hi all.

Hoping to get some ideas on my ongoing brake issues.
99 with ABS and 250k.
For the past year, I've noticed a few times that during panic stops, my brake pedal hardened, which made it pretty difficult to brake. I could not get the truck to lock up on dry pavement.

I took it to my trusted mechanic, who was quick to suggest a new booster. I trust him, and given the vintage of the vehicle, thought that made sense.

However, the issue still persists.

I then went ahead and bled the whole system myself, at which point I sucked in some air at one point into the reservoir. That obviously led to terrible brakes.

Bled the master cylinder based on FSM instructions, bled the system again, and got the ABS to kick on a bunch of times during a snowy morning. I have not bled again since then, but will do tonight, in case there was some trapped air int he ABS actuator that's now in the system.

Other observation I'm having is that with the engine on, foot on pedal, the brake booster will kick in as expected, but the pedal will continue to sink slowly, under pressure, after the initial drop. It won't go all the way to the floor through.

So - in short - new booster, but still having the hard pedal during panic stops. If I feather the brakes, they feel ok. Not great, but ok.
Parking Brake / Rear brakes are well adjusted. 6 clicks.

Thanks all!

Seems like the check valve on the brake booster.

If the replacement brake booster is good then replacing the following could solve your issue if it persists after bleeding the brakes.
It's a pretty easy fix.
VALVE,CHECK [44730-60060]
GROMMET, CHECK VALVE [90480-24015]

Thanks @spartacus . Pretty sure the new Brake OEM Brake Booster comes with a new check valve, no? I took it off, it looks brand new (=shiny), and did a quick blow/suck test.

Can you confirm whether or not a new booster comes with the new CV+grommet?

Thanks!

I had a hard brake pedal and replacing cv and grommet solved it.
I am not sure, but assume that a new oem brake booster should come with the cv and grommet.
Next trouble shooting. Is the brake fluid contaminated or there is blockage in the lines,air in the system? the vacuum hose from the plenum going to the booster has no deterioration or leaks? Rear brakes are adjusted properly and rear axle seals are not leaking on to the drums
Hope you find and fix the issue cause winter is here. if you do find it please post it on the forum to help others.

Lines are bled, rears are adjusted, and as far as I can see, they aren't leaking.
Keep in mind this only happens during a hard stomp on the pedal. Pedal only looses boost then.

Tonight, I will try to get my hands on a vacuum gauge, or switch some hoses around, to see if the booster is getting the right amount of vacuum. Will report back.

@spartacus When your check valve went bad, was your hard pedal intermittent or constant?

THanks!

My brake pedal was worse when stepping on the brakes to stop in a short distance from highway speeds when traffic came to a sudden stop. Then it got progressively worse until the cv fix. it's been good for a couple of years now.

Replaced the Brake Booster Check Valve and Grommet today, and the same conditions persist. Brakes work fine at slow speeds and casual applications, but pedal stiffens up as soon as I simulate a sudden/panic stop.

Any thoughts/ideas are highly appreciated!

Following your hard brake problem with intersest as this has been my problem too. Replaced booster & check valve, checked vacuum, tried a vacuum can. Master cylinder not leaking, brake pads good. As someone else described, feathering the brake pedal for normal stop seems ok. But, in my case, coming slowly to a full stop, particularly going down a hill, I'm standing on the pedal as hard as I can and pedal not going to the floor and hits a hard point.

It's the only thing making me sad about my 2000 4Runner. Have not tried adjusting the booster plunger into the master cylinder, if anybody has thoughts on that as a remedy.

I never experienced this problem. However, I been following this issue. Does your ABS system function properly? Basically does it pulse when the tires slide. As the system is a panic stop would normally activate abs. Being it looks like everything else has been checked or replaced. I am wondering if your abs system itself is causing this problem.

Sent from my SM-J337V using Tapatalk

I am thinking along the same lines as brillo_76 It sounds like the issues are when the abs should kick in. Possibly disconnect the abs temporarily and see if the brakes get hard without the abs. You can also pull and clean all the abs sensors

How about the basics?
Are your rotors and or drums heat hardened or blued? The brake pad and or shoes cant bite into the rotor/drums if it's too hard.
Are your pads and or shoes glazed? Same thing, pads/shoes cant bite into the rotor/drums if glaze is in the way.
When was the last time you changed out or rebuild the caliper and it wheel cylinders? Junk will stick to the pistons, gum up the clearance between the piston and it's bore, robbing braking power.
Are your brake hoses 15+ years old? It was a noticable difference on the bottom stroke of peddle travel when I replaced mines. The hoses had year 2000 printed on them, yikes.

Thanks guys. ABS is working as it should. The hard pedal occurs long before the ABS would ever kick in. Tested the ABS during a recent snow event, and it worked as it should.

Thanks HiLife.
Quick snapshot: Drums are adjusted. Rotors are pretty new. Calipers are off a Tundra, and have been rebuilt a few years ago with new seals, etc. Brake lines are new stainless.

Here's the one things that's so strange: The brakes work fine if I modulate them - it's only if I mash down on the pedal, that it instantly firms up. Sure sounds like a vacuum problem, no? I'll post some odd vacuum readings in a separate reply.

So I borrowed a vacuum pump over the weekend, to measure intake plenum vacuum and brake booster. Here's what I found:

Intake plenum vacuum:
Holding steady at 17 in. It sounds like the norm is 16-20, industry-wide, so even though 17 is on the lower end, this seems right, yes?

Brake Booster Vacuum:
I applied 20 in of vacuum, and let it sit for half an hour. Engine was warned up, but off. Brake booster held the 20 in. Then, I applied the brake once, engine still off, and it dropped to about 10 in (as it should, I believe), and stayed there.

But here's when it went strange: From there, I tried to get the vacuum back to 20, and it would not take. At some point it went up to 15, but I could not get it any higher???

If this was an original booster, I would definitely suspect it to be the root cause. But its brand new OEM (might be OEM refurbished), and all the standard brake pedal tests checked out normal.

Anyone out there with a vacuum gauge/pump that can replicate these measurements?

Thank you!!!!