Aloha Everyone,
I have the hard brake pedal issue after shipping my 4runner from New England to Hawaii. When I picked up my truck I immediately noticed the brake pedal traveled a minimal amount (1.5" or so) and the truck came to a stop very slowly. Scary. There has been times where my idle runs high, over 1.5k rpms, then will go low to 700 rpms (normal). It also has intermittent rough idle. The travel on the brake pedal does not change if the truck is on or off. The brake pedal travel does not change with or without the vacuum hose attached to the brake booster. The vacuum hose to the booster has vacuum pressure. I assume this implies the in-line brake booster check valve is operating properly. I do not have a gauge to tell the exact psi but it holds my fingertip tight to it during engine operation. When I depress the brake I can hear a hissing noise coming from the where brake pedal lever meets the brake booster at the firewall. When I removed the vacuum hose from the brake booster there appears to be some brake fluid (clear) on the brake booster vacuum input nozzle. I have never bled the brakes, flushed the brake system, or added brake fluid since I bought this truck 14 months ago. The only brake issues I've had over the last year is over-travel in the brake pedal and a hard stop; with minor shaking at speeds over 40mph. The brake fluid is an opaque dark brown color. I went through the FSM troubleshooting methods with results as follows:

OPERATIONAL TEST OF BRAKE BOOSTERHINT: If available, use a brake booster tester to checkthe booster operating condition.
1. OPERATING CHECK
(a) Depress the brake pedal several times with the engine off, and check that there is no change in the pedal reserve distance.

- There is no change in the pedal reserve distance.

(b) Depress the brake pedal and start engine. If the pedal goes down slightly, operation is normal.

- The pedal does not go down slightly upon starting the engine. Stays stiff and at the same minimal reserve distance.

2. AIR TIGHTNESS CHECK
(a) Start the engine and stop it after one or two min–utes. Depress the brake pedal several times slowly.If the pedal goes down deepest the first time, but gradually rises after the second or third time, the booster is air tight.

- The pedal does not change in travel doing this test. Same hard brake with minimal travel.

(b) Depress the brake pedal while the engine is running,and stop it with the pedal depressed . If there is no change in pedal reserve travel after holding’ thepedal for thirty seconds, the booster is air tight.

- The pedal travel does not change for this test.

My hope is that the brake booster is not bad because it is a very expensive fix and not an easy part to come by here on island. I would like some help in figuring out if this is the brake booster or perhaps something else. It is covered in rust. Will take and post pics in a few minutes. Mahalo in advance for your help.




Brake Booster

Vacuum Hose

 

Replaced the brake booster and didn't get the depth of the piston right so the brakes were super touchy but now working. Now the brakes keep locking up, barely made it home and now am going to try to bleed the brakes I guess. No replies yet?

 

About all I know about brakes is change the pads/shoes if they get down to a certain thickness, bleed the system in thus-and-such an order, and set the pedal the way the FSM specifies.
After that, I don't know much. I DO know the theory of ABS, and how it was put into practise in certain aircraft. If it was put into vehicles the same, or similar ways, great, I might be some help. Beyond that, best I can say is "I dunno! ".

Could your trouble perhaps be in the master cylinder? Something with it's piston, maybe? The piston's o-rings?? A build up of gunk at the end of it not permitting proper throw of the pedal? Air in it someplace?
I'm just guessing. I could be totally wrong here.

I do wish you all the best luck with this, and look forward to knowing what the solution turns out to be.
BTW: I have been to Kauai a number of time. I think it's most beautiful of all the Islands, I really do.
Pat☺

 

Your post leads me to believe you have two problems, on of which it sounds like you fixed with the new brake booster. the other is that the master cylinder may be leaking out of the seal on the back keeping it from filling you brake booster with brake fluid.
the rod length between the booster and master cylinder is very important as well as the pedal rod length.
If you are sure that the fluid you saw when removing the vacuum line to the booster was brake fluid replace your master cylinder.
If you are not loosing brake fluid you probably don't have anything to worry about, but while you are checking the pushrod length make sure the back.
I imagine your idling issues cleared up with the new booster as well?

 

AK,
Mahalo for your reply. I have a leak somewhere in the line on the front right brake (passenger side). But it is intermittent. I am going to bleed the system next and see if the problem with the brakes locking up persists. It's an older truck so I'm thinking sometime in the next month or two I will just do a full caliper, rotor, break, drum, and shoe replacement. I measured the rod length and it was ~0.6mm which is within spec. So I am confused why the brakes are so touchy. But I also didn't properly bleed the system after either. So I am going to bleed the system and then see what happens.

 

Aloha neighbor (just across the pond right?) the touchy brakes will not be fixed by bleeding. Air in your brake lines will cause a mushy pedal feel and poor braking, not touchy/locking up brakes. I'm not 100% certain about the load sensing proportioning valve (LSPV) as to what air in that line would do, but from what I remember it should be the same as anywhere else in the system, mushy pedal.
When you say the brakes are locking up is it the front or rear brakes? That LSPV has a long rod attached to the rear axle that can get bent and throw off the rear brake proportioning, so if the rear brakes are locking before the front it's adjusted as if there is a load in the back of the truck and it's sending more braking pressure to the rear brakes than it should. The opposite for the front, though it's far less likely. let me know if it's front or back or always 1 particular wheel that locks up.

Just noticed your thread title (ABS) I'll put my thinking cap on while I'm waiting to hear which wheels lock up.
I guess I'll also need to know, at what speeds, on what kind of ground, are you in 2wd or 4wd, how good are your tires, and do you feel the typical pedal fluttering from the ABS operating?

 

Aloha neighbor (just across the pond right?) the touchy brakes will not be fixed by bleeding. Air in your brake lines will cause a mushy pedal feel and poor braking, not touchy/locking up brakes. I'm not 100% certain about the load sensing proportioning valve (LSPV) as to what air in that line would do, but from what I remember it should be the same as anywhere else in the system, mushy pedal.

When you say the brakes are locking up is it the front or rear brakes? I believe it is all 4 brakes. Definitely the front two.

That LSPV has a long rod attached to the rear axle that can get bent and throw off the rear brake proportioning, so if the rear brakes are locking before the front it's adjusted as if there is a load in the back of the truck and it's sending more braking pressure to the rear brakes than it should. The opposite for the front, though it's far less likely. let me know if it's front or back or always 1 particular wheel that locks up.

Just noticed your thread title (ABS) I'll put my thinking cap on while I'm waiting to hear which wheels lock up.

I guess I'll also need to know, at what speeds, Standard 20-30mph

on what kind of ground, Asphalt

are you in 2wd or 4wd, 2WD - Have not even tried to use 4WD yet. This locking happens within a couple miles of driving and is only resolved by manually opening the bleeder valves on the brakes. Last night when it happened at higher speed (~40mph) the brake fluid was so hot it was boiling.

how good are your tires, I have about 6k miles on new BFG AT KO2's

and do you feel the typical pedal fluttering from the ABS operating? I will have to get back to you on this. Right now I am going to adjust the rod from the brake booster, flush the system, bleed the brakes, and see if the behavior persists.

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I think I'm starting to get the picture, you brakes are dragging, you either have the pushrod that activates the master cylinder adjusted too long or the rod connecting your booster to the pedal, make sure you have freeplay at the pedal before the pushrod moves and if that isn't it back off the booster to master cylinder pushrod.
when you say "and is only resolved by manually opening the bleeder valves on the brakes." you are telling a big part of the story, there is trapped pressure in the brake system even when your foot is off of the pedal. that means that the piston inside the master cylinder is not going all the way back to it's "at rest" position which is where the reservoir passages are completely open to the system and pressure can equalize.

 

I think I'm starting to get the picture, you brakes are dragging, you either have the pushrod that activates the master cylinder adjusted too long or the rod connecting your booster to the pedal, make sure you have freeplay at the pedal before the pushrod moves and if that isn't it back off the booster to master cylinder pushrod.
when you say "and is only resolved by manually opening the bleeder valves on the brakes." you are telling a big part of the story, there is trapped pressure in the brake system even when your foot is off of the pedal. that means that the piston inside the master cylinder is not going all the way back to it's "at rest" position which is where the reservoir passages are completely open to the system and pressure can equalize.[/QUOTE]

You were right. I had already fixed it before you posted but wanted to close the loop and thank you. Big mahalo for your help. I went in and adjusted the brake booster push rod. I set it back about ~6mm and replaced the master cylinder. I then flushed/bled the brake system. Went for a drive and worked like new. No lockup. My mechanic friend showed up right as I was pulling in from my test drive and we went out and road tested it. Working perfectly now. Funny thing is he was rebuilding a brake system on another car with the exact same issue so it worked out for us both. In the next couple months I'll redo the entire brake system but I am good to go for now. Stoked.

****NOTE: I have ABS but did not buy the ABS power brake booster. Still worked fine. This question has not been definitively answered anywhere I can find. If this changes I will update the post, but until then....mission accomplished!!!