Master Cylinder Failure - Solved

When I hit the first stop light after leaving home this morning I noticed that the brake pedal had become mushy (but not all the way to the floor), and that the brake lamp on the dash was illuminated. I've been to this movie before, so I turned around and headed home to get a different car.

I just now had the chance to crawl underneath and check it out. The Master Cylinder reservoir was nearly empty, so I topped it off, pumped the pedal for a while, and started looking for a leak. Nothing! Tried some more pumping and more checking...still no visible leaks. Checked the reservoir again, now nearly empty! Hmmm (I thought parenthetically), where's the fluid going?

My suspicion is that the Master Cylinder has failed, but I'm still perplexed because the fluid has to go somewhere (2 reservoirs full is quite a lot). It's not running down the pedal inside the cab. I suppose that the booster may be filling up with fluid, but wouldn't that get sucked into the manifold and start smoking out the exhaust? (Maybe it takes 3 reservoirs full before that happens?)

Does anyone have any other ideas, or better yet, experience with a similar failure?

PS: I don't have the unicorn master cylinder

 

I'd unbolt the master cylinder from the booster to see if it's wet or dry in there, you should be able to unbolt it and reinstall without any effect on the system.

 

Could you also remove vacuum hose/check valve from booster and use something like a somewhat long wire tie or equivalent as a dipstick and check your unwanted fluid level in booster

 

Winner winner, chicken diner! I guess I'll go get a nice new one...

 

I have new calipers and hoses that have been laying around for a couple months because I just didn't feel like opening the brake lines...now I have no excuse.

It's old news, but there are no OEM MCs left in the universe. I wanted Raybestos, but it will be more than a week until I can get it. I chose Dorman because it will be here Monday. One can still obtain an OEM MC rebuild kit, so maybe I'll get that too and rebuild my original...just in case

Problem solved (I hope)

 

I'll consider something like this to evacuate the fluid so hopefully I won't have to pull the booster as well.

 

After a good night's sleep and a cup of coffee, I've decided that I may aw well replace the booster too. Why not, it's only money...I'll get more.

 

Hmmm...I wonder if this is an OEM Master Cylinder?

 

I got rid of mine so I don't have pictures of the markings on the bottom to compare. I do have this picture and it was definitely stamped Aisin 3/4". Yours might be smaller than 3/4" since AC, I can't remember off the top of my head.





Do you have that slave cylinder thingy in-between the MC and booster?

 

Yep.

 

Does it have Aisin or a size stamped on the side?

 

I thought the same thing but when I ordered one it was wrong. Double check the part numbers when you order. I wrote about my experience here.

https://www.tundras.com/threads/wha...t-gen-tundra-today.2558/page-865#post-2118342

Also, you'll need a new paper seal after you replace.

 

Mine says only 13/16" on the top side.

 

For my VIN it shows a MC rebuild kit 04493-60330, which is available from all the online dealers (I chose Ourisman)

https://parts.ourismantoyotaofrichm...ta-brake-master-cylinder-repair-kit-449360330

 

Gotcha, thanks. I couldn't remember the size difference on them between AC and DC.

My Sequoia MC is stamped 15/16".

 

The gasket between the booster and the firewall clearly has an "up" and a "down"...does anyone know which is which?

 

Oops, never mind...I found it. Now the $64 question is why?

 

back in the day “why” usually got me an ass whoopin

Today….I reckon cause FSM says so

 

Lots of shiny new brake parts! I'm using the Motive system "dry". It seems to be OK, but I've had to do it several times to get all the air out. Yes, I bench bled the new Master Cylinder first. Still a bit spongy...I'll go at it again after early tomorrow morning when it's not so hot (now I know why we measure temperature in degrees "F"!!!).

 

UH, where's all your evap shit??

 

2004. Pretty sure EVAP can in that loc was only a 2000-2001 thing. Maybe thru 2002?

 

Really. So much room for activities then!!