Gmt800 master on gmt400 hydrobooster

[Erik the Awful]

I wouldn't recommend just swapping a GMT800 booster in place, unless you swap the entire brake system, including a GMT800 hydroboost. Between quick take-up systems, drum versus disc proportioning valves, and the liability of grinding on brake components to get them to fit, there's way too many opportunities on our trucks to end up with a brake system that feels fine until you get into an emergency situation. There are people on this forum who went to a GMT800 master cylinder and then reverted back when the brake system didn't work right in an emergency stop.

Note that I am not against mixing and matching brake parts to get better brakes, but you need to know what you're getting into. There are far too many people who will tell you "just swap it".

 

[Supercharged111]

I wouldn't recommend just swapping a GMT800 booster in place, unless you swap the entire brake system, including a GMT800 hydroboost. Between quick take-up systems, drum versus disc proportioning valves, and the liability of grinding on brake components to get them to fit, there's way too many opportunities on our trucks to end up with a brake system that feels fine until you get into an emergency situation. There are people on this forum who went to a GMT800 master cylinder and then reverted back when the brake system didn't work right in an emergency stop.

Note that I am not against mixing and matching brake parts to get better brakes, but you need to know what you're getting into. There are far too many people who will tell you "just swap it".

The calipers and rotors are already 800 components, so an 800 master is the correct answer here but he needs to match it to what the brakes came off of. I see now I was a bit cryptic in my first post, but this is what I meant to convey. The 1500s and SUVs waffles back and forth on whether or not they had rear discs/drums, not sure if the 2500s did too, but this is one of the things he needs to watch out for. I seem to recall someone hogging out the bore of a HB to accommodate an 800 master before, I just don't recall how big a diff it was and I didn't get a look at the execution of it.

 

[romanos93]

What donor vehicle did your brakes come from? Specifically, what bore diameter was the master cylinder they're intended to be used with?

What bore diameter master cylinder do you have now? It's not a Quick Take-Up master, so you must have had JD7 or JD8 brakes originally...right?

As said...you need a larger reservoir for the rear brakes.

JB6 brake code in glove box all 4 calibers and discs are from a 2006 suburban 2500.

 

[romanos93]

I wouldn't recommend just swapping a GMT800 booster in place, unless you swap the entire brake system, including a GMT800 hydroboost. Between quick take-up systems, drum versus disc proportioning valves, and the liability of grinding on brake components to get them to fit, there's way too many opportunities on our trucks to end up with a brake system that feels fine until you get into an emergency situation. There are people on this forum who went to a GMT800 master cylinder and then reverted back when the brake system didn't work right in an emergency stop.

Note that I am not against mixing and matching brake parts to get better brakes, but you need to know what you're getting into. There are far too many people who will tell you "just swap it".

factory JB6 brake code all 4 calibers and rotors off a 06 2500 saburban

 

[romanos93]

The calipers and rotors are already 800 components, so an 800 master is the correct answer here but he needs to match it to what the brakes came off of. I see now I was a bit cryptic in my first post, but this is what I meant to convey. The 1500s and SUVs waffles back and forth on whether or not they had rear discs/drums, not sure if the 2500s did too, but this is one of the things he needs to watch out for. I seem to recall someone hogging out the bore of a HB to accommodate an 800 master before, I just don't recall how big a diff it was and I didn't get a look at the execution of it.

i total agree, i know these trucks dont stop the best, im used to it. the only reason i ask for a potential upgrade is that my master started leaking from the front seal. could easily swap in a 400 mc and have decent brakes again but if its worth the upgrade id do it. but a $80 mc vs hydro, mc and valves is a big differnce

 

[Supercharged111]

i total agree, i know these trucks dont stop the best, im used to it. the only reason i ask for a potential upgrade is that my master started leaking from the front seal. could easily swap in a 400 mc and have decent brakes again but if its worth the upgrade id do it. but a $80 mc vs hydro, mc and valves is a big differnce

In your case the 800 piece is neither an upgrade nor a downgrade. That 400 MC new, old, or otherwise does not move the correct amount of fluid for the brakes you said you had fitted to your truck.

 

[1998_K1500_Sub]

That 400 MC new, old, or otherwise does not move the correct amount of fluid for the brakes you said you had fitted to your truck.

^^^ Which is to say the bore size of the GMT800 MC is greater than the GMT400s.

From what I've seen on RockAuto (not measured myself, so... ) the GMT400s used a 1.125" bore MC on non-HD systems and a 1.25" bore MC on HD systems. See disclaimer below. Too, I'm ignoring the "quick take-up" additional piston (1.575" bore AFAIK) b/c it's not relevant to the GMT800 brakes.

Similarly, some GMT800 MCs have 1.338" bores (34mm) and others 1.457" bores (37mm). See disclaimer below.

So the GMT800s, as @Supercharged111 notes, evidently use MCs that move more fluid than the GMT400s... a selection I'm sure GM made so as to match the piston area of the MC and slave cylinders to their design objectives of the brake system.

The stats above aren't supposed to be a complete or definitive list, they're just "for example", as a follow-up to @Supercharged111's post. Other members may have more to add (and I hope they do).

 

[romanos93]

In your case the 800 piece is neither an upgrade nor a downgrade. That 400 MC new, old, or otherwise does not move the correct amount of fluid for the brakes you said you had fitted to your truck.

I'm a bit confused. An 800 would be an "upgrade" if it moved more fluid then the 400?

 

[Supercharged111]

I'm a bit confused. An 800 would be an "upgrade" if it moved more fluid then the 400?

Right now your brake system upgrade as a whole is incomplete. An 800 master on 400 brakes in some cases is not an upgrade per se even if it does move more fluid. It needs to move the CORRECT amount of fluid to both the front and rear channels. Doing something that should have been done in the first place is not allowed to be counted as an upgrade, rather the correct master and all calipers/rotors as a whole are the upgrade.

 

[Schurkey]

I would expect JB6 brakes to have a Quick Take-Up master cylinder, and to have a vacuum booster. Neither of those are present on your truck. One wonders if the hydroboost and non-Quick Take-Up master cylinder have been "upgraded" from a greater GVW GMT400.

JD6 might have Hydroboost, but still with the QTU master.

But GM has not been consistent in the option codes.