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Dual Master Cylinder Upgrade Questions

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92889 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 92889 Quote  Post ReplyReply Direct Link To This Post Topic: Dual Master Cylinder Upgrade Questions
    Posted: 28 Sep. 2022 at 11:55pm


Looking for your recommendations on what dual master cylinder I should use to upgrade the brake system in my CJ2A.


A previous owner had already upgraded all for brakes to 11" drum brakes.


When I built it a couple years ago I installed the brake upgrade kit from "Herm". (Il'l include pictures. Of it new, initial installation and how it looks now.) Herm's master cylinder has no markings. One reservoir is smaller than the other.


Yesterday I had a complete brake failure. Fortunately it was in my driveway at a very slow speed, so only minor damage and no injury. 


I inspected the brake system today. There are no signs of leaks in any of the lines or brake cylinders.  One of the reservoirs was near empty, the other very full. Also when I removed the cap some bubbles were presenting in the full reservoir. 


The exterior of master cylinder is covered in fluid and the paint is all peeling off.


So here is my theory....


My guess is that it was defective out of the box. Its been "micro" leaking out the top seal with every brake application. (Maybe one chamber was feeding the other and then being forced through the top

seal when over full and under pressure.) And finally on brake pedal application #2073 the front reservoir was finally depleted (and air made it to the rear lines).


It's the only way I can think that there is not a pool of brake fluid anywhere but the case is covered in fluid. 


Believe it or not my jeep does not "mark its territory". 


The master cylinder I installed has different sized reservoirs. Can this style be used for four 11" drum brakes, or should I be using a master cylinder with equal sized reservoirs?


What dual master cylinder have you hd success with?


Thanks 


Jon



This is the dual master cylinder that came in the kit from Herm.




This is my installation a few years back when I built it.



This is how I found the reservoir today. Small chamber is empty, big chamber is full. Bubbles were coming up in the full chamber. I had tried the brakes again today before popping the lid off.



Then next two photos are the master cylinder removed. Looks to me like fluid had been slowly leaking from the top seal for a while.



I found this for sale on Amazon. Looks to be the same one that Herm sold to me.



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oldtime View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldtime Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 9:20am
I am not familiar with upgrade modifications that do not use 1942-1979 Jeep components.
But my guess is that one of the small offices in the bottom of your master has plugged with debris.
I suggest you first poking it out with a very fine wire.

Currently building my final F-134 powered 3B .
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mickeykelley Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 9:53am
A few years back I got one from Herm and it was defective and had to be returned replacement has been fine. Don’t remember the nature of the defect now but as I recall it would not bleed properly. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldtime Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 10:19am
I have a fully rebuilt original #945556 available.
AFAIK it should bolt direct to Herm’s “copy cat” mounting bracket.
The bores were slightly re-honed and fitted with an OEM NOS Wagner kit.
Pm me if interested.
Currently building my final F-134 powered 3B .
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Beach Bum Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 1:37pm
I can answer one question, the MC that Herm sells is a 7/8” bore. The larger reservoir is for the front, this is because in the application for which that particular MC was made has bigger brake calipers in front than in the rear. On the stock 9” brakes the front wheel cylinders are larger than the rears. Although I would assume that the same is true with the 11” brakes I do not know that for certain. The master cylinder is a basic GM MC that is often referred to as a “Corvette Style” MC. It is what virtually all the aftermarket products are using. I myself decided that due to the nature of what my Jeep is used for I definitely did not want the MC under the floor so have been working on an alternative. Herm does not build that MC he builds the bracket, the MC is coming from the same place where everybody else’s is coming from (a place where they speak Chinese I’m quite certain. Quality control on anything from China, India, etc. is very poor particularly with cast iron parts. Wilwood makes and aluminum master cylinder that has a cover held down by screws which will fit in that location on that bracket and it will eliminate all your complaints, but you will pay for it. Wilwood also has a MC that uses a remote
 reservoir mounted above the floor (firewall, under dash, etc.). The very small bore MC is used to try and achieve a workable system that does not have the benefit of a booster, That style MC is available in many bore sizes. BTW the wheel cylinders that were installed on the original 9” brakes are 3/4” rear, 1” front.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 92889 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 2:13pm
Thanks for the comprehensive answer.
I need to preface this with "I am not a mechanic."
I was wondering why Herm sells the MC with different sized chambers, I thought that was more of a disc/drum combo where you need to send more force to the discs.

I believe that I have identical 11" drum brakes on all 4 wheels.

I think I have found a suitable replacement.

I did see the Willwood, if my replacement does not work out for some reason I will consider it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldtime Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 3:00pm
11” drums should have 1-1/8” front wheel cylinders and 15/16”rear wheel cylinders.
The jeep split bore cylinder has 1” front bore and 15/16” rear bore.
You do not need to use a proportioning valve with that set up.
Unlike your Delco Remy the Wagner 945556 front reservoir goes to front axle.


Edited by oldtime - 29 Sep. 2022 at 3:03pm
Currently building my final F-134 powered 3B .
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Zero aftermarket parts

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bruce W Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 4:30pm
  The master cylinder with a larger and a smaller reservoir is made for front disc/rear drum brakes. The large reservoir section is for the front disc brakes, regardless of where it is located on the unit. The reason for the larger reservoir: In a “modern” drum brake system the wheel cylinder pistons always return to the same spot when released. The volume of fluid between the pistons remains the same. Brake wear and adjustment on “modern” drum brakes has no effect on that. As disc brake pads wear, they get thinner but are always in contact with the rotor. The caliper piston does not return to its starting point, it stays with the pad, so the volume of fluid behind the piston is always growing greater. The extra reservoir capacity is there to satisfy the calipers’ demand for more fluid as the pads wear. 
BW 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 92889 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 4:59pm
So if I have 4 same sized drum brakes and I want to run a dual master cylinder I should get one with two reservoirs that are the same size?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bruce W Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 5:18pm
  It won’t matter in that case what size the reservoirs are, but I dont think you would want one with a stepped bore, as Oldtime mentioned. Also a disc-brake master will not have a residual pressure check valve in the disc-brake bore, and you should have one for drum brakes. 
BW 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldtime Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 5:58pm
The original design step bore is an advantage because your pumping more fluid into the larger front wheel cylinders.
No proportioning valve needed if you have correct size wheel cylinder front to rear. Early split bore Jeeps never used nor need proportioning valves. Keep it simple!

Here’s the excellent rebuilt Wagner split bore.
Same sized reservoirs front to rear.


Edited by oldtime - 29 Sep. 2022 at 5:59pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bruce W Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 6:15pm
  The original idea of using larger bore wheel cylinders on front drum brakes was to increase square inches of piston area so that more brake application pressure would be obtained with the same foot pressure. Of course this increased flow volume and pedal travel. A larger bore in the master would provide more flow at a lower pressure, giving away what was gained. 
  Proportioning valves were not mentioned, however residual pressure check valves (a whole different thing) were, and they are needed in every drum brake system. 
BW 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldtime Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 6:52pm
Right the residual pressure check valves are located under the output caps.
The 1” front bore simply pushes more fluid volume into the large volumes front wheel cylinders.
AFAIK that was the basic logic behind the design.
IMHO The original Jeep split bore master cylinder is ideal for Jeeps with 10” or 11” drum brakes on all 4 corners.
IMHO The only advantage to disk brakes is “wet braking”.
But drum brakes weigh much less.

Two of my D-225 Jeeps are equipped with original split bore cylinders and 11” Bendix Jeep brakes.
My other F-134 Jeep has the early NOS Wagner single bore master cylinder pumping into the 10” Jeep Wagner brake system with self adjusters. The Jeep 10” Wagner’s have superior drum to backing plate sealing to better eliminate sand, small rock and debris. 
 



Edited by oldtime - 29 Sep. 2022 at 7:07pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldscot3 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 9:13pm
As others have said, a mc with equal sized reservoirs for drum/drum applications is what you want. If I remember correctly, you can find a chart for custom builders that will tell you what bore size works best, based upon the number of pistons it acts upon and volume they displace. However, since this is the 21st century I'd be very surprised if there wasn't a net-based calculator that would let you just plug in the values.

`67 thru `70 chevy pickup with drums all around has a nice cylinder with a 1" bore.


Edited by oldscot3 - 29 Sep. 2022 at 9:16pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Beach Bum Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 11:47pm
Since you are a “non mechanic” I will throw this out just to try and make sure you can get a grasp on it. First, the size of the brake drums does not mean that the same is true of the individual “wheel cylinders” the front wheel cylinders should be larger than the rears (of course, “should” doesn’t mean they “are”). The difference may be that there is so little difference that you can tell from the outside of the wheel cylinder. It’s what’s going on inside where you can’t see it that matters. I think that I would work from the assumption that your wheel cylinders are the sizes that others have said they would have been originally. Also, the size of the reservoirs on the master cylinder do not necessarily mean that the master cylinder bores themselves are different (again, others have explained the reasons for the different reservoir sizes. Residual valves are ALWAYS required when the master cylinder is mounted beneath
the floor because it is too close to the level of the wheel cylinders. Disc brakes = a 2# residual valve while Drum brakes use a 10# valve. As discussed above some master cylinders have them built in but some do not. That is something to look for. I believe that if you call Wilwood (they have spectacular good tech and customer support) and tell them what you are trying to do they will tell you what you need (master cylinder bore sizes etc.) you can then go look elsewhere for the less expensive components that will match the spec’s that Wilwood gives you. There is an amazing amount of expertise available for you here on these forums for sure but the better you can describe what you have the better the advice that you will get. With everybody jumping in here I can appreciate that it can become a mental rats nest just take your time and you will start to get your head around it. Drum brakes are not horribly complicated where it becomes tricky is that with disc brakes there is a fine balance required to get the needed pressure from the master cylinder without using a booster. In many vehicles that is just not possible but the light weight of the CJ will allow getting by with no booster.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Beach Bum Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep. 2022 at 11:51pm
OEM = Original Equipment Manufacturer and NOS = New Old Stock. New Old Stock means that it was manufactured “back in the day” but has been preserved unused since that day.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 92889 Quote  Post ReplyReply Direct Link To This Post Posted: 30 Sep. 2022 at 1:30am
I appreciate your simplified description.
Honestly learning every day.
I bought the "upgrade kit" from ATV because at the time I knew NOTHING about this stuff. I figured he had it sorted and would sell appropriate parts. Maybe he did and I just got a defective one, I don't know, but it only lasted 2 summers worth of operating. Im still confused why the master cylinder he sold me had a large and a small chamber when I have drum brakes all around. Anyhow....

I think I have it sorted. 

I have sourced rear wheel cylinders that have 15/16 bore, front wheel cylinders with 1 1/8" bore and a dual master cylinder that lists a 1" bore and equal sized volume reservoirs. I have a call in to the manufacturer to find out if its a split bore and if so what is the size of the second bore.

I think I'm getting the hang of this.


Edited by 92889 - 30 Sep. 2022 at 1:49am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 92889 Quote  Post ReplyReply Direct Link To This Post Posted: 30 Sep. 2022 at 11:13am
The components I have found are a Dorman M56193 master cylinder. It has a 1" bore (not a split bore). It has equal sizer reservoirs and the fittings are 1/2 x 20tpi which is what already will connect to my plumbing with my banjo fittings. I called the Dorman tech line to confirm that there are residual pressure check valves fore and aft.


I have also found wheel cylinders with 1 1/8" bore for the front and 15/16 for the rear brakes.

So far this is the closest I can find based on the advice here. Is the lack of a split bore going to be an issue in this application?

Again it's a cj2a with 11" drum brakes on all 4 wheels.

Thanks again.

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