I'm at my wits end- Master Cylinder issues..

Thanks for the update!

Any notes on the install? Or pics?

 

Hello Again,

This whole episode is the gift that keeps on giving! Like I said above, the SOS Sequoia MC conversion went pretty smooth and seemed to perform acceptably with no tweaking of the LSPV. SOS sent me another MC to replace the one that leaked, and guess what:?? It leaked too!! And, like that wasn't enough, the Toyota OEM booster/MC assembly that I had ordered "way back when" finally showed up on my doorstep. So, considering the problems with the SOS MC leakage, I decided to just return the SOS stuff and install the stock assembly. That's what I did, and the truck seems to be fine now. Wow, what a nightmare!! Tom at SOS was very nice about returning the kit for a refund, and stated that he had had some leakage problems with his particular supplier's MCs. I told him that, if I ever have occasion to go the Sequoia MC route again, I will just plan to pay extra for an AISIN Toyota OEM MC, just for the added peace of mind.

So ........ I guess this saga is overwith, at least for now. Sorry, no pics from the SOS installation. And really nothing to show in a pic right now - the truck looks exactly like stock (because it IS exactly like stock).

Cheers and good luck to all!!

 

Good to hear from you, Mac.

That really stinks you've been having problems with the MC from SOS. At least you're back to stock at least for now -- without any issues I hope. I didn't have a problem with mine leaking, but mine was ordered and installed well over a year ago and could be from a different source. I was actually concerned at first that the SOS MC I received wasn't stamped with Aisin. It hasn't caused me problems yet, but I'll probably eye it over ever so often.

Not really related, but I've started to look into retrofitting one of the fancy hydro-boost boosters onto a vacuum booster truck like ours. I found a company that sells the parts for a conversion, to be paired with an even larger(!) Wilwood master cylinder. I'm still looking into this and may do it at some point as a way to upgrade my brakes even more without going to bigger wheels. If I find more information or pursue this, I'll include some info on this thread.

 

Des2Mtn ---

Thanks for the reply. I quite enjoy the back & forth, and the updates on what folks are up to with their trucks. I had no complaints about the SOS update, other than the fact that they have a MC supplier who apparently does not have a robust QA system. In the industry I retired from, Aerospace & Aviation structural component manufacture, a QA system that allows a defective part to get shipped is a huge no-no. When you let a bad part slip out the door, it's a HUGE deal, and the FAA, NASA, the prime contractor, your QA system certification body, and many other people get very excited and very testy.

On both of the SOS MCs I dealt with, the fluid dripped out of the cavity on the side of the reservoir, where you install the level sensor. It wasn't a gushing leak, just a steady drip, but enough to just about empty the reservoir overnight. Completely unacceptable for a key brake component. I have no idea how the manufacturing process for the plastic reservoir goes, but I suspect some kind of secondary bonding or fusion process with a detail-part (the "well" for the sensor) that's molded separately from the body of the reservoir and added during an assembly operation. However it works, it's a process that needs some attention.

The hydro-boost system sounds interesting, and I hope you have good luck with it if you decide to go that route. I would be interested in hearing about your progress, although that sounds like something that's never going to happen with my truck. As long as I can haul my camper around in the back-country, and have good enough braking to be reliable and safe, I'm happy. Back in my sports-car days, it would have been a different story, but those days are probably gone for good - other priorities, limited budget - you know the story!

This has been fun. I will keep an eye on this thread and see if anyone has any updates along the way. Many thanks to you and everyone else. My best!!

P. Mc.

 

I am selling my used master cylinder with booster. Before I put it on eBay wanted to give first dibs on this site since you guys been really helpful. It needs to be rebuilt. Let me know.

 

Is this the website you used? Can you confirm if you were ever able to receive the unit? I'm thinking of getting a 2004 Dbl Cab for my son but remembered this thread. It seems the 2004 Tundra to avoid is the one with stability control.

 

FREE OEM Toyota unicorn master cylinder available for diagnostics / rebuilding - you pay the shipping charges. I missed my core return window.

Former 2006 Tundra Double Cab 4x4 owner here. I struggled with ongoing brake issues similar to many of you and replaced most of my brake components over the course of a few years. I replaced the unicorn master cylinder with a rebuilt BrakeBest 11-3300 master cylinder from O'Reilly after finding my brake booster was doubling as a brake fluid reservoir / swimming pool earlier this year. I still had spongy brakes after several bouts of brake bleeding. I have my original master cylinder available to anyone interested in tearing into it do determine its failure point and potentially rebuild it - just message me and pay me shipping charges and it is yours!

Location: Minneapolis, MN

 

1st post for me here and I'm bringing this back up. I recently bought an 05' DC 4x4 ABS non-VSC as a hunting truck and have been doing some needed maintenance. I have this issue. Anyone come up with a viable work around or is the only option to drop 1K on a new assembly? I've looked at the Sienne option but it looks like that part is now 1K too. I've heard rumblings of using T100 booster and MC with cutting 1/2" off the shaft but have questions on whether that will work for my model. I appreciate any help provided. Thanks Mike

 

I seem to have fixed my leaky unicorn piece with a $2.62 seal!
Pulled master cylinder and saw leaking fluid. So I pulled the snap ring off the unicorn piece and removed the shaft (never uncoupled unicorn from master cylinder). service manual said if shaft is dropped it can't be reused, though they may have been referring to the inner shaft between the unicorn piece and master cylinder. I was careful regardless.
Saw that the oring was good and leak was coming from around shaft. Pulled apart the spacers on shaft to see shaft seal and saw it was kinked sideways in its nest. It was obviously the source of leak and everything else looked fine so didn't disassemble further. But if you got fluid leaking into booster then its gotta be either the oring or shaft seal. Hopefully your leak hasnt been so bad for so long that the shaft has damage.


I happen to live in the oil patch so there are gasket stores or hydraulic shops on every corner. I found the fastest caliper in the west at Gasket Services in Odessa, tx. He explained its a common U-cup seal but the material, EPDM, is less common.
11/16" x 1" x 5/32" is the closest standard replacement size. But could only find that size in Nitrile or some other material you do not want to use with brake fluid.
16mm x 25.5mm x 4mm seems to be the correct metric replacement size. Again couldn't find any in EPDM.
Ended up using a 5/8" x 1" x 3/16" EPDM gasket at the O ring store online. Part no 6226-48
https://www.theoringstore.com/store...FGMXzDJGy4ttUAE_1EECqv21LFJmPV1eLSHDp-A5bmT1C
There was also a nearly microscopic part no on the original seal. it was of no help, so ill just add it so you dont waste your time trying to read it. A23 (A28?) 132291-19020 (the 2nd numbers 3 & 9 could maybe be 8 or 0)

Sizes are ID x OD x Thickness
The 5/8 ID is a smidge tighter than its original metric size, but its nicely snug and shaft slides freely. 1" OD is a smidge large but fits nice and snug. The 3/16" thickness is too thick by 1/32 or more, and sticks out past its surround by a hair. This extra thickness makes the shaft preload the master cylinder plunger by that much more. The extra preload might not affect anything but I was worried about interfering with whatever valves and passages are inside a master cylinder so I did my best to sand down the back and front faces of the seal. EPDM doesnt sand well. After an hours of grinding down the faces by hand with some find sand paper, I had reduced the protrusion by maybe half. But it also weakened what protrusion was left and made it more pliable and easier to squish flat. It still didnt sit flush but pretty dang close.
I reinstalled shaft and master cylinder and everythings working like it should now. Though Im keeping the spare seals in the glove box in case. I also just completed repair today so I'll update if it fails.
The part is only $2.26 but the site has a $5 minimum. Plus I paid $30 for 2 day shipping. So $35 and 36 hours later I had normal brakes again.

 

Talk about a first post! Appreciate the thorough info!

 

Thanks. Maybe someone can use the details to find a seal that fits better.

It's been over a week, including hundreds of miles with a full load and mountains, and brake is as good as ever.
I just bought the truck and it needed a ton of parts. Aside from reseating the wheel sensors to get the truck to stop mooing when coming to a stop, this ended up being the cheapest fix. Glad the thread was here to save me time looking for parts.

 

Pictures of the offending unicorn seal.

Mine rolled over inside the housing causing a leak into the booster.

It was deformed from the pressure so I removed the seal and let it sit for an afternoon and it regained its shape to a point. These pictures are after sitting and starting the reassembly.

I expected to find holes in the seal but all were in good shape.

It seemed to have bled ok and will see how long it lasts.

Disassembly is trivial with only 1 large c-clip.

I’ll post again if this fails.

 

Good luck, I hope it holds for you.

 

Good quality first post! Welcome to the community.

I'd be curious if any numbers were embossed on the seal, it may point to a manufacturer or OEM part number to buy a replacement, in the event perforation or tear does occur.

Toyota is typically using (at the time of our truck production) 10-digit hyphenated part numbers, like 12345-67890, a lot of times the 9th digit is a "C", and sometimes the 6th digit is an "A". Digits 1-5 are usually the part category, the latter 5 are the individual parts / their variations.

 

There is a number that's extremely hard to read and couldn't find any parts that match the number. Closest that came up from a similar part number was an entirely different seal.

 

Maybe take a pic with your phone and then zoom in on said pic. My eyes aren't what they used to be and this always worked.

 

Did that and like 50 other things to read the number. Also had another person double check my work.
I took a micro camera and shined a micro light on the numbers which helped illuminated the edges of the imprinted numbers and i could read them that way. in my comment detailing my work, post #189, I list the part number that I found on the seal, though it was very hard to decipher if a number was a 3, 8 or 9.
Was unable to find a part for sale that matched, but anyone is welcome to try using the numbers. In the end a simple 3 dollar non-Toyota replacement seal fixed it just fine.

 

I couldn’t see any numbers visible to the eye.

If I have an issue again, and I find the seal I’ll let the powers the be here know. This thread is quite useful!

 

It's nearly microscopic and on the inside lip, inside the channel of the seal.

 

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With only 91K on my 2005, I will have to keep this issue in mind.



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If you have a regular cab then your master cylinder doesn't have the added unicorn part.

 

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Thanks! Good news, for a change.



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