2004 GMC Master Cylinder Resevoir ?

2004 GMC Master Cylinder Resevoir ?

Postby bigdaddie [OP] » August 2nd 2012, 11:42pm

Hi Folks, i have enjoyed your forum for a while now, first time i have not found an answer to my question..........

I love these vehicles, i now own a 2004 GMC safari AWD, 147,000 miles, my first was a 1996 GMC AWD that was totaled by a little Ol' Lady rear-ending me, folded the van half way up the chassis......anyway....

I have a leak in my master cylinder reservoir, its actually loose, i can wiggle it a little, and i'm loosing fluid at the base where it mounts to the Master cylinder....

My question is, the reservoir sets into 2 rounds holes on the master cylinder, and 2 pins go through the master cylinder to hold the reservoir in place. I've tried pulling these out with plyers, and they don't move, i'm afraid to try and use a center punch, seems that would expand them to the point that i couldn't remove them..............

Any Help??

Thanks
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Re: 2004 GMC Master Cylinder Resevoir ?

Postby Corsemoto » August 3rd 2012, 2:20pm

Find or modify a punch (or an actual roll pin punch) that just fits over the edge of the pin so it won't try and expand, penetrating oil may help loosen. Once removed, you may be able to match up the two o-rings that seal the reservoir to master cylinder. If you do, make sure to use o-rings suitable for brake fluid - EPDM (not Buna-N or Viton).

If not you can buy a new reservoir which comes with o-rings and retaining pins.
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Re: 2004 GMC Master Cylinder Resevoir ?

Postby bigdaddie [OP] » August 3rd 2012, 4:11pm

Corsemoto wrote:Find or modify a punch (or an actual roll pin punch) that just fits over the edge of the pin so it won't try and expand, penetrating oil may help loosen. Once removed, you may be able to match up the two o-rings that seal the reservoir to master cylinder. If you do, make sure to use o-rings suitable for brake fluid - EPDM (not Buna-N or Viton).

If not you can buy a new reservoir which comes with o-rings and retaining pins.


thanks, i will try the larger size punch, i have a set, just totally forgot about them.

I already purchased a new reservoir from RockAuto, comes with the new O rings and Pins.

Thanks for the help.
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Re: 2004 GMC Master Cylinder Resevoir ?

Postby bigdaddie [OP] » August 3rd 2012, 10:23pm

Maybe i need to ask another question relating to this, at least it's still a brake problem,

I replaced the reservoir, thanks for the tip, it was fairly easy, i think the O ring on the from portion was what was bad, but i'm glad i replaced the entire reservoir.

But i still have a problem with my dash warning lights, The ABS and Brake warning lights come on after i apply pressure to the brakes a couple, maybe three times.

I thought the problem was the leaking reservoir, causing me to loose fluid and pressure to the system that was originally the reason for these warning lights, evidently not.
I also just replaced the front brake pads, not the rotors, they were still in good shape, but the pads were just about gone, especially on the inside pad.

Could this ABS warning light be caused from just dirt and dust from the old pads, or maybe a bad sensor??
If a bad sensor, how do you check the sensors to determine if one or both are bad??
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Re: 2004 GMC Master Cylinder Resevoir ?

Postby chevymaher » August 3rd 2012, 11:56pm

Bleed the brakes maybe you got air in a line. That will do it.
Clean the sensors with brake cleaner. Metal shavings interfere with them and can cause it to.
I don't know if the newer vans have it or not, but a plug at the bottom of the fluid reservoir. Fluid level sensor. Dirty conection where it plugs in if it has one.
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Re: 2004 GMC Master Cylinder Resevoir ?

Postby Corsemoto » August 4th 2012, 2:02am

Definitely start by bleeding brakes to make sure there isn't air but if both ABS and brake lights are on, it is sounds like it is ebcm related which is not uncommon.

Here is some more info on testing ebcm and related sensors:

http://www.modulemaster.com/store/instructions/ABS/Chevy-Truck_Astrovan_ABS.pdf

https://www.astrosafari.com/viewtopic.php?f=45&t=13496
Last edited by Corsemoto on August 4th 2012, 4:08pm, edited 1 time in total.
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Re: 2004 GMC Master Cylinder Resevoir ?

Postby bigdaddie [OP] » August 4th 2012, 3:53pm

thanks for the information.
I hadn't driven the Van since i installed the new reservoir, so this morning i took it for about a 20 mile drive.

The Brake light and ABS light prior to today would not come on right away after starting the van, i would have to engage the brakes maybe 3 or 4 times, then the warning lights would come on.

This morning, as soon as i started the van, they were on. I turned the van off and restarted, still the same.

Prior to installing the new reservoir, my brakes were a little soft, i knew i was loosing pressure in the system because of the leak.
This morning, the Brakes are very firm, secure, no hesitation or softness to them what so ever.

I will first check the ABS wheel sensors, hopefully that is what the problem is, becuase of the firmness of the brakes, i don't think i have any air in the system.

thanks again for the help.
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Re: 2004 GMC Master Cylinder Resevoir ?

Postby bigdaddie [OP] » August 4th 2012, 3:59pm

Corsemoto wrote:Definitely start by bleeding brakes to make sure there isn't air but if both ABS and brake lights are on, it is sounds like it is ebcm related which is not uncommon.

Here is some more info on testing ebcm and related sensors:

http://www.modulemaster.com/store/instructions/ABS/Chevy-Truck_Astrovan_ABS.pdf

http://www.modulemaster.com/store/instructions/ABS/Chevy-Truck_Astrovan_ABS.pdf


Corsemoto, i do have specific question concerning your info, the PDF file is great for removal and installation, but how would i test the unit that is on my van now?? :ty:
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Re: 2004 GMC Master Cylinder Resevoir ?

Postby Corsemoto » August 4th 2012, 4:09pm

The second link above was incorrect, I updated it but there is more info in this thread:

https://www.astrosafari.com/viewtopic.php?f=45&t=13496
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Re: 2004 GMC Master Cylinder Resevoir ?

Postby bigdaddie [OP] » August 4th 2012, 6:36pm

thank you, the in depth information provide on this site is unbeleivable, thanks again.
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Re: 2004 GMC Master Cylinder Resevoir ?

Postby Corsemoto » August 5th 2012, 12:42pm

Sure thing, hope it helps.

It's best to be able to scan for ABS codes but most shops will charge for this. When I had mine scanned, it showed a code for a fault with ebcm (don't remember the exact code) but I still wanted to test and rule out other things first, like wheel speed sensors.

When I had my module repaired by Atlanta Speedometer Repair, they told me that when the ebcm fails it will usually have other failure codes stored for things like faulty wheel speed sensor but that doesn't necessarily mean that item is faulty. This was the case with mine, they noted all the stored codes in ebcm (including a wheel speed sensor fault) and told me this is fairly common. Installed repaired ebcm and everything worked great, no problems with sensors, etc.
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