The problem occurs when you hit bumps while braking, or if you get on the brakes to hard. It seems like the brakes lose pressure entirely and the pedal goes to the floor. It kicks back in when the ground smooths out, or if you are braking hard it slowly grabs again. At first i thought it was sliding, but it definitely is not. The truck appeared to be equipped with abs but the previous owner seems to have cut the abs wire/sensor at the hub. Another thought i had is maybe the brakes need to be bled and have fresh fluid put in them, I have not tried this yet(Plan to this weekend along with a dozen other things), but does anyone have any insight?

Its rather scary when it happens. but i didnt think much of it until i nearly rear ended someone..with my younger brother in the car. Now i need to fix this asap.

 

The bleeding and fresh fluid would be my first attempt at fixing it as well. The fluid could be very old and not good anymore.

 

I'll take a home run swing here thinking you have an early model truck with RABS.

Sounds more like normal operation of a RABS equipped vehicle, one where a previous owner installed from hubs for a vehicle with four wheel ABS and cut off the unused leads. Is there a yellow ABS lit up all the time or just when the truck is first started?

 

Just when the truck is first started


What is Rabs?

Edit: I feel really stupid, it makes sense to me now..Rear ABS

I am looking at the options to help with this, it appears that i can do a bypass with a brake proportioning valve, is there a way to keep it from tripping the light if i did that? maybe as simple as a resistor or as complex as an entire circuit (im an electronics major if it is electronic i can probably figure it out)

 

Brake fluid does not go "bad" after a period of time, at least not in any way that would cause the system to behave the way you describe.

Is your master cylinder reservoir full?

It would really help to know what kind of vehicle we're talking about.

 

I can't help you with bypassing the electronics side.

I was never a fan of RABS as its a passive system, not active like 4WABS. All it can do is lock the hydraulics and dump, then return to whatever pressure you're applying. It does the best that it can, but as you stated when the rear wheels come off the ground in reaction to a bump, the rear brakes lock out until the wheels get back up to speed and the rear pressure is released. Slow in operation compared to 4WABS.

Any change to the system gets you to own the liability in case you get into a severe accident where an accident reconstructionist gets involved, and the plaintifs lawyer starts to get wide eyes (I used to do accident reconstruction). So you need to consider that before you decide to reengineer the brakes.

While not dealing with the bump issue, installing a single circuit manually adjustable prop valve in series with the RABS can reduce the rear lockup condition, permitting the owner to adjust the rear brake output to the load being carried in the vehicle. Or an older same size master cylinder with the screw-in prop valve in the rear outlet port can also be installed. Those are a few ways for an owner to address the issue but you will be still taking responsibility for the changes.

I used to go into a long soap box statement on why not to do this but that would get drowned out by the masses.

The other way to somewhat address this is by changing brake pads so you have a higher friction material in the front and lower friction in the rear. Going with a front material like the Hawk LTS or some of the Poterfield offerings will do this. Selecting the rear material is a little harder, and would need to have an edge code rating of EE or EF (the production line pads were EF). The F hot rating on those was low on the range to meet that F rating. Depending on how the pads were replaced in the past could have already altered the brake balance towards the rear and making this situation worse then it came stock.

 

I was working on a couple otger (mainly glow plugs) things and decided to check and see if it was rabs. i did not see the module where i thought it was supposed to be.(drivers side frame rail under the master cylinder?) is there a way to check and see if its a rabs vehicle any other way?

 

You could look in the front wheel wells to see if there are connections for the ABS for the front wheels?

 

You might want to check to make sure that all of your brake lines are clear from pinch points like shocks.

 

It was 15-18 years ago for this vehicle .... if your front ABS sensors were not connected to a 4WABS then the yellow ABS light would be on full time due to the controller not getting the signals.

We had all the various Ford pickups and vans in the fleet at that time.

Edit: This shows RABS to be under the drivers battery tray where the 4WABS is normally as well.

 

I do not ghave any pictures, but i can get them tomorrow. I dont know if i mentioned it, but the truck is a 2000 f250 4wd

 

Edited above.

If I remember correctly it was 2001 when they only used 4WABS.

 

I will look into that tomorrow. could it be as simple as fresh fluid, bleeding all the lines and that module?

 

Excellent advice. You may have to pull the fender well liner back to check. They should come in from the back side of the fender well.

Excellent advice as well.

Not disagreeing. However, there is another situation in which this can happen.
If one or more of the front wheel sensors has been disconnected or is not working, the truck can exhibit the same characteristics. A work around to hide this used to be to unplug one of the fuses that controls the ABS. IIRC there are three with one being a low amp. If that one is unplugged the speedometer and cruise will not work. If one of the other remaining fuses is unplugged the 4WABS is disabled completely and you have standard brakes.
I think the ABS light goes out when any one or all of them are pulled.

So, check all fuses to make sure none are blown and or removed.

Again, this is accurate unless one of the fuses is either blown our pulled that influences the ABS directly or indirectly.

Check your owners manual for the referenced fuses and locations.

 

If there was a lack of power from any of the fuses, thereby an inoperative RABS, he would be complaining about rear wheel skidding, not a drop in the pedal due to the ABS trying to prevent a skid.