FYI- Later model GM truck Stabilitrak and service brake system soon due to leaking master cylinder into booster.

69hurstolds

69hurstolds

Master Mechanic
Jan 2, 2006
398
204
43
#1
Who'd have thunk it? My 62K mile 2010 GMC Sierra Crew 1/2 ton, 2WD, 5.3 Flexfuel truck is dependable as hell. Just a few months ago I replaced the original battery in it. Besides oil/coolant changes and a new set of tires, everything else it factory supplied. Even the brake fluid.

I usually check the oil at least every time I get gas and do a cursory check of belts, and other underhood levels and stuff. That was last done about a week ago. As usual, I added a little windshield wiper fluid and noted the brake fluid level was a little lower than "Max" on the reservoir. No biggie, with 62K miles on it, I would expect the volume to drop a bit to make up for normal brake wear.

So, the other day I'm going around town and "bing bing bing" idiot alarm goes off, and on the DIC it shows "Stabilitrak OFF" and flicked back and forth between that and "Service Brake System Soon". The stabilitrak triangle with a skidding car symbol is lit up on the dash. I get to where I'm going, shut it off, and went into the building. When I came back out and cranked up the truck again....nothing. Everything seems normal. I pop the hood and start looking around and noticed the reservoir level above MIN, but a whole lot lower than last week. Great. I look everywhere for a leak. Nothing. Hmmm.
 
69hurstolds

69hurstolds

Master Mechanic
Jan 2, 2006
398
204
43
#2
Go back home, the light comes on again. Same thing. Get home, plug in my scan tool, and NO CODES! Whatever it is shows to be intermittent and not a hard set.

Do a little research and come to the conclusion that the level switch is LIKELY bouncing around enough to engage the master cylinder low level input which certainly would give a service brake system soon light along with turning off "active braking" (aka Stabilitrak).

I almost said hell with it and refilled the reservoir to call it a day. But the question was still bugging me. Where's the leak? Where's the fluid going? Nothing at the rear wheels, nothing along the frame, nothing in the ABS block of crap, and nothing leaking at the front or under the truck. Further internet searches led me to a revelation about GM trucks of that era the the rear seal of the master cylinder can leak right INTO the brake booster filling it up and not having any external leaks. This still could mess with the Stabilitrak as the flashing brake fluid in the booster could mess with the vacuum (sensor on booster check valve monitors that) so I have to get it fixed.

Thus I pulled the master cylinder loose from the booster and sure enough, the back of the MC was wet, and looking inside the booster there was a puddle of dirty brake fluid all piled up in there. OMG.

Read up on the situation and the best thing was to replace the booster and the Master Cylinder. On my truck, Rockauto.com delivered both plus a gasket for the firewall for a total of just over $203.00
 
69hurstolds

69hurstolds

Master Mechanic
Jan 2, 2006
398
204
43
#3
Here's some pics. Note the MC with the red arrow showing current brake fluid level.

And in the hole in the front of the booster, you can see where the brake fluid is puddling. I probably could drain and try to save the booster, but fugg it. It'd probably corrode up stuff in there and screw me at the time I needed brakes the most.



master-cylinder-leaking-jpg.94583
brake-fluid-in-booster-jpg.94584
 
69hurstolds

69hurstolds

Master Mechanic
Jan 2, 2006
398
204
43
#4
I got to give a shout out to rockauto.com. When I found out all this crap early afternoon yesterday, I hopped on rockauto and started searching for the parts. Luckily, they had everything I needed and it was all GM ACDelco stuff. Loaded up the cart, my 5% discount code, paid the bill and it was on its way. And it arrived today about 24 hours after I ordered. Unfortunately, the MC's are NOT rebuildable. I think if I just had a seal kit I could make it work. Ain't like the old days.

But with that, here's what I'm finding interesting now about the part number labels. Apparently, in an effort to thwart Chinesium parts being peddled as Genuine GM stuff, GM is now putting some security holograms on their part number labels. GREAT IDEA! (until someone in China copies that)

I haven't bought any new GM parts lately, but this is the first time I've seen this hologram marking. Pretty cool.

IF you have a GM truck that has a mystery loss of brake fluid that is equipped with Stabilitrak (JL4) and looks like the stuff I have under the hood (vacuum disc/drum brakes, nothing special) and your booster p/n (located on the bottom on a sticker) is either 20877277 or 20896308, then order ACDelco p/n 178-0800. If you have the AFM coded master cylinder 20845345, you'll want to order ACDelco number 174-1159. The firewall gasket is GM# 15974046.

Check out these cool holograms on the part number labels...
img_2314-jpg.94585

img_2315-jpg.94586
 
pagrunt

pagrunt

Royal Smart Person
Sep 14, 2014
2,330
1,150
113
Shelocta, Pa
#5
But with that, here's what I'm finding interesting now about the part number labels. Apparently, in an effort to thwart Chinesium parts being peddled as Genuine GM stuff, GM is now putting some security holograms on their part number labels. GREAT IDEA! (until someone in China copies that)
That is until they start printing them in China so they can have them there.
 
69hurstolds

69hurstolds

Master Mechanic
Jan 2, 2006
398
204
43
#6
That is until they start printing them in China so they can have them there.
Yeah, I was thinking they got a Chinese printing company to make their new labels. :p
 
GP403

GP403

Administrator
Site Admin
Moderator
Feb 25, 2005
2,901
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Inola, OK
#7
you should have tagged RockAuto so they see it (I just did!) :banana:
 
motorheadmike

motorheadmike

Comic Book Super Hero
Nov 18, 2009
2,622
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Ottawa, Ontario Canada
#9
Nice!
 
69hurstolds

69hurstolds

Master Mechanic
Jan 2, 2006
398
204
43
#10
you should have tagged RockAuto so they see it (I just did!) :banana:
Thank you. Didn't realize you could do that here. Yes, they have always been fast to ship. It helps I live close to one of their warehouses. But even when they're not the lowest price, which most times they are, they certainly are one of the fastest. Even faster than Amazon. And I just paid the normal shipping rate, no overnight charges of a bazillion dollars.
 

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