strange brake issues, sometimes hard, sometimes not, brake pedal

ya, by let go I mean it gets softer and works normally as it should without being hard, the brakes work great the whole time and would put you through the windshield if you wanted them to.

nothing in the pedals at all, everything works fine there, completely took it apart and greased any pivot points and pins.

 

just went try it, I unplugged the abs, had abs light on in dash "obviously" but the brakes work fine now, plug abs back in they are hard again.

whatever it is, its definitely the abs unit that is doing it so maybe its a mechanical fault with it not doing something it should when its plugged in

and im not trying to disregard your advice CodeSeven, im trying to learn about this stuff, I always want to know why about everything I work on so if I ever deal with something similar I have something to base my troubleshooting on.

I was getting conflicting info from the dealership who said point blank it was an electrical issue with the abs circuit board that caused it ????????????


something else that caused me confusion was I recently was trying to read about my 4runner abs/booster/master cylinder all-in-one combo unit, and it has some sort of pressure assist pump they kept talking about. I still don't understand about the design and function of that system very well and the more I read the more confused I got.

 

Ok. So disconnecting relieves the problem. That is very unusual unless it was coincidence. Up to you to pursue that issue. Honestly I'd just disconnect the module and carry one. Actually, that's what I did.

 

I've been around quite a few Toyota's for quite a few years and this is the first time I've heard of one failing. Doesn't mean it doesn't by any means, I'm just thinking it's gotta be pretty rare. Wonder if the dealership diagnosis equipment is able to pick up that kind of failure? Or did they have to do it all the old fashioned way? Just curious

 

Do you know if any petroleum based liquid may have gotten into the brake system? When a system is contaminated you can get some strange issues that occur due to the rubber components in the system swelling.. it's easy to check get some brake fluid on your clean finger and run it under water if it's pure brake fluid it washes right off if there is any trace of petroleum products the fluid will bead up causing bubbles as it washes off

 

another issue about the dealer is they only want to replace the expensive items so they can "certify" your brakes work. that doesn't necessarily mean that's whats wrong. in fact, im more convinced the pump isn't bad and they just want to screw you over.

just disable the module and be on your way. save yourself 3k. or call NAPA and ask them if they know of any independent toyota specialists and have them diagnose the issue.

 

I don't know I never heard of one going out either. the service manager guy said its the first time his mechanics ever had a bad one.

my guess is, when the older mechanic guy stepped in, I bet he knew to unplug it to check it like CodeSeven said to do, when that fixed things, they found the problem

 

I mentioned earlier, I bought a used one that came out of a running wrecked vehicle for $65 and free shipping, same year, model, engine, and part number, so heck, I figured I cant do better then that for an original replacement

whats the best way to to purge the lines when you swap them?

I saw some stuff on internet saying just pull abs fuse or unplug the abs and then I just bleed the brakes as normal ? is this correct?


I never new you had to disable abs for proper brake bleeding

 

this abs video does match my situation somewhat, I also had one wheel pull on the front so I changed the brakes and this video reminded me about the dark fluid in the system that I replaced. when I drained and refilled the system with fresh fluid, I suppose its possible with all that dark brake fluid, it "may" have carried along some sediments or something inside the lines which "could" have clogged up the abs system.

https://www.youtube.com/watch?v=ZYpyN1LvddA

 

When under ABS braking condition the module will close off the line to the skidding wheel. Alowing the valves to control the pressure to the brake. If it didn't it wouldn't be able to lower line pressure. Because your foot would just add more pressure into the line. If you go on a dirt road about 45mph and hammer the brakes hard. Putting you into all wheel abs activation. Your pedal will be hard as a rock while ABS is controlling brakes. As soon as the ABS releases control. Your pedal will feel normal. So its very possible something electrical is causing the valve to shut a line off. IE: bad ABS control module.

 

what do you think happened to it?

when I plug it in it seems (im guessing) to be restriction fluid flow causing the hard pedal, when I unplug the abs system it seems to be ok

the only thing I can guess is the dark color (like watered down pepsi) of the old brake fluid I flushed out a few months before I started having pedal issues, must have been the root cause of the issue. I think the one wheel pulling on the front that caused me to put new brakes all around, may have been the beginning stages of whatever the abs failure is. I remember thinking there was absolutely no reason I found for having any sort of a brake problem except one side pads were only half way worn down, while the other side pads were about completely gone so I just attributed it to that

 

A malfunction in the electrical part is intermittently energizing this solenoid, The main one in the video.

 

cool, thanks. I love learning about new stuff and I go crazy not knowing why things fail

 

So you said you have a "one wheel pull" when braking. Whichever side is opposite of the pull will be the one that has no braking power due to the abs module blocking off the line. Have you checked that specific sensor and matching rotor? I may have missed this in the thread.

I had a ford ranger that had similar issues with ABS. It only had rear abs but would do all sorts of weird things if I slammed on the brakes, with a lot of pull to one side. I just unplugged it and drove without ABS since it was cheaper and I know how to stop a vehicle without it.

 

this explains it fully

new pads and rotors up front, new brake hoses, new drums and shoes in rear, master cylinder replaced twice with 2 brand new ones, brake booster was replaced with a salvage yard unit and then replaced two more times with 2 brand new ones, the boosters all came with check valves installed so that counts as 3 check valves also replaced, brake pedal linkages were looked over carefully and nothing strange found, seemed to be functioning without any obvious binding.

before the pedal issue I had a random pull to the left from front brakes when stopping, so that's why I did the 4 corner total brake replacement and hoses, that "seemed" to cure it, but then a few months later I started to notice a funny feel in the brake pedal like it "gave way" sometimes which is the point I started replacing the master cylinders and then when that didn't cure it, the boosters

 

ok the replacement abs unit came in today and I just got it fully installed before the rain started so im done for the night, but what is the best air bleeding method to use? the old unit had fluid in it so it isn't bone dry full of air but of course it has trapped air in all the lines now I need to purge out of the system.

I saw "somewhere" online that the abs will "self purge the air out of the lines between it and the master cylinder. is this true and how do I "trigger" it to do this?

I do understand that the "exit" lines coming out of the abs system, are still done the old fashioned way of you just have to pump the pedal to push the air all the way through the lines to each separate wheel, and in the case of the rear wheels, just to the load sensing valve.

 

According to the FSM there's no special procedure when replacing ABS unit. Just bleed the system at the wheels and LSPV valve like normal.

 

FINAL UPDATE AND FINAL RESOLUTION

ok, the rain held off long enough for me to get the system bleed today.

the pedal is soft compared to how it was, like its only needing 25% of the pressure it did before with abs unplugged and 75% less then when abs was plugged in.

truck brakes perfectly now and it was a simple fix with a $65 salvage yard abs unit rather then the estimated $4k repair bill for dealer to do it.

I never knew just how hard that damn pedal was until it was fixed, "wow holy shit" is the only way to describe how different it is now and I feel like its braking much better so im guessing it wasn't using "all" of my brakes before because it stops so much better now.

FINAL THOUGHTS:
if you are having strange brake pedal issues and its not the master cylinder then its either the check valve for the brake booster or you should start looking at the abs system for a problem. the booster (as I had originally thought) is either good if it holds vacuum or bad if it has a vacuum leak but otherwise its not going to ever fail or need to be replaced.

 

Glad you got it fixed.

 

kinda feel bad after being a mechanic doing all my own work including engine and tranny rebuilds for 45 years, and I totally missed not looking at or even thinking about the abs system.

its like it never even entered my mind I had one, and I literally sat there saying to myself "I replaced the whole brake system, what else can it be" and was stumped. but obviously I left out one main component I guess im getting senile in my old age lol