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

So you changed the master cylinder and the booster in hopes of fixing the issue, right?

 

ok I really should recap what has been done to the truck so far.

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

 

what's a timeline of events?

original issue? ~> front brakes? ~> rear brakes? ~> master cylinder? ~> booster?

 

brake job was done a year ago, about 3 months after that the pedal would "give way" at stop lights about once every other week, after a couple months it started doing it about once a week, about a month later it happened an average of once each time I drove it. this is the point I started replacing master cylinders and then boosters.

the pedal gradually got stiffer over time so I never really noticed it until it started to give way, then I realized it was a lot harder pedal then any other vehicle

 

I just picked up the truck with the "official" diagnosis

definitely some type of electrical issue in the circuit board on the abs system so it needs to be replaced

early estimate of $1500 was way off, actual cost of oem replacement abs system is $2400 plus a guestimate of $200-$300 more for labor to install, purge air from brake lines, and fully test the system to certify its working properly. for them to do it they said it requires they replace the booster as well which is another $900 because they "being a dealership" cannot recertify the abs system using remanufactured parts for liability reasons, and the booster I have is a cardone reman unit

after doing some reading it seems like its most often just corroded solder on the circuit boards with a pretty high rate of success to fix it if you just redo all the solder points on it. the bad part is it doesn't look like ours comes apart with screws, it seems to be a glued on cover so I cant try the re-soldering trick to see if I can fix it.

the good part is I am seeing these units on ebay for under $100 listed as "used/working" so that's not too bad at all considering new costs $2400 lol, just have to bleed the entire brake system after which is a bummer.

I am pretty disappointed in myself for not realizing the abs system was the only component I never looked at. to be honest it escaped me that I even had one

 

Try simply unplugging the ABS unit or removing the fuse for a bit and see if the issue comes back. If it doesn't, just leave the ABS disabled. I've always heard that our trucks are unsafe with ABS enabled anyways. My ABS has been disabled for the entire time that I've owned it. 4 or so years.

 

well after looking at it for 2 days the dealership determined it was 100% at fault so im going to go with their diagnosis on it, especially since every other item in the system has been replaced but that.

as for unplugging it, it has a pressure pump that assists hydrolic pressure to make the system work, if it is unplugged its like pulling the vacuum assist from the booster.

it was explained to me the system wasn't working and that is why I have a hard pedal with very little travel to it, in essence they are telling me it is acting like it is unplugged. unplugging the fuse is different then unplugging the abs because the pump still adds pressure if the fuse is pulled, the fuse just disables the abs system actuators but still lets the abs pump run

im still learning about how all this abs stuff works so I may have some of it wrong but the general premis was, unplug abs - no, pull abs fuse - you can do that

 

Unless Toyota is doing something different it is my understanding that the ABS system opperates only when the control unit senses a wheel approaching "lock up" then the ABS pump modulates(blocks/dumps) the fluid pressure to the effected wheel(s). I could see where if the control unit randomly senses a "false" lockup condition and the ABS pump randomly modulates the fluid pressure that you could experience a normal pedal then a hard pedal when the control unit closes off that wheel(s) hydraulic line.

I would check in the for sale forum here and see who has a vehicle they are parting out and see if you can make a deal for the ABS system. I am assuming this is your daily driver so you dont have time to fiddle with this until it is done. I look at it this way... the unit you have is "broke" and if you have a know good unit avalible I would experiment with resoldering the board to see if it works, but that is me and I have the time and other trucks to drive while I play with it. Sorry to hear how much of a P.I.T.A. this has turned into.

 

ok, like I said im not in clear full understanding of the workings. from what you say then, it sounds like the system is shorted and shutting off fluid pressure all the time causing the hard pedal with limited travel then "randomly" it lets pressure flow and that's the "give" I feel in the pedal.

I found one on ebay from another 04 3.4L v6 taco with exact same part numbers for $65 and free shipping so I bought it, but it wont arrive until next week, so it means I have to wait even longer before I can sell the truck

 

What would be the point of an ABS system if your driving and your electrical system fails and you're coasting unable to stop because the ABS doesn't have any power? Yes, you can disable/unplug your ABS module and have your system still work. The ABS system works only by pulsing open chambers that are attached to each line to each wheel to relieve pressure. If it's disabled then it can't open the chambers, but it will still allow for full pressure to pass through from the master cylinder / booster. It's up to you. You can spend the nearly 3 grand or see if just disabling the ABS system suffices.

Also, Have you ever heard one of those ABS systems going off? It's like having a mini Harley Davidson at full throttle under the hood right next to your engine. I can't imagine that it's a faulty ABS module like the dealer says. If you're not hearing a high frequency rumble while braking, your module isn't pumping. I think something else is blocking/siezing.

Have you re-checked your brake job? Did someone else do it and maybe forget to put the brake pads in right or they're the wrong ones that get caught up in the caliper?

 

as stated earlier, im not fully schooled on the abs system but..... it has a high pressure hydrolic pump driven by an electric motor that assists in pressurizing the brake lines so they apply more power to the brake pads when applied.

its pretty simple logic is if you shut off power to it and don't add this extra hydrolic pressure to the system then you need to press on the brakes harder to get the same stopping power.

im not trying to reinvent the wheel here, but I don't see where reducing the pressure applied to the brakes is a good thing

there are 2 functions of the abs system, one is to increase the line pressure which is what the pump does for you, the other is the solenoids shut off fluid pressure to some wheels as needed if the sensor detects wheel slippage.

I am in agreement not having the abs function shouldn't matter, I am not in agreement that doing without the fluid pressure assist for the system is a good idea.

I was told if I unplugged the abs system I would get constant abs warning lights on the dash and start throwing CEL codes. im trying to sell the truck so im not looking to do hack repairs, I want it fixed the way it is supposed to work.

I ordered another abs system from ebay for $65 and when it gets here next week it should fix my issues.

 

Everything you just said here is the exact opposite of what an ABS pump actually does... And trust me, it's my job know this. But if you don't believe me, go watch some youtube video on how a pump works or actually do research before suggesting others they're wrong.

 

don't get all silly, I never said I didn't believe you, I said I didn't know much about these systems several times, I can only say what I am told about them and apply basic logical assumptions after that. plus, I have no way of knowing what you know or if you are just someone commenting on things you read on the internet

if the pump and motor isn't there to provide extra brake force then why even have a pump? what purpose does it serve?

im sure they wouldn't have a pump to increase pressures if it wasn't needed for something.

since you know this stuff what electrical fault in the unit would make it give me a hard pedal with limited travel like this?

 

if I remember correctly The pump only pumps fluid back to the reservoir. The solenoids allow fluid to the pump reducing line pressure - taking the place of pumping the brakes for you during an emergency stop. I'd try unplugging it and make sure the symptom goes away.

 

Oh dear god, keakar... You were JUST TOLD 3 times how the pumps work! The pumps act as relief valves and frees each wheel that it thinks is locking up. It feels for lockup through the wheel speed sensors. That's literally all it's legally allowed to do!

Imagine an empty syringe connected to your calipers. When the ABS module feels one wheel is locking up, it pulls on that syringe and relieves some pressure, effectively allowing the wheel to begin moving again. That is all that an ABS pump is allowed to do!

 

And it only works during a panic stop. It does nothing during normal brake application

 

it does. the video you watched was for the original mercedes patent where they had 2 sensors in the vacuum chambers that allowed them to have those "Features". our trucks don't have anything like that cause were not all bumbling idiots

 

if you want to find out. drive about 10 mph in a tight circle and start braking. you'll probably trick the module into thinking a wheel is locking up and it will turn the pump on. i get this a lot when im parking my 96 civic. I start turning into the parking spot and hit the brakes then the pump will start relieving pressure. sounds like something is grinding. kinda scary if you're not expecting a noise like that.

 

Lots of great info in this thread. Thank you Keakar for asking your questions and thank you gents for taking the time to answer and explain so a layman like me can learn. Seriously, good info.

 

There is no electrical fault that can give you a hard pedal with limited travel. The brake system is designed to be entirely mechanical and be actuated by you're depressing of the pedal.

The issues that can give you those symptoms though are things that have already been mentioned in this post. Bad parts like the master cylinders, vacuum booster, and vacuum leak (which can be verified by watching the fuel trims). I think it's more likely that a screw fell out from under your dashboard onto the hinge mechanism and is preventing you from depressing the pedal.

My main recommendation would be to remove the tires and inspect the calipers pads and rotors and make sure they were installed correctly.

Then I would say get a scanner and see if your fuel trims are above 5% at idle after being warmed up. That will indicate you have a vacuum leak.

Another question I have is when you say the pedal would "let go", what do you mean? Do you lose your brakes or does it become extremely easy to press the pedal and stop the vehicle?