Gmt400 brakes-master cylinder upgrade, residual valves, disc/drum MC differences. So much confusion.

Discussion in 'Axles + Brakes' started by Ryan cin, Nov 25, 2020.

  1. Ryan cin

    Ryan cin Newbie

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    The brakes on my 97 2 door suck. I read that you need 10lbs of residual pressure on the rear drums, but I took a gamble and bought a gmt800 master cylinder without knowing if it had the valve (part number Dorman M630031) pedal feels better, but still not right. Today I discovered my back brakes were barely working, if at all. Takes about 3/4 second of an empty hub slowing down, so my front brakes are doing all the work. Don’t know if this was a problem before I changed the mc. Are all the guys that do this upgrade effectively deleting their back brakes? I don’t understand how the topic of residual valves and pressure is ignored. I know the 05-06 1/2 tons had rear drums, but couldn’t find the right master cylinder anywhere. What’s the deal?
     
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  2. Ryan cin

    Ryan cin Newbie

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    I know somebody knows what’s going on
     
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  3. SUBURBAN5

    SUBURBAN5 I'm Awesome

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    There busy shopping for deals lol. I'll chime my 2 cents. EXTREMELY LIMITED though. I agree with you some gmt800 did have disc in front and drums in rear. That's why I bought a gmt800 mc. Now there are technical stuff I'm not sure about. But to my understanding it depends on your brake set up. Some guys have different sizes and setups which is why this mod works for some and not others. Mine is a 2wd 11" drum set up with JB6 set up. Which I'm told is pretty good set up. Better mod would be to switch to hydro boost. I just dont wanna go through that whole process. Most I'll do is police wheel cylinder and maybe 2500 calipers. If not maybe powerstop rotors and calipers. Because I cant make up my mind lol I'm doing ac delco new oem hardware on all wheels.
     
  4. SUBURBAN5

    SUBURBAN5 I'm Awesome

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    Make sure all you lines are good btw. Any restrictions or kinks... look at your brakes hardware and the adjuster
     
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  5. Ryan cin

    Ryan cin Newbie

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    Thanks. Sounds about right. Hydroboost sounds like a huge PITA. I think the master I put on has too large of a bore also (combined with it being a disc/disc master). I don’t know how gm managed to screw these brakes of so bad on all these trucks.
     
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  6. Curt

    Curt I'm Awesome

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    Short answer is: If you have rear drums, then yes. By swapping over to a disc/disc MC you've effectively made them 80% useless when you step on your brake pedal.

    In a disc/disc MC, the residual pressure in the lines is about 2 psi front and 2 psi going to the rear brakes vs. 2 psi front and 10 psi rear pressure in a disc/drum MC. Due to the pressure difference, the rear brakes cannot engage as quick since the brake line pressure needs to build to 10 psi before the rear drums start to engage. This results in increased brake pedal travel before the rear brakes actuate (i.e. longer stopping distance, and a spongy pedal feel) If you want to fix your braking setup I would either swap out the disc/disc MC back to a disc/drum MC, or install a residual valve in-line of your rear brakes to keep brake pressure at 10 psi in the rear.

    If you want, I do have my 1 year old OEM ACDelco disc/drum MC I removed when I swapped over to rear discs (probably about 7k miles on it) just sitting in my garage. Oddly enough I planned on throwing it out months ago but never had the heart to do it since it was so new. If you're interested, I'll give it to you for $20 + shipping cost.

    If you want to keep the nicer looking MC reservoir and not throw away the money on that shiny new MC, then you might want to consider installing one of these to fix your issue: https://www.summitracing.com/search/part-type/residual-pressure-valves?N=brake-line-pressure-retained-psi:10-psi
     
    Last edited: Nov 27, 2020
  7. FastFreddy

    FastFreddy OBS Enthusiast

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    Use an adjustable proportioning valve and then you can set it where you want it

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  8. arrg

    arrg I'm Awesome

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    The GMT800 master cylinder firms up the pedal because it has more piston area than a GMT400 master cylinder. This also results in less clamping force at the brakes. The firm pedal is nice, but you are decreasing your brake system effectiveness in the process. That said, I have the GMT800 on my truck, and my rear brakes work just fine. Start by making sure they're properly adjusted, then try to bleed the wheel cylinders.
     
    Last edited: Nov 30, 2020
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  9. Supercharged111

    Supercharged111 I'm Awesome

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    The GMT800 isn't the only master with more piston area. I really don't see why it's such a popular upgrade. I ran one and yes, my truck stopped better, but nowhere near where it does with the hydroboost and matching HD master cylinder. On the first test drive I slammed on the brakes and the ass end came out a little. The other thing nobody's brought up is the fact that OP likely has the crappy 8.5" diff with it's crappy never adjusted properly rear brakes further muddying the waters. My setup is a mostly GM matched configuration. The HB, master, and front calipers all match. It's just my oddball 6 lug 9.5" rear that doesn't, but based on feel it seems to match nicely.
     
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  10. MatthewRB1616

    MatthewRB1616 Newbie

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    Y’all got me worried this is my issue. I went with a M630283 MC off amazon, when I look it up on summit I believe it says It’s for 4 disc can anyone confirm this? If it is can someone give me a part number for a disk/drum MC
     

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