GMT800 MC Swap Gone Wrong, It Sucks Bad

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Supercharged111

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I snagged a GMT800 master at the boneyard yesterday and threw it in and it didn't do anything until I was damn near on the floor. It feels like the pin is just too short so now I don't know WTF to do. I've already bent my lines for the GMT800 master and I don't want to bend them back to throw my old MC in and rebend them when I get a good MC. Has anyone ever had this happen before? When it grabs it grabs, but there is no pedal travel left for the brakes to work. It's downright dangerous as it sits now so I can't even take the ****** to work. I've never known a master to fail like this, only to stop holding pressure altogether. I'll be SUPER pissed if I break down and buy a new one and have the exact same problem.
 

GaryCCR

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This might sound silly but, did you bleed the lines? If you pump the pedal, does it hold pressure? It could be a bad MC or air in the lines. What year/model is your truck?
 

Supercharged111

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Guess I forgot to mention I'be already gone through 2 quarts of DOT4 so yeah I'be bled the **** out of it and there's no air coming out at all.
 

GaryCCR

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If you pump the pedal, does it hold pressure? What year/model is your truck? Did you bleed the ABS module?
 

Supercharged111

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98 with ABS delete and TBI prop valve. The problem ended up being that a wiring harness support bracket landed itself between the mc and booster. Not sure if that was factory or accident. I just noticed that there was a small gap between the new mc and booster when I went back out to quadruple check everything. Once I moved that bracket between the mc and prop valve bracket (thus sinking the mc farther into the booster) the truck stops much better than before. It would never lock a tire on dry pavement before but I had them barking at 50 no problem. This stack of brackets is a combo I believe Chevy never offered and is unique to my bastardized braking system. I'll see what kind of adjustment the rear brakes will take tomorrow to see how much more I can get. Below you can see the 2 brackets that are mashed in there.

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EppNation

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Pretty sure most trucks have a bracket hanging off the MC studs for one thing or another. I have one that supports all the PS tubes in/put of hydroboost.

Every application I've seen though, they are in between MC and nuts. They are always last to go on before a dab of blue loctite and the nuts.

Having to daily drive through a brake diag isn't fun. Glad to hear you've made progress!

Sent from my Nexus 5 using Tapatalk
 
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