GMT800 master cylinder swap problem

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slowburb

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A braking setup is a complicated system. Piles of R&D has gone into engineering. It isnt a system that you can add a part or a few parts and notice a dramatic improvement. I don't know how or what really happened when I panic stopped my truck but it is not happening again. I don't even need to know. Anew older thread on this message board put the pieces together for me. It is dangerous to ride around with a mismatched combination of parts. Your calipers, rotors and pads need to match your master cylinder and booster and pedal, and your drums and shoes, and your proportioning valve. Then of course all parts must be in good repair adjustment and working order. This all factored together allows you to controllably apply proper brake bias to stop your truck safely. Too much or not enough effect from any of these parts compromises performance and in this case safety.

JB6 is the RPO on my glove box sticker. It designates the GVWR of my truck at 7200# and that I have a vacuum brake booster.
 

michael hurd

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A braking setup is a complicated system. Piles of R&D has gone into engineering. It isnt a system that you can add a part or a few parts and notice a dramatic improvement. I don't know how or what really happened when I panic stopped my truck but it is not happening again. I don't even need to know. Anew older thread on this message board put the pieces together for me. It is dangerous to ride around with a mismatched combination of parts. Your calipers, rotors and pads need to match your master cylinder and booster and pedal, and your drums and shoes, and your proportioning valve. Then of course all parts must be in good repair adjustment and working order. This all factored together allows you to controllably apply proper brake bias to stop your truck safely. Too much or not enough effect from any of these parts compromises performance and in this case safety.

JB6 is the RPO on my glove box sticker. It designates the GVWR of my truck at 7200# and that I have a vacuum brake booster.

Ding ding ding, we have a winner. It is a complete SYSTEM.
 

Scottlavice

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^ So true.

Both of my trucks have been in the family since they were brand new and I've never thought "Man these brakes aren't good." As long as everything is maintained and bled properly they stop just fine in stock configuration. I never really understood why the GMT800 master swap gets so much love...
I've been working for GM in R/D in Detroit for 26 years. The GMT 400 brake system that you speak so highly of is one of our engineers most admitted failures, the DOT scored it with a "D" you must be the luckiest GMT400 owner in the history of GM. With that said you are very wrong that brake system is and was inferior. We knew it was from the start but it was cost effective....
 

df2x4

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I've been working for GM in R/D in Detroit for 26 years. The GMT 400 brake system that you speak so highly of is one of our engineers most admitted failures, the DOT scored it with a "D" you must be the luckiest GMT400 owner in the history of GM. With that said you are very wrong that brake system is and was inferior. We knew it was from the start but it was cost effective....

Where did I ever speak "so highly" of them? All I said was that they work fine stock. Which I stand by 100%.

Are they phenomenal? No. Are they bad enough that I'm going to start swapping parts? No. Like I said, as long as everything is bled and maintained properly, they're fine.
 

Scottlavice

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Where did I ever speak "so highly" of them? All I said was that they work fine stock. Which I stand by 100%.

Are they phenomenal? No. Are they bad enough that I'm going to start swapping parts? No. Like I said, as long as everything is bled and maintained properly, they're fine.
No just the way I read it it sounded like you were all for them and the upgrades were not necessary which is not true we wouldn't have revamped the entire braking system if that was the case I can't even tell you how close the federal government was to making GM Recall the entire brake system 96 up. The scariest part is some brake systems worked better than others from the factory and they knew this
 

JMiller

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I put a NBS MC on my 90, and I just never thought about needing a residual valve. And since then I have been too lazy to put one on. Stops great and haven't experienced the awfulness he reported. Not saying that's not the problem, or that one should not install a res. Valve.

I think I'm seeing the reasoning here but just want to clear up why to do this. Does it just increase pedal feel so that you aren't half way to the floor before you get any brakes? Or does it improve performance other than just the pedal?
 

JMiller

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Having driven some of these from new, they don't all exhibit that symptom.

Many I drove had hyper sensitive brakes with OEM brake pads and shoes after a rainfall though.

Likely, long travel is a symptom of air introduced into the brake system and not properly bled out. I assure you long travel is not normal.

I have spent many hours vacuum bleeding these system. New master cylinders, new boosters, new brakes all around, properly adjusted as well. Pedal issues remained. The only vehicle I've ever had pedal issues is a GMT400. I've never tried the NBS parts too attempt to resolve the pedalissue. Maybe that's the answer. I have driven so many 400's in an out of the shop over the years I've never met one with "good pedal".

Also, on the 7-13's. I've had some few in the shop that at very slow speeds inside the shop the brakes are very "grabby", no smooth creeping up to the bumper of the car in front of you. It wants to lunge instead of creep. I've always written it off to high idle (cold start), and very grippy high pressure discs.
 

JMiller

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A braking setup is a complicated system. Piles of R&D has gone into engineering. It isnt a system that you can add a part or a few parts and notice a dramatic improvement. I don't know how or what really happened when I panic stopped my truck but it is not happening again. I don't even need to know. Anew older thread on this message board put the pieces together for me. It is dangerous to ride around with a mismatched combination of parts. Your calipers, rotors and pads need to match your master cylinder and booster and pedal, and your drums and shoes, and your proportioning valve. Then of course all parts must be in good repair adjustment and working order. This all factored together allows you to controllably apply proper brake bias to stop your truck safely. Too much or not enough effect from any of these parts compromises performance and in this case safety.

JB6 is the RPO on my glove box sticker. It designates the GVWR of my truck at 7200# and that I have a vacuum brake booster.

Very well said. All things can be improved upon though. Not to say that mixing up all these model year parts is the correct resolution to the problem. I would like to think that there is a way to get some decent pedal out of these things without completely replacing the system including lines, valves, blocks, everything on each corner, booster and mc. That's the only resolution I've ever thought of. Call Baer Brakes. Talk to a rep. Spend upwards of 3k on a completely new system. I almost forgot to mention new pedal setup.

Then all that's left is the wife. If you are a bit more lucky than I am, and i consider myself a pretty lucky man. My wife goes along with most reasonable ideas. Somehow I don't think she's on board for spending a few grand to get a better pedal feel. However if you wife is cooler than mine and is like oh **** yeah you can spend 5k on one phone call to Baer Brakes then I think I would take that route.
 

Tavi

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Adequate brake system on these trucks is not what I would call it. With several panic stops in several different models. I was less than impressed. 89 c1500 had pedal travel to nearly the floor before it would slow. And another inch before lockup.

After removing the ABS from my 92 k3500 drw, it was a massive improvement.

The desire to upgrade to "anything" better is entirely understandable. My 04 suburban has great pedal feel and stopping distance. With nearly 350,000 on the clock.
 

90halfton

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Very well said. All things can be improved upon though. Not to say that mixing up all these model year parts is the correct resolution to the problem. I would like to think that there is a way to get some decent pedal out of these things without completely replacing the system including lines, valves, blocks, everything on each corner, booster and mc. That's the only resolution I've ever thought of. Call Baer Brakes. Talk to a rep. Spend upwards of 3k on a completely new system. I almost forgot to mention new pedal setup.

Then all that's left is the wife. If you are a bit more lucky than I am, and i consider myself a pretty lucky man. My wife goes along with most reasonable ideas. Somehow I don't think she's on board for spending a few grand to get a better pedal feel. However if you wife is cooler than mine and is like oh **** yeah you can spend 5k on one phone call to Baer Brakes then I think I would take that route.
I just tell her after the phone call:D
 
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