Hi,
Im not new to FTE i just forgot my old password and user name, and used this handy facebook login.

I used the search function on the forums and have any luck on what i was looking for.

I'm having a problem with my rear drum brakes locking up when I press the brake peddle. I've had this problem before, but ignored it because it wasn't too bad. It would just release after taking my foot off the brake peddle.

1998 F-150 XLT supercab 4x4 front discs, rear drums 4.6L v8 175k miles

I cleaned my master cylinder and bled all brake lines so they have fresh fluid. Now that my brakes work way better, my rears just lock up. I put new brake cylinders on, still locking up. I went to a junkyard today and replaced the proportional check valve, brakes still locking up.

Could I maybe have did something wrong when cleaning the master cylinder? I'm sure I put it back together correctly. Anything else i should look at? bad lines maybe?

I'm about to take it to a mechanic and see what they say but I thought I'd ask here before i fork out $$$.

Thank you,
Paul

 

Replace the rear flexible brake line, the one between the frame feed and the rear axle. It has a rubber line attached to a block; the two rear axle brake lines screw into it. The block is mounted to the rear axle on the driver's side, and is held in place by the vent tube.
Over time, the insides of the lines swell shut internally. The brake pedal pressure is enough to push fluid to the rear wheel cylinders and set the brakes, but the shoe springs cannot overcome the pressure to return the shoes and unlock the drum.
Usually this happens over time as the rubber lines get older, and your truck isn't that old, but if you have replaced all the other components, I would replace this before seeing a mechanic.
If this turns out to be it, then I would replace your 2 front lines as well, just to make sure all is well.

 

also check your park brake cables. if the shoes contact the drums just a little,your rears will lock on you. hope this helps.

 

Well I took off the rear brake line between the frame and rear axle, the line seemed fine. I blew air with my mouth both ways and there wasn't a problem. The parking brake light is ON but but I don't have the parking brake peddle down at all. I'll take the drum off and inspect ONE more time. (i've done it 4 times now) Thanks for the replys

 

ok so i ended up having to replace the master cylinder completly, even tho the old one showed no signs of wear. weird. Thanks for your input banjopicker and schtoke.

 

did replacing the master cylinder work. i have the same exact problem

 

I too have a '98 and am having the same problem, it starts around this time of year when it's cold and wet. If I touch the pedal the slightest bit, the rears lock up. It gets a bit better as the truck/brakes warm up, but the overall problem has been worse this year than in years past, so I've got to do something. I have been told that the locking up is caused by humidity/wet conditions between the shoes and drums, and I think I even read somewhere on a Ford TSB about replacing both the stock drums and shoes would fix the problem.

If anyone has a place to start, rather than just replacing stuff, that'd be much appreciated....ie I don't want to replace the master cylinder and the rear brakes if the problem only lies in 1. Any more comments would be appreciated.

 

My 98 started this a year ago about a week after I did rear brakes. I replaced the drums, shoes, and all hardware. They release fine, but lock up super easy. If I take the drums off they are as shiny as a mirror (sorta). I dump out the dust, ruff up drums with #100 grit and put back on. Problem solved....but only lasts about 10 days. I'm convinced it is the budget shoes I bought. I really should get some other shoes...go with the better ones...have the newer drums turned...and that should do it...I think. Any thoughts on this?

 

What does master cylinder have to do with rear brake lockup? Someone please explain.

Master cylinder's job is to generate pressure in each brake branch, equal pressure if functioning normally. Each branch has (one front + one rear). If MC caused lockup of one branch, it would make sense. But the rear lockup?

What causes unequal activation between front and back is metering valve and proportioning valve. Are they not more suspicious?

 

I suspect some of these trucks have RABS ? Two possible problem areas, the speed sensor on top of the differential housing, or the ECU module under the dash. Between the two, they control hydraulic pressure to the rear brakes. A fault in either may cause rear wheel lock up. Seems like there should be a code stored regarding this, but they may not be picked up by some scan tools.

No ABS light on ? If I remember correctly, yellow brake warning lights are electrical problems, red lights are hydraulic( like low pressure or fluid).

Dave

 

One thing else I thought of. Although I haven't worked on one in a long time, brake proportioning valves sometimes have to be reset if there was a major problem in either front or rear brakes. In fact it's been so long since I had to do this, I may not be entirely accurate. I think, to reset the valve, if you have one, you turn the key to "on", depress the brake pedal all the way, then turn the key off before you release the brakes. I'm sure someone will correct me if I have this backwards or something.

Dave

 

is the proportioning valve electronic or mechanical? where exactly is the rabs sensor? I don't have any check abs light comming on.

 

Inspect the lines and parking brake cables and all the hardware.

If you find nothing unusual, replace the rear shoes and drums (or have the drums resurfaced).

There is a TSB on rear brake lock up being cause by moisture sensitive linings.

I had the same problem on my 97, changed the shoes and drums and the problem was solved for about a year...it's starting up again now but it's too cold out to deal with it now.

My inspection is due in March anyway, so I'll just deal with it until it gets warmer.
I got mid-grade linings from autozone. I'll try the top of the line ones next time.

 

All 4 tires are ABS, not just the rear. I highly doubt it is a sensor because it only does it when it gets cold/humid/wet outside. As soon as the brakes warm up, they're better. And in the summertime, there is no problem with the rears locking at all. I think I'm going to lean towards getting new/premium shoes and drums.

 

I've used ceramic pads for practically as long as they've been available. Much less problems all around, especially when towing a load, which I do almost all the time. I get 2-3 times the life vs. semi-metallic linings, though the cost is around $55.00 to $60.00 per fronts or rears.

Podaso- your problem could still be a harness issue with water getting in a crack or connection. I've not encountered brake shoes/pads sticking because of moisture, probably because I always use ceramic linings maybe 90% of the time ( unless they have a problem with cost). Can't go wrong just upgrading either way, either it fixes the problem with locking up, or at least no brake replacement for a long time.

Dave