Please diagnose my brake problem

I have a 1998 Dakota, with rear wheel antilocks. This truck runs very well, has full power, and never gives me problems. When driving normally, the brakes perform very well. My problem occurs when I have to brake hard to avoid the dolt that pulled out in front of my and hit the brakes. The first time this happened, a minivan pulled out and hit her brakes. I pushed hard on the brakes, and had plenty of room to stop, but the truck just slowed down. I stood on the brake, but the truck did not skid, it just continued to slow, and I could not skid, which would have stopped me long before I got near her. It was dry pavement. I missed her by an inch. This scared me, so I took the truck into a brake shop. They could find nothing wrong. However, I determined that one caliper may may sticking, and I replaced that caliper, and both front rotors. The truck braked evenly before, and after bleeding was braking just as I would expect, even and without excessive pedal force. The next time someone pulled in front of me, I did skid. So, I thought I had tagged it. Last night some the same thing happened as before. It was cold and wet, and somebody pulled in front of me and stopped. I had plenty of room to stop, but just slowed down again, and I was standing on the brake.

I have done some research on hard pedal, but the only thing I can guess is that the check valve on the booster is not working all the time. I would not suspect the master or the booster because the brakes work so well in nornal conditions. What are your thoughts, please?

It is unlikely to be the check valve causing failure of the brake booster as it traps the vacuum in the booster if the engine should stop producing vacuum, i.e. engine dies, allowing a couple of assisted brakings before the reserve is used up. You would probably notice the lack of boost if the engine vacuum was not getting to the booster, i.e. blocked or broken vacuum hose. It could be a fault in the booster that is not allowing ambient air to get to the pressure side of the diaphragm or a broken diaphragm allowing air to bypass directly to vacuum side. A broken diaphragm would show up as continuous hiss while the brake pedal was applied.

The only other thing would be the ABS but I cannot see how rear ABS would materially affect the stopping power of the vehicle because most of the braking power is in the front disc brakes. The rear ABS is engineered to keep the truck from swapping ends when there isn’t a lot of weight in the back.

Post back to us when you find the solution to this problem. I am curious about what it could be.

How/where does the ambient air get into the booster? Could this be a clogged valve somewhere???

Maybe this is a matter of the anti-lock brakes working properly to prevent the wheels from locking up and therefore preventing the truck from going into a skid. This is what anti-lock brakes are designed to do. The idea is that they allow you to steer during a panic stop because the wheels keep turning. Locked wheels are going to move in the direction of the skid and not necessarily where you want to go. The downside of anti-lock brakes, as you are discovering, is that a panic stop requires more distance to bring the vehicle to a complete stop.

Some pick ups have a load sensor in the frame around the bed area which senses your load and tells the anti-lock computer how much lock up is needed for the back tires during heavy breaking. Or, your anti-lock computer is not getting the signal. I’d have those two things checked.
But, lets be realistic here. It sounds like you hard-break a lot. Are you one of those drivers who can’t see beyond the hood of the car, never lets a merge happen, or follows too closely just before making a lane change? Switch to decafe.

I tested the antilock back brakes on ice, just to see if they worked, and they pulse the pedal. This never happens when I have to stop quick. And I don’t have front antilocks!

Nope. Cannot agree with you. I drive slower than most and get passed all the time. I try to keep adaquate distance between me and those in front of me. The fact is that I have plenty of room to stop, but it’s like the truck just isn’t trying. And these incedents happened over a very long range of time. It’s not that they happened this week. But when it happened the first time, it got my attention, and it’s been in the back of my mind ever since.
I have no load sensor. And wouldn’t my antilock light come on if there was a sensor problem?

I’m relatively new to this forum so I cannot claim to know any of the “personalities” that regularly contribute; but it does seem that there are a lot of rude and disrespectful comments among the participants. I don’t know, Beefy Norm, if you and shansen are friends and you are just engaging in a little friendly poking here, but from a newcomer’s perspective, I don’t enjoy the insults.

Norm is a idiot. The regular posters know it, so just ignore him.

did you do a brake job just before this started happening?

i had this same problem with my daewoo. the brakes #$%^&ed. big time.

i found that using the cheap brake pads, and using cheap rotors diminished the brake performance alot.

after fighting with it, and getting frustrated i switched, using the premium pads, and good (made in usa ) rotors helped immensely.

I did after the first occurance. New front pads, rear shoes, and front rotors, made in usa. Next I’m going to crawl around underneath and check for pinched tubing. That’s all I can think of…

Atmospheric enters the booster under the dash inside the cab. There is a filter surrounding the rod going from the pedal to the booster inside the sleeve the rod goes into. When the pedal is pressed the air is valved in to the pressure side of the diaphragm. If you have the engine off, you can usually hear the gulp of air being admitted when you press the pedal, sort of a ‘shush’ sound. I have never experienced this filter getting clogged because the cab is usually not a dusty enviornment.

After thinking on this problem, I thought of one possibility that might be occuring. It would depend on whether the engine computer uses a Manifold Absolute Pressure sensor and not a Mass Air Flow sensor. If there is air bypassing the diaphragm that air would cause the engine to power up making braking less effective on a MAP system. With a MAF sensor, the engine would probably stumble and/or die as the computer will not compensate with fuel for the additional air. The engine would be driving the wheels while you are trying to stop. The next time this problem appears, bump the transmission into neutral if it is an automatic. You don’t mention whether this is an automatic or manual transmission. With a manual transmission, you probably would have the clutch released so the engine would only rev up if the problem exists and I would be off base. The fact that the ABS is not operating under severe braking on dry roads could be because the engine is driving them and keeping them from slipping as would normal occur when the bed is unloaded.

Just an idea that I have only heard as a possibility.

It’s an auto. I will check this out, thank you!

Looks like it has a MAP sensor. So, what is bad, the sensor or the booster? If the sensor was bad, would the check engine light come on? BTW, it does not…

check your brake hoses for internal collapse,they may be acting like a check valve on hard brake apply.also metering valve(hold off) and the propotioning valve(limiting)

How do you check the hoses for internal collapse?

buy a gauge and look at the pressure,at the calipers,and see what it reads,you can install an inlne gauge if you so choose for the rears.quick check is apply hard braking in your bay and crack the lines loose at the hose above the suspect area. 2 man op.

It’s also possible that a flex hose is ballooning, preventing full pressure from reaching the brakes. Start the engine, have someone stomp on the brake pedal, and look at them.

I missed the part about a hard pedal (I assume from your mention of it that you experienced a hard pedal each time you’ve had the braking problem.) A ballooning hose would cause a soft pedal, so that’s out.

Make sure the vacuum line to the booster isn’t collapsing or kinking. Frankly, if this was my truck (coincidentally, I have a '99 Dakota with rear ABS), I think I would replace the booster for peace of mind. And then test the heck out of it to ensure the problem was fixed.

I checked on Alldata. Your truck does use a MAP sensor. The MAP sensor is not defective as it would be seeing the right manifold pressure and telling the computer to add more fuel. That is what it is supposed to do.

The situation with the MAP sensored fuel computation is that any air that gets into the manifold via the throttle body or elsewhere (bypassing the throttle body) will power up the engine. If you had a MAF sensor, the bypass air flow would also bypass the MAF sensor so the computer would not supply extra fuel and the engine would probably stumble or die.

If you do get an engine rev when the brake pedal is pressed, the problem would probably be in the booster. Either the diaphragm is split or something in the valving is allowing the vacuum valve and the air valve to be open at the same time. But, don’t go replacing the booster just on speculation. One test I thought of is to idle the engine in neutral and press the brake pedal as hard as you can. If the engine does not rev up, my idea of bypass air is wrong. Do it a couple of times, especially if you have the ‘failure to stop appropriately’ condition again.

Hope that helps. Let us know if it is the booster and if that cures the problem.