Brake Issues- Master Cylinder?

Guys, need your help. My brake pedal is spongy, which I know is a typical issue with Tacomas. One thing I noticed though... if I slowly press the brakes the pedal will go to the floor. I suspect the master cylinder is bad. The truck will stop reasonably okay when I brake normally, but it feels soft. No brake fluid leaks detected after thorough check. Brake fluid replaced every 2 years.

2006 PreRunner DCSB with 95K miles.

 

Air in the lines?

 

I wonder... but they were bled using a pressure bleeder. I guess I can try again and see.

 

Honestly, this is probably normal .... our brakes are weird sometimes. I know one can feel spongy sometimes, and firm other times.

Maybe check the brake booster as well. That could be bad.

 

the pedal to the floor is not normal. check for air first. then look at the master.

 

You have to be careful when bleeding Toyota, their reservoirs look like 1 part but it's actually 2 separate tanks that share a high volume overflow. As the brake fluid level drops beyond a certain point the front and back no longer share the same fluid reservoir storage. This is a safety feature, if there's a leak in the rear, the fluid will drop and eventually isolate the rear lines from the front allowing the rear fluid level to deplete entirely but leave enough fluid in the front to function to bring the vehicle to an emergency stop, and vice-versa.

Typically what ends up happening is you can see the front reservoir and it looks like it has plenty of fluid when in reality you've been pressure bleeding air into the lines for the empty rear reservoir. This air then gets trapped in the ABS block and the fluid begins to bypass the block meaning you won't be getting air out at the bleed screws but your system is slap full of big ol bubbles.

You can never let the fluid level get below the minimum line when bleeding, this ensures both the front and back divided reservoirs maintain enough fluid.

If your pedal wasn't going to the floor before you bled your brakes then I suspect your master cylinder is fine.

If the pedal was fine before I can almost guarantee you have air trapped in your ABS solenoids and, unfortunately, the only way to bled those is via a dealer or a shop that has the all data tool that can actuate the ABS solenoids while bleeding.

Takes about an hour and a half, cost is about $250.

Seen this and fixed it as a tech, and also done this to myself as a short sight even when I knew better because I was tired and pressure bleeding by myself.

Ended up with similar if not the same symptoms as your own.

Good luck, come back and post what ultimately was the fix when you git 'er figured.

 

Here is what I found out. I went ahead and replaced the master cylinder and all the brake hoses. I chose to do this because the truck is 12 years old and rubber degrades over time. Overall, it most likely was air in the lines, but replacing the parts helped me learn a little more about my truck and I now have the peace of mind that the core components in my brake system are new.

Five things to remember--

1) Removing the spare tire to replace the rear hoses is a MUST. Don't even try it without doing this first.
2) Doing a two-man bleed is better than just pressure bleeding. The force of the pedal pushing the fluid out works better to remove air than the paltry flow of a 15psi pressure bleeder. If you are simply trying to replace the fluid in a system that doesnt have air in it, a pressure bleeder is better though.
3) Before replacing the hoses, bleed one wheel so that the pedal goes to the floor and have your buddy keep it to the floor while you replace the hoses. This will lock the fluid and keep the mess to a minimum.
4) In replacing the master cylinder- read Key-Rei's post above. I accidentally let my cylinder get too low and had to re-bleed all the lines again. Check your fluid level after every 2 or 3 cycles of bleeding. It may be a pain, but its cheap insurance. The front brakes will bleed out way more fluid per cycle than the rears.
5) Found out that using Techstream to try to "Air Bleed" my 2006 truck ABS showed that "this utility is not available for this vehicle". The shop manual doesnt mention anything about bleeding the ABS either. I am not sure if I need to force the ABS to fire off and then re-bleed or if its fine doing a traditional bleed.

I went through almost two 32oz bottles of brake fluid to be sure I got all the air out, as well as re-bleeding due to a mistake.

 

Thanks for sharing the conclusion.

So many never return to share the final results of a problem.

 

I bet its the master cylinder, I replaced mine last year and my spongy pedal problem persisted so I replaced everything else I could think of and still my spongy pedal problem persisted so out of exasperation I replaced the master cylinder a second time and the problem went away. Now my brake pedal is solid as a rock.

 

I'm having this problem right now I'd had it bled from 2 different shops they both said the master cylinder was bad

 

did you ever figure out the final cause/fix?

 

no the master cylinder was changed out but the problem still is there I'm going to change out the brake booster next

 

But did you bleed the M/C on the truck according to the instructions of the FSM? I would not throw more parts at it until this were first done.