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

not really sure but the vacuum to the brake booster seems weak. I have no obvious signs of any vacuum leaks, the hose is fine and it good shape, no sign of intake leaks anywhere, but it just feels like the suction is weak.

in past vehicles it seems like they had strong suction to the brake booster that holds onto your finger as you try to remove it from the end of the hose, yet the taco suction is not as hard as I would think it should be.

maybe its regulated to be that way but does anyone know what amount of vacuum it should be pulling???

 

Curious as to why you felt the need to check. Are you pulling a code, having a high idle, or other issues? If there's no problems, I wouldn't sweat it.

 

lol ok but remember, you asked

im having "random" brake booster issues as though the vacuum assist "comes and goes" with 2 boosters replaced already (one from salvage yard and one bought as refurbished) and both with varying degrees of the same problem, I ordered another reconditioned one from a different company but I am seeking all avenues of checking what else it might be.

I swapped the check valve in the refurbished one that was hard almost every time I pressed the pedal and now it acts up much less then with the check valve it had, which when tested, had a stronger check valve spring, so that got me thinking maybe I don't have enough vacuum to overcome the check valve spring?

master cylinder was replaced prior to replacing the first booster as well as linkages at pedals are all in good working order, move freely and I greased them all.

brake works fine 1, 2 ,3 times then the 4th time its hard and if pressed on gives way, next time it might be hard on the first time pressing on brake pedal. it can go days or even a week with no issues then start acting up again.

 

Ahhh see that makes it much more clear why you were wondering lol. It could also help diagnose. Unfortunately...you're way past my level already haha. Good luck!

 

my thought on the randomness, brought about by the fact that changing out the newer check valve with my older one with the weaker check valve seal spring, does "alter" how its acting.

so I am going ahead and replacing the booster again (this will be the second reconditioned one purchased) but in the back of my mind I am thinking "maybe" its right there sitting on the edge of overcoming the check valve spring seal so "at times" its not getting the vacuum assist it should even though it always has "some" vacuum in the booster. maybe could it be that for some reason I am just an inch or two below what is needed to overcome the check valve spring?

the trucked had worked fine for 2 years before this brake issue came up literally out of nowhere.

can someone out there with a 3.4 v6 take a vacuum reading on the brake booster hose so I can compare what they get to what I see on mine?

it would help greatly to eliminate this as something for me to be concerned about

 

I can check mine tomorrow but my vac gauge is at work. Sorry I can't do anything tonight man

 

appreciate it man, I just want to know what it should be to compare so I can rule it out

because next week when the other booster gets here, if its still got issues im totally lost and may jerry rig the check valve to stay open just to see what that does

 

how fast are you pushing the brake to test it? Like are you pumping it 5 times in a row to test it?

 

nope just normal driving

if I sit there and pump repeatedly but not fast, it works normally, then after sitting for a moment, its back to randomly being hard pedal without vacuum assist

when the pedal is hard if I stand on the pedal it releases then "usually" but not always, it works fine for the next try or two

 

I have been looking online for solutions and found this https://www.yourmechanic.com/article/symptoms-of-a-bad-or-failing-vacuum-brake-booster-check-valve

on that page, this description "seems" to suggest my problem is with the check valve

1. Brake pedal is difficult to engage
When the vacuum brake booster check valve is working correctly, applying pressure to the brake pedal is easy and very smooth. When the check valve is not working correctly, the operation of the brakes becomes much more difficult. Specifically, the pedal goes from smooth and soft to aggressive and very difficult to press down. This is due to the excess pressure inside the master cylinder, which the check valve is designed to regulate. The inconsistency of the brake pedal is a warning sign that a potential safety issue exists with the brakes and should immediately be inspected by a certified mechanic.

but then I have tried replacing the master cylinder with no change to the condition and then I tried putting on 2 different boosters and each one came with its own check valve on it so, including my first one I took out, how can the 3 boosters or the 3 check valves all be bad?

logic dictates the odds are so far against it im forced into looking for some other cause

 

I wonder if it's something to do with the line itself, like how rubber brake lines will lose a little of their inner lining and it blocks flow one way, acting like a check valve itself.

 

naa its a 3/8" hose and its double walled and only 6" long so its not closing in on itself. but I did check flow at the nipple of manifold just to be sure and its certainly not a strong vacuum so I removed the nipple to check it and it was clear of any obstruction.

another strange thing is on older cars I have worked on unplugging the hose to the booster either causes the engine to kill of just barely run yet when I pull it off the engine idle speed changes a little but it still runs fine

 

I have had a check valve fail and give random hard brake pedal.

 

it should be engine manifold vacuum and that depends on how tight your motor is... above 17 would be "normal".

I just went through this on my 2002 Prerunner 3.4. What I had was a 2 part issue. The first part was the seal on the back of the master cylinder (MC) was torn where it seals on the aluminum protrusion off of the back of the MC. This caused a zero reserve vacuum when the motor was shut down. The brake pedal immediatly "jumped" up/back when you had your foot on the pedal when you turned the engine off. Replaceing the seal semi-fixed the problem. The second problem was the booster had a small leak and would not hold vacuum evan after the MC rear seal was replaced. A rebuilt booster from O'Reileys solved the problem. This seal gets torn because everyone wants to not disconnect the MC from the lines so they dont have to bleed the system. Attempting to slide the MC back into the booster they tear the seal UNLESS they remove the seal from the booster where is sticks when you take the MC off of the booster.

I would disconnect the booster vacuum line, take the check valve off and try to blow through it back into the intake manifold... there is a possibility that there is so much crap at that connection/fitting that vacuum is being restricted.

The pushrod distance from the booster push rod to the pushrod receptor in the MC is a hyper critial measurement... if the booster push rod is too long, the MC never seats and the brakes are slightly on all of the time. If it is too short, you might feel a slight click when it contacts. Toyota's measurement is 0.00, they want it exact.

Reserve vacuum should be 3 to 3 - 1/2 pedal pumps before it is depleted.

 

the thing isthough, 2 "used" check valves had the random hard pedal and the one that "appears" to be a brand new check vale on the rebuilt booster had a harder spring (had to blow harder for it to open) and that one had the hard pedal almost all the time. makes me think its the check valve causing the "condition" but maybe the valve itself isn't the problem?

the gaskets seals are good on the master cylinder and the booster and I make sure to rub a little grease on all contact surfaces when I put it together

all 3 check valves function as they should but on the new one you need to blow a little harder for it to open

I have no vacuum leakage at all, I can go out there 3 days later and its still holding vacuum if I pull the check valve out.

I checked and measured the push rod distance to be sure it was set the same as the original one was.

hmmm, never tried blowing into the manifold fitting, I just made sure it had suction/vacuum. I took the fitting off of the manifold and it was clean and clear. I checked and its just an open hole into the manifold there.

I sprayed some carb cleaner in the nipple at the manifold with engine running to clean and clear the passage of any "buildup" it might have in there. after that, if I don't notice increased vacuum from it. I think that kinda eliminates the idea I had that the vacuum might not be strong enough but I still plan to borrow a gauge to check it

engine has never been apart at all for any reason and only has 141k miles so there should be no issues with the engine itself that I know of.

 

im praying I just got a bad rebuilt booster and this last booster I ordered solves all my problems because I just cant explain why this is happening

 

ok, booster replaced, truck still has the same issues

so I have determined (after 3 different boosters) its not likely to be the booster, or the check valve, or the vacuum supply, this gremlin has been acting up off and on now for over a year now with no clear evidence of what part is bad.

as it turns out, I now think it has something to do with something ini feel a little "thump" in the linkage brackets where they rub together when manipulating the pedal by hand. it does this every time but the pedal is only hard to press "sometimes" and when that happens it "releases" the hardness in a non repeatable pattern as though the pedal "gives way"

it seems to be a "notched" like feel where the two metal brackets rub against each other to leverage the booster rod but you feel it by hand but not when using your foot so this might be a "normal" thing?

the lower bolt and nylon washer are fine on the brake pedal lower lever, the pivot has no play in it and it works very smooth and effortlessly but im unsure how to take the rest of it apart so that's as far as I could investigate.

this is the lower lever part I am talking about https://www.toyotapartsdeal.com/oem/toyota~lever~brake~pedal~47112-35040.html

the little thump I feel by hand is only in the tip end of that linkage where it appears to just be a flat metal surface rubbing against flat metal surface.

I really don't want to have the dealer charge me a couple hundred dollars just to look at it because I know those assholes are gonna want to tell me I need an oem master cylinder and booster before they even try to find the real problem but at this point im lost, tired and so disgusted im sick of trying to guess what is wrong

 

UPDATE:

I just talked to dealership service guy and he said they are still doing some testing but they had to find and "old guy" to work on it to figure out the problem lol, and they think the problem is something to do with the ABS actuator system itself or its electronics.

rough quote is in the neighborhood of $1500 for a new one so I told them to be absolutely sure that's it then don't fix it and ill leave it like that.

my plan, assuming they confirm that really is the problem, is I can buy a used one on ebay for around $100-$150 and fix it myself, really isn't much different then replacing any other part, just bolt in and plug in wires, and the electric part should come off and be replaceable if that is the part they tell me is bad. if its the motor that went out, I think the only way to fix that is to replace the whole unit which requires needing to disconnect and bleed the whole brake line system.

just sucks waiting to see how big the labor fee is going to be they are charging me to find the problem

I just want to put as much of this info out there as I learn it, for anyone else that might be going down this same "mystery brake issue" rabbit hole I have been dealing with for a year now.

 

the only thing left for me to suggest is a real pain in the rear and that is to plug the outlet ports on the master cylinder with the correct size crimped off brake line. Have a buddy press the pedal slowly and bleed the air out just as you are doing the final tightening and start her up and see if it still has the intermittant issues. It is a simple process of elimination. If the push rod that comes out of the booster is too long the master cylinder spool valve will not seat properly and uncover the "fill" port and get a full load of brake fluid and it wont work right.
Very, very tough to diagnose over a forum and there are a lot of assumptions made. Do the rear shoes have enough brake material, are they adjusted properly, are the rear drums wore, are the backing plates that the shoes slide on smooth... no gooves where the shoes pads ride, is the mechanism that the parking brake uses opperating properly... there are quite a few levers/bellcranks/cables that get rusty, are any of the flex hoses cracking.

 

if you do have to replace the hydraulic portion of the abs, there may be a special bleed procedure with a scan tool. Just food for thought...