Brakes seize when applying slowly

[AntleredRuin]

This issue started Friday, and I can recreate it, even while in park, so it's not my imagination.
The brakes work great with normal braking, but if I slowly apply them, like pulling into a parking space, there's a slight hissing noise and then they firm up, like if the truck was turned off. I can stomp them to still stop, but it's hard.
If I let off the brakes entirely for about a second, they go back to normal.

I've had the TBU upgrade for almost 2 years, and I changed my pads 2 months ago. I didn't bleed the brakes after the pad change, and I introduced no air to the system.


Something that may or may not be related. I had a hole in my exhaust between the cat and the rear O2 sensor, which I patched Saturday. It did throw a lean code Friday afternoon, though, and roughly around the same time is when I noticed the brake issue.
I don't think that should matter, though. Vacuum shouldn't be related to engine A/F ratio.

I'll try to bleed the brakes tonight to see, but does anyone have any ideas of what to look at?

 

[LittleCaesar]

This issue started Friday, and I can recreate it, even while in park, so it's not my imagination.
The brakes work great with normal braking, but if I slowly apply them, like pulling into a parking space, there's a slight hissing noise and then they firm up, like if the truck was turned off. I can stomp them to still stop, but it's hard.
If I let off the brakes entirely for about a second, they go back to normal.

I've had the TBU upgrade for almost 2 years, and I changed my pads 2 months ago. I didn't bleed the brakes after the pad change, and I introduced no air to the system.


Something that may or may not be related. I had a hole in my exhaust between the cat and the rear O2 sensor, which I patched Saturday. It did throw a lean code Friday afternoon, though, and roughly around the same time is when I noticed the brake issue.
I don't think that should matter, though. Vacuum shouldn't be related to engine A/F ratio.

I'll try to bleed the brakes tonight to see, but does anyone have any ideas of what to look at?

Are the brake lines original?

 

[SaulG]

Maybe time for a booster depending on wich system you have.

 

[AntleredRuin]

Are the brake lines original?

The brake lines are original. They fit perfectly when I did the TBU so I left them. No issue when bleeding them though, so I know they're not clogged.
Also the brake fluid amount in the master cylinder is perfectly topped off, so there's no leak anywhere.

Maybe time for a booster depending on wich system you have.

That was my guess too. What do you mean by which system? It's a 1999 Limited, which I believe has the different ABS than the '96-'98.


I'm also seeing it could be the check valve, which would be far nicer. Really don't wanna buy a new brake booster.

 

[thegipper]

I would replace the brake booster check valve first, it's pretty cheap (20 bucks ish). Mine started to fail but it would only happen on cold starts on a cold morning like when I was backing out of my driveway. I replaced the check valve and it hasn't done it since.

 

[JohnMc]

I had the *exact* same thing as thegipper had. On *very* cold mornings, no vac assist until the engine bay warmed up some. And 4Runner brakes don't do much at all without power assist, lol. Replaced it, works perfectly. It's cheap and easy to swap that.

If that doesn't help I'd just double check that it is getting good vac at the booster end of the line (pull it off and put your finger on it), if so, the booster itself is the likely suspect.

 

[nevada]

This issue started Friday, and I can recreate it, even while in park, so it's not my imagination.
The brakes work great with normal braking, but if I slowly apply them, like pulling into a parking space, there's a slight hissing noise and then they firm up, like if the truck was turned off. I can stomp them to still stop, but it's hard.
If I let off the brakes entirely for about a second, they go back to normal.

I've had the TBU upgrade for almost 2 years, and I changed my pads 2 months ago. I didn't bleed the brakes after the pad change, and I introduced no air to the system.


Something that may or may not be related. I had a hole in my exhaust between the cat and the rear O2 sensor, which I patched Saturday. It did throw a lean code Friday afternoon, though, and roughly around the same time is when I noticed the brake issue.
I don't think that should matter, though. Vacuum shouldn't be related to engine A/F ratio.

I'll try to bleed the brakes tonight to see, but does anyone have any ideas of what to look at?

the booster is bad. a seal has failed and its leaking vacuum, that is the hissing you hear. no vacuum means no assist, thus the hard pedal.

 

[Airmapper]

Have a similar issue I was debating posting about, minus the hissing. Strange what rare times anything goes bad on my Runner, seems others have run into it as well.

I replaced the check valve/grommet to no effect, but at around $50 it was worth a shot.

Booster itself is my next target I suppose. Good luck to the OP I bet you we have the same thing going on.

 

[AntleredRuin]

The brakes started failing completely yesterday when I was about 5 miles from home. I was able to limp home safely standing on the pedal each time. Never higher than 15 mph, always a good 200 feet between me and the car in front of me, flashers on, yada yada.

I checked the check valve, and it's still one way (can suck but not blow) so it's fine. The grommet was still nice and clean rubber, no cracks.


Shopped around for a Brake Booster, and no parts store near me had one in stock, and the dealerships didn't either. I called 15 junkyards and only 2 had them in stock (they usually pull the booster first thing and keep them in the warehouse), and they were $100+ with no warranty or return policy.

I found a 5-star rated eBay special for $60 from a 99% positive ratings seller with 1 year warranty and 90 day money back guarantee, and free 3 day shipping so I took my chances.
New Brake Booster for Toyota 4Runner 4 Runner 1996-2001 446103D700 723650019573 | eBay
I've had very good luck with eBay specials before so hopefully that streak continues.


Also got a new muffler because the patch started failing already too. When it rains it pours.
It'll all be here Friday.

Have a similar issue I was debating posting about, minus the hissing. Strange what rare times anything goes bad on my Runner, seems others have run into it as well.

It is funny how someone always seems to be having the exact same issue at the same time. Obviously you can search for your issue and find something, but the timing is always very coincidental.

 

[AntleredRuin]

I got the new booster in this afternoon.
Nearly a perfect swap for not being OE. The only real issue was the 'hinge-pin' thing on the end near the pedal.

The holes on the new hinge were slightly too small for the pin. I was able to simply unscrew the old hinge, however, and install it in place on the new booster. I used a carpenter's combination square to make sure it was adjusted properly.

Installed fine, did not have to crack the brake lines. Just pulled the master cylinder out of the way carefully.

1.5 hours, all in.
Brakes great now, no issues. I guess we'll see how long it lasts, at $60.

It started raining so I couldn't do the muffler.

 

[AntleredRuin]

Wow ok, so eventful Sunday.
I got the muffler installed, perfect fit. Very slightly throatier, but really only if you're listening for it. Not annoying at all. It started pouring while I was halfway done, so I got soaked. I felt like a great big lightning rod, holding that muffler in the middle of a storm.


The brakes however...ooh different story. They were fine at first, with a quick jaunt around the neighborhood. Firstly, once I got up to 45+mph, I started feeling a weird rattling/vibration. I hadn't touch the tires or suspension, etc, but I realized that the drivers side caliper was seized, I guess from the earlier standing on the brakes in order to stop. When I jacked up the truck, I couldn't even spin the tire by hand, they were so tight.
I had to pull the caliper and take the brake line off to release pressure, then press the pistons back in with a c-clamp. The passenger side was fine.

Also, during the extended test drive, the brakes started to firm up again. Not near as bad as before, but definitely not normal.
After I released the pressure on the caliper, I re-bled the brakes (thank God for speed bleeders), but the issue remained. The check valve still blew fine, so it looks like this cheap-ass part was a bust. Luckily they had a good return policy, backed by eBay, etc.

I ordered one from Advance Auto for $330+ core charge, and it'll be in today. I'll post back.

 

[nevada]

the push rod is adjusted wrong. its too long and not letting the brakes fully release. to clarify, the pushrod between the booster and MC. you may have the same issue with the new one unless its adjusted properly.

 

[AntleredRuin]

the push rod is adjusted wrong. its too long and not letting the brakes fully release. to clarify, the pushrod between the booster and MC. you may have the same issue with the new one unless its adjusted properly.

Yup, I thought that too, but the pushrod on it was literally adjusted as far back as it could go. I even removed the entire rod and took off the retaining nut so it could thread in further, but no go.


I put the new (remanned) Advance Auto booster in last night, and now it's fixed.
I bled the front brakes again with a whole pint through each caliper and it brakes amazingly now.