My 2010 has just over 200K miles and recently, after doing the rear brakes AGAIN (lots of salt here), when applying the brakes there is an area of nothing engaging, then it engages aggressively all at once. This is new and I have not changed driving style.

I also have the pump on the MC running a LOT now and I never remember hearing it before.

I believe the master cylinder is failing?

Can anyone help me diagnose this. The pedal does slowly go down when pumped up (Classic MC test), but it has always done that to a degree.

I have blead all four corners several times, like I have done many times throughout ownership. Something is wrong and I do not trust it, so I'm not driving it.

I would greatly appreciate any thoughts or insight.

Thanks

I tried a few Google searches in an attempt to find a resolution.
"Toyota master cylinder pump runs continuously"
"Toyota abs pump runs continuously"
"5th generation Toyota brakes grabbing"
"Toyota 4runner brake master cylinder forums"

I didn't find anyone reporting no brakes followed by grabbing brakes. It appears only the 4th generation has reports of a continuously running pump. I don't have anything specific to offer except for the following suggestions.

Make sure battery/alternator is OK because the braking system has electronic components.

Check brake hoses because mileage is greater than 200K. Clean abs hardware if dirty, check wiring for loose connections, etc.

Factory manual lists a multi-step Techstream diagnositic procedure which is not a DIY process (to protect the solenoids, Techstream turns off power automatically after 2 sec). According to a 5th gen forum post, the brake system and ECU stores codes without triggering a brake warning light. Have you checked for codes?

Bled the brakes again today, but I believe the MC is on its last leg. I have to push it way down to stop......then it stops very aggressively.

The pump is running every time I hit the pedal also, which I don't think is normal?

So, can I grab just any MC setup and run it, or does the computer system require a specific match?

Thanks

After reading your reply, I thought of trying one more Google search ...
"toyota master cylinder loud pump sound" I can't hear my pump, so I figure your pump is really loud.

Found this post: Brake Pedal Suction Noise?

Be sure to click "more results from toyota-4runner.org ... however, most of the hits are on the 4th generation forum.

Also click "more results from ih8mud.com ... this is a forum in Australia. One guy posted loud sound recordings ... Booster Pump / Accumulator Noise diagnosis | IH8MUD Forum ... check out some of the other ih8mud hits offered up by Google

I would think the ECU expects to "see" a 5th gen master cylinder.

This is kind of a guess, but could be failing brake booster perhaps? I’m not sure if MC is integrated into the booster on our trucks.

Shoot me a text nine one two six five six four four zero five, I will give you access to mitchell pro demand, you will probably find your answer there.

@Argyle Edison , I’ve had a similar problem for the last 12 months on my 2019 T4R Off Road.

What model do you have? The brake systems differ between models mostly due to crawl control electronics.

Suspicion is master cylinder or booster pump assembly.

Although, the electronics are also suspect as it improves somewhat after a zero point calibration and crawl control calibration. It’s not all related to air in the system or spent fluid.

Please keep us posted on your solution, I will as well.

- Mark

The master cylinder, brake booster pump, booster motor, accumulator, and ABS valve assembly are all one unit on 4Runners from 2001 on. I have some experience with earlier systems as I had one fail on my 2002 4Runner, but in a different way than you describe. My issue was a failed pressure switch in the ABS assembly which was not replaceable, which caused the pump to run longer than it should and overpressure the accumulator leading to a horrible dying seagull squawking sound on cold starts. The fix for me was replacing the whole assembly for ~$2,100 new.

Land Cruisers of a similar vintage also had a lot of problems with both the master cylinder and booster motor/pump/accumulator failing and I've seen some reports of issues on 4th gens and FJs as well. There are lots of documentation and comments on the ih8mud.com forum about the systems in the Land Cruisers.

FWIW, at least on the older systems (and I assume the newer ones too hopefully) there is a master cylinder rebuild kit (~$50) which can be used to replace the seals/pistons in the master cylinder portion of the system. Additionally, the booster motor/pump/accumulator can also be purchased and replaced separately (~$1,200). Finally, the whole stinking unit can be replaced ($2,100).

Here's the parts diagrams/prices for a 2010 4Runner. Hydraulic System for 2010 Toyota 4Runner | Boch Toyota South



It looks like on these newer trucks a lot more of the parts are individually available and replaceable than on my old 2002 system.

Hope you're able to figure out what's wrong with your truck.

Thanks everyone, I REALLY appreciate it. This runner has been awesome and still looks and runs fantastic. I have no intentions of ditching it anytime soon.

With 200,000+ miles it isn't a surprise that things are wearing out, and I really can't complain. I'll be ordering a used (likely) MC and going from there once I figure out which ones are compatible.

You guys rock

I have an SR5 with a 9/9 build date. No crawl control.


Does anyone know if a 10-12 (or other years) 4x4 v6 SR5 MC assemblies are interchangeable?

How thoroughly did you bleed the hydraulic system during your brake job? (regardless of answer, if the problem began after the brake job then that's where the problem likely is, e.g. lots of air in one or both rear calipers).

That's a legit question.

I have done it three times now and feel confident there is no air i the calipers, but I do not know if or how the ABS system has been effected, if at all. I have been a mechanic since I was 16 (I'm 48) in some capacity (in the military also) and I bled them the old fashioned way as I have done many times before on this vehicle.

I certainly could be missing something with the system complexity.

You have air somewhere in a system, pump sense no pressure build up and pump it in. Beauty of 4runner 5th it bleeds by itself. Dealership service using software can make bleed system. I recommend complete overhaul all brake calipers and after that bleed system. Bleeding is easy, just have big bottle of brake fluid. Have small hose to brake caliper, and other end of hose into bottle with fluid thru bottle cap. Make it tight, couse you have to keep bottle upside down fluid cover hose end inside bottle. affix the bottle like that, open bleed valve and turn ignition - no starting engine. Observe action inside bottle. Cut bottom of plastic bottle if need to add or remove fluid. Go around from wheel to wheel. Add fluid to main cylinder. May need two rounds. Conventional bleeding procedure not good for T4R5.