GMT800 master

[Ol' Drippy]

Has anyone ever tried to use a GMT800 master cylinder in their truck? I have a 72 Suburban that is getting an explorer rear end with discs and I like the thought of using a master designed for them that is aluminum and easy to find at a parts store. Would I need the 800 booster as well? Has anyone tried?

[jumpsoffrock]

I know thread-starters don't like being told not to want the thing they want, so I don't mean to step on your toes, but if the weight of the MC is the problem, and solving it with aluminum is the answer, you shouldn't be swaping out MC's often enough for it to matter, it's not exactly something you do every couple months.
Secondly The MC's for these trucks are common enough to be found at probably any parts store.

Also I find the aluminum MC plastic reservoirs are kinda trading one problem(weight) for another(weak reservoir that can be hit and broken easier)

Sorry for the rambling. You maybe outta go to a junkyard and see if you can't find a GMT800 and measure the rod throw, and look at bolt patterns.

[Ol' Drippy]

I'm not worried about the weight, really I just prefer the newer aluminum style masters because they don't rust up and look terrible after a few months. I guess I will be trying something new then because it doesn't appear anyone has tried this yet. Time to hit the junk yard!

[Keith Seymore]

I was a brake development engineer at the Desert Proving Ground for GM full size truck from 1990 - 1994.

I figured that would be a good time to go from the cast iron master cylinder on my Chevelle to one of the aluminum/plastic types.

I tried every single one in the GM parts book that seemed like it would work (including obscure variants like C car limo). Most of them seemed like I could get the pedal force good but the travel was too low (the effectiveness seemed to occur after too much initial travel).

The one I liked the best was for an L Van (aka AWD Astro van) but the reservoir was so small I had to jack the rear of the car up at the end of the driveway in order to fill/top it off.

In the end I just went back to the cast iron unit and never looked back.

My $0.03.

K

[Keith Seymore]

Originally Posted by Ol' Drippy

Has anyone ever tried to use a GMT800 master cylinder in their truck? I have a 72 Suburban that is getting an explorer rear end with discs and I like the thought of using a master designed for them that is aluminum and easy to find at a parts store. Would I need the 800 booster as well? Has anyone tried?

You might not realize but there are probably four or five (or double that) GMT800 brake master cylinders in the parts book.

There may be different masters for each of the brake systems (ie, JB3, JB5, JB6, JB7 and JB8) and they might also proliferate between vacuum boost and hydroboost if there is a difference between the QTU (ie "quick take up") units vs non QTU (this affects the bore diameter at the end of the m/c and therefore the interface to the booster). Be aware that in some cases the front reservoir is plumbed to the front brakes; in some cases the front reservoir is plumbed to the rear brakes, so you may need to accommodate that.

You are saying you are going to rear discs, which probably throws another variable into the equation.

You might check with brake system vendors like Wildwood or Tilton to see what they recommend. They also have calculators so that you can determine the desired m/c bore size based on front piston diameter, rear piston diameter, proportioning (meter, split and slope) and maybe even friction material.

The first thing you'll need to know is the piston diameter(s) in those Explorer rear disc brakes, if you choose to do the calculations.

I have all the brake charts starting with the 1969 model year but I'm not sure how new they go. I'll check tonight and see what it shows for the GMT800 era.

K

[Keith Seymore]

Originally Posted by Ol' Drippy

Has anyone ever tried to use a GMT800 master cylinder in their truck? I have a 72 Suburban that is getting an explorer rear end with discs and I like the thought of using a master designed for them that is aluminum and easy to find at a parts store. Would I need the 800 booster as well? Has anyone tried?

What kind of Suburban?

C10, C20, K10 or K20?

Do you know the GVW?

K

[Ol' Drippy]

Wow, thanks for all of the research Keith! The Suburban is a 72 3/4 ton 2wd drive but I am not planning on using any of the original brakes except maybe the booster. I am converting it to 5 lug using 73-87 front spindles with the HD 1.25 in front brakes and the explorer rear brakes. I have to re-do the brake lines anyway so I don't mind switching them up. I would prefer a factory master cylinder that I can find at a parts store and is less expensive. Other options include a 90s impala ss master and the master from a 79-93 Dodge D-150 which has similar stats to the original master.

[Ol' Drippy]

I think the explorer rear brakes have dual 46mm pistons.