Is it definitely my master cylinder?

[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

 

[mattman]

No fluid loss?

 

[dgoll75]

No fluid loss?

No sir none at all.

 

[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.

 

[mattman]

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]

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]

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]

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

 

[dgoll75]

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]

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]

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.

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]

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

 

[dgoll75]

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]

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]

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....

 

[AZ4Running]

Yeah this is not a booster issue. You would have a really hard pedal and it would be hard to stop. You have a 97 so you have a normal booster and master cylinder not the crazy expensive electronic integrated abs unit master cylinder.
First be sure you have bled all the air out of the master and the brakes. Start at the 2 lines on the master then go to the right rear, left rear, right front then left front, get plenty of brake fluid. Check inside the rubber dust boots on the brake calipers and the rear cylinders, you can have a leak in one but not see any leaks or major fluid lose. If all that is 100% then check to be sure you properly adjusted your rear brake shoes when you replaced them. Replacing all the springs, cleaning and lubing or the adjusters and contact points is a must. Adjust the rear shoes until the drums go on sung but not hard, parking brake should fully engage in 3 to 5 clicks. Rear drum brake adjusters just dont work as they should even in good working order they dont adjust out enough to keep up with shoe and drum wear. One of the reason cars and trucks have disc brakes on the front if not all 4 wheels. Good luck.

 

[LittleCaesar]

Have many times did the previous OP(s) change the brake fluid?

If it isn't a failing hose, I can't offer any more possible solutions. You may try to have a buddy press on the brakes in P engine running while visually inspecting all the lines and connections.

 

[dgoll75]

Yeah this is not a booster issue. You would have a really hard pedal and it would be hard to stop. You have a 97 so you have a normal booster and master cylinder not the crazy expensive electronic integrated abs unit master cylinder.
First be sure you have bled all the air out of the master and the brakes. Start at the 2 lines on the master then go to the right rear, left rear, right front then left front, get plenty of brake fluid. Check inside the rubber dust boots on the brake calipers and the rear cylinders, you can have a leak in one but not see any leaks or major fluid lose. If all that is 100% then check to be sure you properly adjusted your rear brake shoes when you replaced them. Replacing all the springs, cleaning and lubing or the adjusters and contact points is a must. Adjust the rear shoes until the drums go on sung but not hard, parking brake should fully engage in 3 to 5 clicks. Rear drum brake adjusters just dont work as they should even in good working order they dont adjust out enough to keep up with shoe and drum wear. One of the reason cars and trucks have disc brakes on the front if not all 4 wheels. Good luck.

I agree its not the booster. I am going to tear down my drums today and check both cylinders. I hope that its just a random instance where one has failed but not leaking enough to see any fluid. Its kind of my last resort.

 

[dgoll75]

In total I replaced the master and both rear wheel cylinders. Still the same. I bled forever seeing no air bubbles. Still the same. Disabled the ABS via the fuse removal. Still the same. Rear shoes look fine, e-brake system and front calipers look fine. I have spent about 7-8 hours on this and no resolution. I am going nuts..... HELP :frusty:

 

[mattman]

Seems like with the pedal sinking, the loss of pressure almost has to coincide with fluid loss. Maybe you're right that it is a really small leak. What about pulling the wheels and pumping the brakes to make the pedal sink 10-20 times and then inspecting? Hopefully if everything is clean you should be able to see evidence of even a small leak.