Manual Disc master cylinder

[Andy4639]

Originally Posted by 54blackhornet

ANDY we aren’t interested in a part number for later vehicles. We are interested in the proper cylinder for a 71/72 truck with non power disc brakes l Appreciate your project converting from drums to disc but totally different topic. As confirmed by the above information, there are two different cylinders per GM’s engineering

True, but if the stores don't carry it then what you gonna do? As so many of us have posted that they all say one master fits everything for those 2 years, C-10,20,30 manual are power doesn't matter.
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I'm trying to give you all info to get what you want. If you look at the part it's got the larger opening for the longer rod as what manual brakes use. I just got back from installing the master on the booster and it worked perfect.
Just because LMC say's it's 71-72 doesn't mean it was. They could have done what I just did and now use it as the stock parts. I'm not saying they do are not but I wouldn't be surprised if they don't.
If LMC can get it then I would think any parts store could so why don't they list them?

[54blackhornet]

Andy they all don’t say that ! You have to dig deeper for the truth.

[dmjlambert]

I'm thinking that the issue is the parts stores may carry exactly the correct part and can have it for us tomorrow, but computers and documentation about what is the correct part number for the application has been lost. Probably because there is not a lot of demand. There's probably a dozen people in the country right now that may be interested in manual disc brakes on their old truck, and 100,000 people interested in Chinese disc conversion kits with drop spindles.

I am mostly interested in C-10 because that's what I have. I recall reading somewhere that the heavier duty trucks always had boosters even with drum brakes, because they have to stop a heavier gross weight and manual brakes just don't cut it. I didn't retain that info, I just read it and moved on, because I have a C-10. So, until I see evidence otherwise I don't believe in C-20 and C-30 manual brakes of any kind. And I have doubts that a master cylinder that was made for a C-20 or C-30 would have come out of the Chevy factory on a C-10, because I doubt they mixed them. Sooner or later somebody with a 71-72 C-10 and manual brakes will be able to chime in and see what bore size they have and other details.

[Steeveedee]

Originally Posted by 54blackhornet

It is amazing that something so vital to folks safety is so confusing ? The one part that fits all mentally don’t work in this situation.

Agreed. If we can get enough people with manual and power disc to take measurements (and hopefully the ones who do this are using unmolested trucks). We can compare them and maybe figure it out.

[jocko]

Originally Posted by Steeveedee

How far the push rod mounts on the brake swing arm from the pivot point for both power and manual brakes. I always remembered that the manual master cylinder had a smaller bore for more hydraulic force. I've also seen here that the push rod mounts in a different place. If we can get some measurements, maybe we can establish some ground truth.

Steeveedee, if you look closely at pic #1 from the Assembly Manual, although they don't show the two connection holes on the brake pedal, it is fairly clear that pushrod clevis for the non-power brake system (top part of pic) is connected closest to the brake pedal pivot point, whereas the pushrod clevis for the power brake system (lower part of pic) is connected furthest from the brake pedal pivot point. This makes sense when you consider it takes more leg power to push non-power brakes, so there is more mechanical advantage when the clevis is connected closer to the pivot point. Power brakes don't need this much mech advantage because the booster is doing the work. Hope that helps.

Originally Posted by Andy4639

As so many of us have posted that they all say one master fits everything for those 2 years, C-10,20,30 manual are power doesn't matter.

Andy, I hope you will find the below helpful. The bottom line is that the C10 MC is simply not the same as a C20/30 mc, no matter what the parts stores state. And, more importantly, it DOES really matter how you hook it up, depending on whether it's a C10 mc vs a C20/30 mc. If you do not route your lines as pics 2 & 3 from the 72 service manual describe and show (i.e. C20/30 front port to the back of the combo valve), the metering in the combo valve will lead with front brakes first, which is the opposite of how it is supposed to work. You make a great point about it being hard to find the parts - I recall having a hard time too a few years back when I redid my 71's system top to bottom. I "think" what may be making it hard to find the right setup for your truck is because it may have started as a mix of parts to begin with. The mc you removed from your ramp, in an earlier post, that was connected to the booster, looked like it was a non-power mc on a booster - and you mentioned you had to do a little re-wickering of the intermediate rod to make it work. Was the booster added to your ramp's system at some point? i.e. is it an original power brake truck? It may have had a mis-matched mc/booster combo - that indeed can be resolved to some extent with a custom length intermediate pushrodrod coming out of the booster. It's also possible that the different booster mfgrs had different length intermediate pushrods - both Delco-Moraine and Bendix made boosters for this era for the factory. Anyway, who knows, these trucks are old and many of us have systems that work that aren't exactly configured how they were born. But I did want to share the info below about the combo valve and C10 vs C20/30. If you get the red underlined part wrong, then the green underlined metering won't work correctly. Could result in nose dives or early brake lockup on the fronts - or never really using the rears, or who knows.

[Steeveedee]

You know, Jocko, I stared at that pic enough that I was just getting confused. It absolutely makes sense for the holes to be in different places. But when I went out and looked at my truck, the hole is pretty close to the pivot. It has power brakes from the factory. I suspect that the difference may not be as big as is shown in the pictures. Maybe that is what was throwing me off.

[jocko]

Good question Steevee. I've had some trucks that had two holes and they weren't as dramatically spaced out as the pic above implies - the upper hole was about like the upper one in the pic above, the lower one was only about an inch, maybe two, below. Above was the only ref I could find in the crazy Assembly Manual, and it was somewhat generic, i.e. not specifically 71-72, etc. My 66 did have two holes - it was where I first stumbled across all these little weird brake system nuances - I was converting from single to dual reservoir and non-power to power - but accidentally bought a non-power mc, not realizing it was wrong until I went to put it all together and discovered the piston pushrod hole depth difference. I ended up skipping the booster and installing a non-power system because that was the mc I had. Was kind on the fence about the booster at the time anyway, so didn't mind

[Andy4639]

Guy's I'm not arguing the point about different master cylinders for the 71-72. I just stating that the computer guy's don't know enough to tell you this difference. They rely on the computer to tell them. They are not 71-72 gooroo's that spend all their spare time on these trucks like us.
So if the computer doesn't state it for the 71-72 model year then they don't know. But when the computer states there is a difference for the 74 to 86 model then they can look for the right part.
The master cylinder in my last post is for a C-20 truck of the 74 to 86 model years. Is it a 71-72 model also, who knows but it does work are at least it bolts up any way. Time will tell if it works are not.

My 71 C-10 is factory disc brakes with booster. I have changed it over to full disc now though so it's no help.

[54blackhornet]

Boy has this thread gone off topic ? I went to Napa this morning and returned with part # M36280 . This is a MC that replaces the original GM master 29881. This is a new, not rebuilt master. As you can see it has the deep recessed hole necessary for NO POWER FRONT DISC BRAKES. (Attention ANDY) ! It has the large front reservoir, small rear reservoir as per the original GM design. I hope that this information might help folks looking for the proper master in the future. I will not have the opportunity to install it for a week or two..

[54blackhornet]

Can someone reorient the photos

[jocko]

here u go

[54blackhornet]

Thanks Jocko !

[dmjlambert]

Specs I see on the Internet say it's 1 1/8 bore size. Is the large reservoir for the rear drum brakes? Is the combo valve for C-10 something also available from a car parts store?

[54blackhornet]

You can probably find one on buy sell forum section.

[68Gold/white]

The larger reservoir is always for the front (disc) brakes.

[68 TT]

Originally Posted by 54blackhornet

I had a heated discussion with the owner of my local NAPA store about this. He didn’t even believe me about 71 model year trucks having disc brakes up front. As far as his books showing only one # for both power and manual, he’s stands firm. He did ask me for the number cast into the master I have presently installed. I looked but can’t find a #. Can anyone provide a number off theirs ?

Some of the confusion may come from the 1971-72 C20/C30 not having a manual disc option like the C10 and only being available with power disc as it was standard equipment on them.

The old parts books probably had two columns to fill for the C20 just like the C10 had and put the same power disc part number in both spots.

[BigBird05]

You do realize that you are arguing over a picture in the service manual that is hand drawn by one or more illustrators. Humans make mistakes.

[Steeveedee]

Originally Posted by BigBird05

You do realize that you are arguing over a picture in the service manual that is hand drawn by one or more illustrators. Humans make mistakes.

When I worked as an engineer, and was really careful to get that spreadsheet to show the performance curves as accurately as possible, we'd send that information to the graphic arts people. What we got for presentation was something just a little different. Not cheating, just artwork. We also had artists who painted pictures of our satellites...which I will generously describe as not "anatomically correct". I call it "artist's deception", as opposed to "artist's conception". I've made many an engineering drawing, and I expect that that drawing shows precisely what is going to be seen by the people who have to do the work. Maybe it's different on an automotive assembly line, since these vehicles generally aren't launched into space. Elon Musk notwithstanding. My take is that the difference in the distance in the holes on the brake lever arm is a bit exaggerated for the line workers. Not a big deal, since the parts come down the assembly line labeled for the vehicle that is going to be so equipped.

[dmjlambert]

So, has anybody outfitted their 1/2 ton truck with manual disc brakes, or has a 1/2 ton truck with factory manual disc brakes, who can give more information such as the bore size and part number of the master cylinder? Is there a part number of a combo valve or proportioning valve from GM/ACDelco (not junk yard but auto parts store source)? Did you guys who were working on this a couple months ago get it done and can give an update?

[ltdmstr]

Originally Posted by Steeveedee

But then I see where people do all kinds of crazy stuff like putting Corvette master cylinders on their rat rods. It says it's for a Corvette, it's got to be good, and NO engineering know-how may have been applied. People get hurt that way, sometimes badly.

Doesn't matter much what the vehicle is if the rest of the parts are the same or similar. What's the problem with putting a Corvette master cylinder on a rat rod if it has basically the same brake setup and the weight is close? It's no different than how car companies share parts across a number of brands or vehicles within a brand. I just put a brand new hydroboost unit and Corvette master cylinder on my '68 C10 with disc/drum brakes to replace manual drum/drum setup. Should work fine. Certainly way better than the stock setup. I'll know in about a week or so when it's back on the road.

[Greasey Harley]

I am just about to do this exact thing to my '68.
This is what the local Federated Auto Parts Store got me.
They also didn't show 2 different PN#'s. I ordered this for a 72 C10 with a l6 and no options. I sure would like to figure this out before the truck gets tore down.
I'm thinking about sending it back and getting this instead: https://www.speedwaymotors.com/Alumi...nder,8905.html
---> Subscribed <---

[dmjlambert]

I wanted to bring this back up. Can we confirm the factory never produced a 1/2 ton 71-72 truck with factory manual disc brakes, or nobody can provide a picture of what the master cylinder looked like or provide any evidence it happened? If power brakes on a C-10 were an option, they must have been such a popular option that 100% trucks got the option. Is that right? Or is it too long since 1972 and every truck in the years has been messed with to add the power brake option by going to junk yards or parts stores?

[BigBird05]

Just a couple of weeks ago I removed a master cyl from a 71 that didn't have power brakes and before that saw a 72 in a junk yard that didn't have power brakes.

[Steeveedee]

If it doesn't say anything about a booster on the SPID, it came with manual disc brakes...unless there was an option that ended up including power disc brakes without calling them out on the SPID. The reason I say that is because I have 16.5" wheels on my truck, but it doesn't say anything about them on the SPID. I suspect that this was because they came with being a Longhorn or some thing like that. I don't see any indication that the 61A alternator should be on my truck either, according to the SPID. Maybe that came with UY1 Camper Wiring? It would make sense.

[dmjlambert]

Originally Posted by BigBird05

Just a couple of weeks ago I removed a master cyl from a 71 that didn't have power brakes and before that saw a 72 in a junk yard that didn't have power brakes.

Thanks, so there is some evidence. One of these days could you measure the actual bore size of the master cylinder off of a disc brake non-power brake truck and perhaps post the part number of the master cylinder? I suspect it is 7/8 but don't know. I will be converting to disc brakes in the future, and I will have to start out with power brakes since it is not possible to just go get OEM parts appropriate for a non-power setup. As this thread proves, there is no documentation available on non-power setups. My goal is to eventually get going to a non-power disc brake setup, so when I find out the actual master cylinder bore size and preferably part number, I will be able to remove the booster and get back to non-power. I prefer the feel of non-power brakes.