Is it definitely my master cylinder? - Toyota 4Runner Forum

dgoll75 · 01-21-2015, 06:29 PM

Heres the scoop: I currently have 166k and I did Tundra front brake upgrade, new rear pads, and rear axle seals roughly 6000 miles ago with no issues. Fast forward to early today when after a complete stop my pedal faded and I started to creep forward. Happened a couple times throughout the day but oddly to me not every time I stopped. I drove about 40 miles today total. I got under her when I got home and see no leaks or any evident problems. Definitely my master cylinder? And if so whats the best replacement "how to" thread you can recommend? Thanks

mattman · 01-21-2015, 07:21 PM

No fluid loss?

dgoll75 · 01-21-2015, 07:23 PM

Quote:

Originally Posted by mattman

No fluid loss?

No sir none at all.

mattman · 01-21-2015, 07:31 PM

Hmm - that is odd that it was fine for so long and I haven't read of master cylinder failures on these. Although it's possible for sure. Hopefully someone else can chime in on how to isolate whether it's the MC or booster.

mattman · 01-21-2015, 07:46 PM

You know best how it felt before and after the work. I guess it doesn't cost you anything to bleed everything again and see if there's any change or if you get any more clues

dgoll75 · 01-21-2015, 08:01 PM

Exactly: ODD. And to add to the confusion it was not before and after work with a long sit time. I drove the 40 miles off and on all day, truck never sat more than 1 1/2 hours today. So I guess bleeding is a start but I am not sure how air would have been introduced into the system for no reason.

HammerHead · 01-21-2015, 09:03 PM

Does your emergency break work? I've been having problems with the rear adjusters. I've had to readjust 3 times since June and have been good for the last month. Knock on wood.....

LittleCaesar · 01-21-2015, 09:26 PM

How did the rear wheel cylinders look when all the rear brake work was performed?

dgoll75 · 01-21-2015, 09:57 PM

Quote:

Originally Posted by HammerHead

Does your emergency break work? I've been having problems with the rear adjusters. I've had to readjust 3 times since June and have been good for the last month. Knock on wood.....

Just checked and it seems to be working well. Gets tight after about 6-7 clicks. Wheel cylinders seemed fine too. And I just did a couple tests I read on another of our threads moments ago; I counted as the pedal sank and it took about 45 seconds to the floor, I also tested the vacuum pressure into the booster and its strong. Now I am pretty positive that I am dealing with a master cylinder... I have never done one.

kammando808 · 01-22-2015, 02:52 AM

Damn I'm in the same situation, no leaks anywhere to be found and I've bleed the brakes more than enough times, although if I hit the brakes fast I stop no problem but if I casually drive and do a lot of stops my pedal slowly sinks to the bottom...I'm guessing my mc is also goin out and would like more info on changing it out

amalik · 01-22-2015, 02:57 AM

Quote:

Originally Posted by dgoll75

Heres the scoop: I currently have 166k and I did Tundra front brake upgrade, new rear pads, and rear axle seals roughly 6000 miles ago with no issues. Fast forward to early today when after a complete stop my pedal faded and I started to creep forward. Happened a couple times throughout the day but oddly to me not every time I stopped. I drove about 40 miles today total. I got under her when I got home and see no leaks or any evident problems. Definitely my master cylinder? And if so whats the best replacement "how to" thread you can recommend? Thanks

How do your front pads look in terms of thickness? You mentioned a replacement of rear pads (Was this a typo? Rear has shoes)

Also -- You should be able to get the MC tested for free (And get a quote) from a service shop like Meineke or Midas/etc. It's worth a shot. My understanding is they test it with shop air/liquid/etc and tell you if it has a leak. Kind of like a leakdown test on cylinders. Maybe someone who wrenches for a living can chime in a little better.

Quote:

Originally Posted by mattman

Hmm - that is odd that it was fine for so long and I haven't read of master cylinder failures on these. Although it's possible for sure. Hopefully someone else can chime in on how to isolate whether it's the MC or booster.

The brake booster utilizes engine vacuum to assist in making braking (pushing the brake pedal) much easier. A really tough pedal would be a symptom of the booster going bad. That's how you can differentiate, IMO. OP stated his pedal is not hard, but rather sinks to the floor and the truck isn't braking.

dgoll75 · 01-22-2015, 09:14 AM

Decided to replace MC. Any good "how to" easy to follow thread suggestion?

dgoll75 · 01-22-2015, 09:18 PM

This is Nucking Futs.... I just replaced the master cylinder with a BRAND NEW not rebuilt unit and bled all 4 corners a bunch. ORIGINAL SYMPTOMS STILL HAPPENING. Exactly the same, Brakes working briefly than fading to the floor within about 30-45 seconds. Same results whether engine is running or not albeit maybe takes 50-60 seconds when motor turned off. I am leaking NO fluid so it can't be the rear wheel cylinders I don't think, and the e-brake works perfect as well. HELPPPPPP>>>>> PLEASE!!!!

LittleCaesar · 01-22-2015, 09:38 PM

Two other possibilities come to mind:

1. Failing brake hose, but not totally gone yet.

2. Bad ABS modulator, where a valve is opening just enough. But I have learned that if they fail, the valve usually stays open.

dgoll75 · 01-22-2015, 09:50 PM

Quote:

Originally Posted by LittleCaesar

Two other possibilities come to mind:

1. Failing brake hose, but not totally gone yet.

2. Bad ABS modulator, where a valve is opening just enough. But I have learned that if they fail, the valve usually stays open.

1.I doubt it. I am in N FL and its pretty much never been off road, no leaking anywhere, and there is zero rust anywhere on any hard lines on the truck.
2.I did disable the ABS to test the theory, still the same....

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