jmlcolorado

I was cruising down the highway this morning. Got off the highway and noticed there was a little bit of unusual drag on the truck when coasting (nv5600).
Within about 3/4 of a mile, I noticed the drag was going p more and more and the brake papal was getting pretty hard.
I immediately started looking for a parking lot to I'll over in and by the time I found one and went to pull into it, the truck had TONS of drag on it and I had to drop into 1st gear just to get into the lot.
I got out and walked around the truck and found both fronts were very hot and crackeling (sounds). The rears were noticeable hotter as well, but not as bad as the front naturally.

This seems like a master cylinder issue, or maybe a vacuume pump issue?
Steering didn't seem to change.
The truck has been sitting off for about 15 minutes now and the pedal is still very hard like its applied pressure.

Help?

jmlcolorado

More info. They ruck has set for about half an hour now. The brakes free'ed up a little bit, but still have drag.
I pulled the abs fuse as a shot in the dark, nothing has changed.
It seems that the system has applied full pressure to the brakes and is not allowing it to be released.
Gotta buddy that will tow me home, and to get it on the truck, I'll likely have to open the bleeders on all 4 corners to let the brakes loose.

Steering seems to be okay, but the steering box is coated in fluid lightly. Can't see where that leak is coming from as the entire box is evenly covered.

jmlcolorado

Even more info.....
I found another larger fuse for the ABS brake pump. Pulled that fuse and now everything has free'ed up. Obviously, I have lights ablaze on the dash now, but all seems to be okay to drive home.
So with this new info, could the pump be bad? I'm not educated in the abs system very much.
The truck wasn't throwing any codes when everything was locked up, so I don't think it's a sensor issue. Is there a bleed back valve to let pressure off the pump that could go bad?

Lary Ellis (Top)

I had a caliper lock up like that due to the rubber brake hose rotting on the inside......It would act like a valve, pressure with my foot on the pedal would force fluid past the obstruction but it would not flow back out leaving the brakes on.

I believe in replacing those hoses every 50,000 miles especially now that all the cheaper Chinese rubber is in everything.

jmlcolorado

Funny you say that cause I just replaced a rear caliper about 600 miles ago due to a heavy leaking seal. While bleeding the brakes, I blew up the drivers front rubber hose.
I blew up the passengers hose about 6k ago, so both fronts are replaced, and the rear is in order too.

I suspect that I didn't bleed the brakes properly by opening the bleeder screw and compressing the cylinder when I did my brakes last, leading to contaminated fluid (though it looks clean as a whistle int he reservoir) and this has led to some failed brake components.

Hodge

That would only affect the wheel with the collapsing brake hose, not all 4. Plus, removing the ABS fuse wouldn't alleviate it. I agree that those hoses need to be replaced, but his problem isn't there. Obviously, it's in the ABS system. It applied and kept pressure.

If your brakes weren't bled properly, you would have had an issue before 600 miles. It's something else.

jmlcolorado

All signs point to a faulty hydraulic control unit.
I've read that these motors sometimes get stuck in the run position with a lot of folks. I tried plugging in the fuse again for the pump whth the truck off, but didn't hear the pump running. I didn't attempt to drive with the fuse back in, cause, well to be honest, locking up brakes while driving isn't much fun.

It still kinda seems to me like its not the motor itself running, but more of a pressure release valve getting stuck and holding pressure. But that doesn't explain the fact that the brakes broke loose after loosing power to the motor

Guess once funds are available, sometime in the next 5 years, I'll have to replace the control unit.
I'll probably have to take care of it before my next emission test since the idiot lights are all lit up and I'm not sure if they do a vehicle Safty inspection when they do emissions in Colorado.

jmlcolorado

If I could use profanity on the forum to express my frustration right now, I would!!!
Driving down the highway again this morning, the fuse for ABS pump has been unplugged for a few couple days with no issues.
This time, I was on the highway for 20-25 miles, with no brakes applied. This time it seemed to slowly apply the brakes slowly with no input what so ever. Just started noticing drag. Pulled over and here I am again, with my brakes locked up.
BOOOOOOO!!!!!!


Any thoughts?

jmlcolorado

Well I was stuck on the interstate. So in a scramble to get off the highway at the nearest exit, i pulled the 'ABS ignition power switch on' fuse.
Nothing. Still locked up. So I opened the bleeder on the drivers side front. Small pressure release let them all free up and the brakes are working like normal again
I got off the highway and pumped the brakes vigorously to SW if they would lock up again. No dice.
This is strange.

jmlcolorado

So with both ABS fuses removed, and the symptoms still appearing, this seems to me that it is not the ABS module, rather something mechanically controlled.
Could it be the proportioning valve?
Possibly the master cylinder?

jmlcolorado

If I had air in the system (from replacing the rear caliper, or the drivers front brake hose) would that cause a hard brake?
I always assumed that caused a soft pedal.

patdaly

Air would indeed result in a soft pedal, Hate to say it, if it were mine, I would replace the front Calipers and brake hoses first, then if that didn't do it, I would suspect the Hydraboost unit. I personally have never experienced one do it, but there have been reports of them failing and doing similar stuff, though rare.

jmlcolorado

But air in the line wouldn't cause a HARD pedal right?
Both front lines have been replaced in the last 10k. The drivers side just within the last 1k.

I have a service manual that shows testing procedures. I'll be performing the tests tonight and I'll advise of the results.

Totallyrad

Might be it, might not.

jmlcolorado

HYDRO-BOOST BRAKE BOOSTER:

With the engine off depress thebrake pedal several times to discharge the accumulator. Then depress the brake pedal using 40 lbs. of force and start the engine.The brake pedal should fall and then push back against your foot. This indicates the booster is operating properly. With the engine off, I can pump and the pedal is solid hard. Apply pressure, and as soonas the engine starts, the pedal falls to its normal position then kicks back slightly. I would say this test passes. If the brakes lockup again, I will dothis test again to see if there is a difference.



ACCUMULATOR LEAKDOWN

(1) Start the engine, apply the brakes and turn the steering wheel from lock to lock.This will ensure the accumulator is charged. Turn off the engine and let the vehicle sit for one hour. After one hour there should be at least two power assisted brake application with the engine off. If the system does not retain a charge the booster must be replaced. Im assuming that by “two power assisted brake applications” they mean that the pedal should be slightly soft? If that’s so, there is no difference between 1 hour from shutdown or 1 minute from shutdown. The pedal is stiff both times. I honestly don’t ever remember the pedal being “assisted”ever the entire time I’ve had the truck, with the engine off. Being a NV5600,my startup sequence, getting in the truck, foot on brake, foot on clutch,place into neutral, start truck. The brakes have always been hard before starting.



(2) With the engine off depress the brake pedal several times to discharge the accumulator. Grasp the accumulator and see if it wobbles or turns. If it does the accumulator has lost a gas charge and the booster must be replaced. Zero movement here. The accumulator is solid as a rock.



SEAL LEAKAGE

If the booster leaks from any ofthe seals the booster assembly must be replaced (Fig. 48).

I see no discernible leaks anywhere around the master cylinder, hydro-boost assembly or accumulator.



CONDITION POSSIBLECAUSES CORRECTION

Brakes Self Apply:
1. Dump valve faulty. 1. Replace booster. Crap.
2. Contamination in hydraulic system. 2.Flush hydraulic system and replace booster. Crap.
3. Restriction in booster return hose. 3. Replace hose. Isn't there a screen that I can check for contaminates in?

Booster Chatter, Pedal
Vibration Not a symptom.
1.Slipping pump belt. 1.Replace power steering belt.
2. Low pump fluid level. 2.Fill pump and check for leaks.



Grabbing Brakes
1. Low pump flow. Its full 1.Test and repair/replace pump.
2.Faulty spool valve action. How to test? 2.Replace booster.


I guess the next step would be to flush the entire system with fresh fluid (maybe with a different color so its easy to tell when each corner has been flushed?) and go from there unless someone has another idea.