K on them. They probably had another 20K or more left. I put on new pads, but the rotors stayed on as is. The rear drums went back on without doing anything to the rear shoes. (I too have RABS-II)

I think what all of us are experiencing is a matter of rust forming between the metallic pads and the rotors, especially on a glazed rotor. It's only when it rains that I have the issue, or after washing, etc, whatever gets the rotors wet when they're not hot. I left my truck parked outside for about 6 months while driving my old T-Bird to work, and the fronts locked up completely with rust between the pads and rotors. The right front wouldn't budge until I pulled the wheel, and rapped it a couple times with a large rubber mallet. A C-clamp to compress the calipers just a little will work too.

I'm thinking about pulling the rotors and having them turned because the new pads are squealing a bit, which is apparently normal when installing new pads on rotors that haven't been turned. I also bled everything, not because of air, but because of some really nasty looking fluid (the fluid that was put in it at the assembly plant in 1992).

What I've learned to do with my manual, is feather the brakes when backing out my drive in the morning (they only lock going forward). Then, try to get enough speed up the first time and brake with the clutch in to warm the rotors and clean the rust spots off. The next time I use the brakes, I typically feel a slight pulsing until the rust polishes off completely. Then, I'm good to go until I leave it sitting in damp weather again. Since I have the M5OD, I typically gear down (all the way to 1st sometimes) and use the brakes very little (as indicated by 137K on the original factory pads). Most of the time I can get out of my drive, down my street, and onto the next busier road without ever touching the brakes. This lets me get some speed up. Since the pads never really back off the rotors completely, they'll polish up just fine before I have to use them.

If the RABS warning is coming on, it may very well be a loose connection at the VSS. Watch your speedometer very closely for any fluctuations at low speed. Better yet, go under the truck, and unplug/clean/replug the connector at the VSS. The same signal that drives the ABS system also drives your speedometer. NOTHING has been done to the brakes on this truck since I've owned it (November 1992), except the pad replacement and fluid replacement I did last month. My RABS light came on a couple times, but I just unplugged and reconnected the VSS sensor, and it hasn't done it again since.

 

I believe in Old_Paint's post. I have the same problems but mine might only happen once a month. It must do with moisture and condition of all brake components combined. We all seem to be unlucky enough to have the right combination. Mine happens only on the rotors and like I said isn't frequent. Great topic.