Dual master cylinder question

[Matt Harwood]

After five tries and five failures to get a satisfactory rebuilt single master cylinder for Melanie's 1956 Chrysler, we are reluctantly going to switch it to a dual master. Probably not a bad idea from a safety standpoint since Melanie drives the car daily in the warm weather, although I'm not necessarily a "all brakes must have dual master cylinders to be safe" kind of guy. But I digress.

 

My question is about the dual master cylinder size. Our original master cylinder has a 1.125-inch bore (1-1/8). I can get a dual master cylinder also with a 1.125 bore. Will that be adequate or should I try to find a 1.25-inch bore instead? On the one hand, the master has to move a fixed amount of fluid because the wheel cylinders are the same; whether it's going through one hole or two shouldn't matter, right? On the other hand, I'm wondering whether the master cylinder needs to be bigger because it will now be pushing fluid through two lines instead of one, effectively splitting the amount of fluid being moved by the piston. If it is moving X amount of fluid through a single line, is it moving X/2 amount of fluid through two?

 

My gut says that going with the same sized master should result in the same amount of fluid being moved at the wheel cylinders. But my gut has been wrong before. Thoughts?

 

 

Edited March 9, 2022 by Matt Harwood (see edit history)

[Ronnie]

I think the answer to your question might be complicated depending on how you intend to connect to the dual master cylinder lines to your old brake system. Do you intend to just Tee the two master cylinder lines into the one original line? Or are you doing a split front/rear brake system or maybe a diagonally split brake system?

[Matt Harwood]

We're going to do a front/rear split.

 

Part of the rub is that 1956 Chryslers use two wheel cylinders per wheel in front, so it takes quite a bit of fluid. More akin to a disc setup than drum, I'd think.

 

 

Edited March 9, 2022 by Matt Harwood (see edit history)

[F&J]

10 minutes ago, Matt Harwood said:

I'm wondering whether the master cylinder needs to be bigger because it will now be pushing fluid through two lines instead of one,

No, if all wheel cylinders stay as they are now, and if the brakes are in the same exact adjustments, then the total "volume" of pressurized fluid is exactly the same with a single VS dual master cylinder.

 

  In other wording, the prior total fluid "volume" went through one circuit before, but then half of that prior total volume goes to each of the dual circuits with the new dual master. 

 

If you went with a bigger bore M/C, much more pedal pressure will be needed to do the same stopping distance.  1/8" bore difference is a lot when dealing with hydraulic systems of any type. Also the bigger M/C bore will make the compressed pedal sit much higher.   (if a cylinder 1/8" smaller than stock was used, that would make the compressed pedal much closer to the floor).

 

 

I would guess the stock master is under the floor? If so, it uses a residual valve for two reasons at the output fitting, one is to keep a slight amount of residual pressure in the lines. That takes the micro time lag out of brake application and secondly, keeps the line fluid from backing back up through the master cylinder if the cylinder is below the floor. >>>So, when you choose a new dual master, it must be rated for "4 wheel drum brake systems" as then these should have a proper psi rated residual valve placed inside BOTH outputs.  A disc/drum Master could have too high of a psi rated valve in the front circuit, so that would cause front drum drag/overheating.

 

 

5 minutes ago, Ronnie said:

I think the answer to your question might be complicated depending on how you intend to connect to the dual master cylinder lines to your old brake system. Do you intend to just Tee the two master cylinder lines into the one original line? Or are you doing a split front/rear brake system or maybe a diagonally split brake system?

There is no volume change at all in any scenario, no matter if diagonal, or F to Rear, or of it was "teed". 

 

A tee'd system; This eliminates any benefit of a dual master "unless" the original single master went bad on a stop. It won't help with a bad wheel cylinder orhose or bad line.

 

 

2 minutes ago, Matt Harwood said:

Part of the rub is that 1956 Chryslers use two wheel cylinders per wheel in front, so it takes quite a bit of fluid. More akin to a disc setup than drum, I'd think

Does not matter if it's twin cylinders when switching to dual master (and still using all original F&R cylinder sizes).   Another thing is that some people  believe front disc uses more fluid movement than drums because the front reservoir is larger.  It's larger to be able to "not go empty" during the total normal wearing out cycle of the disc pads.  The Disc piston is huge and needs more fluid in reserve as the piston runs much further extended with worn pads

[joe_padavano]

F&J is spot on. If the M/C bore is the same and pedal ratio is the same, the pedal travel, force, and volume of fluid will be the same. I actually purposely used a slightly smaller M/C bore when I converted the manual brakes on my 62 to dual cylinder M/C to increase brake force. Pedal travel is slightly greater but braking is dramatically improved.

[Mikefit]

Matt this a 2 edge sword. Think of it as the ratio of the piston dimensions large piston master cylinder takes more foot pressure to stop there should be fluid in the lines at all times I would sure look at all wheel cylinders be sure they are corect installed proprietary also have the shoes be replace with hard linings I would try and find the problem before changing components just my thoughts   mike

 

 

[kgreen]

Matt, would your proposed dual master cylinder have the same piston sizes in each bore?  Some dual cylinder masters might have a different bore between front and rear brakes to apply a different force, ie increased force on front brakes.

[F&J]

1 hour ago, Mikefit said:

I would try and find the problem before changing components just my thoughts

He is the second one to suspect that the "bad" master cylinders might not be bad.  I would think the odds of 5 bad ones is slim to none.  The other poster deleted his post, but warned of bleeding issues on those cars.

 

A single master is about the most simple part of a car, easy to take apart if needed, and very easy to test on or off the car.  Just buy a block off fitting for the outlet, or make one from a piece of line & fitting soldered shut.

 

Bleed the master at that block-off fitting and then simply feel if it's "liquid locked".  It should be brick hard, totally solid feel.  The piston should not move at all if it is bled properly.  It will feel frozen solid if it's good. You can test the one on the car now which is the best option, or test another one that you removed before, by putting it in a bench vice.

 

 

[Bloo]

Dual cylinders are wildly overrated. A single cylinder has one seal... well technically two I guess that could cause you to lose your brakes. A dual, depending on design, has at least five, so it is much more likely to fail.

 

Rear wheel drive cars need a front rear split like you are planning. When it fails, you will in theory have 2 brakes. If it's the rears that still work, they are pretty much useless. Imagine a 2 wheel brake car, and then imagine it as heavy as a 56 Chrysler. On the other hand if you have the fronts it will stop almost as good as it ever did, just have a low pedal and feel grabby.

 

So you are something like 2-5 times more likely to have a failure, and when you do, a 50% chance of having any usable brakes, and that is if the pedal has enough travel to engage the remaining 2 brakes. By design they need more travel than single cylinders. The thing is, many conversions (and a few factory setups) don't have enough travel to engage the remaining 2 brakes during a failure, especially if there is a power booster, and the guy who converted it doesn't even know(!). On the other hand he isn't really worse off either, because he would have had no brakes during a failure before he converted it. Yeah. WILDLY overrated.

 

@F&J nailed it. Ideally bore size should stay the same. If you enlarge the bore, it *might* help with the travel problem, if you have a travel problem, by shortening the stroke, on the other hand the cylinder might be deeper or maybe have more volume between the pistons, so maybe not. One thing the bigger bore will do is make the pedal effort harder. Probably too hard. Especially since the Chrysler brakes have no servo action. They are indeed more like discs. Never lose sight of the fact that one size up from an original size is a bigger change than one size down. Pi * r squared. I'm thinking it would be way too much.

 

Edited March 9, 2022 by Bloo (see edit history)

[Ronnie]

1 hour ago, F&J said:

There is no volume change at all in any scenario, no matter if diagonal, or F to Rear, or of it was "teed". 

 

A tee'd system; This eliminates any benefit of a dual master "unless" the original single master went bad on a stop. It won't help with a bad wheel cylinder orhose or bad line.

If both lines of a dual master cylinder were teed into a single line, the dual pistons would be capable of moving about twice the volume of fluid with the same amount of pedal travel. I would agree the flow would not change.

 

However, if you teed the dual master cylinder into a single line, the hydraulic pressure applied to the line would be greatly reduced, maybe by 50%, when the same amount of pressure is applied to the pedal with your foot.

 

Simply put, teeing a dual master cylinder into a single line results in short pedal travel and a hard pedal with less stopping power compared to a single piston master cylinder with the same size pistons. 

[Ltc4748]

Another item to consider with the dual wheel cylinders upfront and singles in the rear. The amount of pressure tries to be equal at all points since all lines are together after a single residual valve. With one side of the system having 4 cylinders and the other side having only 2 cylinders it may affect the braking ratio if the travel of the dual cylinders is greater due to design. if the wheel cylinders push on the front shoes the same distance from the stationary/anchor point of the shoe as the rear ones I would expect the distance traveled to be the same as the rear under the same pressure applied. One thing not mentioned was the wheel cylinder size, are they the same from front to back? The rear cylinders are open on both ends. A one inch cylinder open on both ends will have the same amount of force applied by fluid pressure as 2 one inch wheel cylinders that are only open on one side.  I have not done a system like that for a few years but I remember that adjustment of the brakes anchor pin eccentrics (if the 56 is like the 40's Mopars) at the anchor pin can make or ruin correct operation of the brake system. You can put adjustable residual valves on the system to modify application of the front brakes but that would be difficult to judge.

[Matt Harwood]

Thanks for the feedback, guys. I've seen a number of other '56 Mopars with dual masters so I'm probably over-thinking it (although there's no way to know how well those systems work).

 

We have had the brakes working really well, but each time the master cylinder eventually fails. I have purchased them from different sources, but I suspect there is only one rebuilder doing them all and he's doing a sub-par job. We had one fail on the bench (which we couldn't return because the shop said we put fluid in it and therefore it was "contaminated") while the rest worked at first then eventually started to go mushy, after which all the bleeding in the world wouldn't bring them back. Another replacement master and it works fine for a while, then eventually starts to leak--the firewall paint is a mess and that really makes me angry. We've had the system working properly, but the master is most definitely the problem. The only reason I'm switching to a dual is to have a different source for a master cylinder that hopefully won't fail (as often). 

 

We will make new lines going fore and aft from the master, but if I use a proper drum/drum master I don't think I'll need residual valves. If so, we'll plumb them in at the master. The master is mounted on the firewall and has power assist, so pedal effort isn't a problem. I kind of want an OEM master instead of an aftermarket one with lots of plastic just to make it look as inconspicuous as possible. 

 

Thanks for the confirmation that my initial assumption was the right one. I'll get a dual master with the same bore size as stock. 

 

 

[Bloo]

If the bore size is the same, it will stop the same. You won't be able to tell the difference. Yes, the residual valves are already in a drum/drum master. They are under the tubing seats.

 

 

[nickelroadster]

You will need a proportioning valve also.  Make sure you talk to brake people who have done this sort of thing.

[Bloo]

I have done this sort of thing. Nothing else is needed here because it is a four wheel drum brake system, all stock, and was just running on a single piston. None of the disc conversion cautions apply. Proportioning on a 4 wheel drum system like this is set up by changing the rear wheel cylinder bore size. It shouldn't need changing.

[F&J]

33 minutes ago, nickelroadster said:

You will need a proportioning valve also.  Make sure you talk to brake people who have done this sort of thing.

It is very easy to understand why many people assume that a 4 wheel drum brake dual cylinder car or truck needs a proportioning valve by just looking at factory stock 1967 to early 70s systems with 4 drums. 

 

All factory dual master cars and trucks from 1967 onwards were also mandated to have a brake failure warning light, and that typical warning unit with the warning switch, definitely looks a lot like a proportioning valve, right down to both units having multiple lines, and the same brake warning switch with single wire.  ( "most" cars/trucks, but some like VW had the brake warning switch right on the master itself).

[nickelroadster]

Sorry if I got that wrong on needing a proportioning valve.

[TexRiv_63]

17 hours ago, Bloo said:

If the bore size is the same, it will stop the same. You won't be able to tell the difference. Yes, the residual valves are already in a drum/drum master. They are under the tubing seats.

 

 

Order a new master if it is available, and double check that the residual valves are actually there. Insert a small screwdriver or straightened paper clip in the tube seat hole, it will only go in a short distance if the valve is there.

[Frank DuVal]

Have you tried putting a kit in one of the failed masters to see what the problem is? Pretty simple setup in there. Seems easier than re-piping for a dual master.

 

Hmm, well, I see your pain, not a lot of listings...Rock Auto is of no help, they do not list a kit. Maybe Mobileparts on this forum has one?

 

These parts look similar?

 

https://www.summitracing.com/parts/dhb-tm13621

[hook]

5 hours ago, Frank DuVal said:

Have you tried putting a kit in one of the failed masters to see what the problem is? Pretty simple setup in there. Seems easier than re-piping for a dual master.

 

Hmm, well, I see your pain, not a lot of listings...Rock Auto is of no help, they do not list a kit. Maybe Mobileparts on this forum has one?

 

These parts look similar?

 

https://www.summitracing.com/parts/dhb-tm13621

Also have the new master cylinder sleeved with stainless or bronze.

[Dandy Dave]

I've had brake stuff done buy this company in the past. They sleeve the old unit. I have not had any complaints. 

https://www.applehydraulicsonline.com/

[Dandy Dave]

I've been told, on duel master systems with a proportioning valve at least, that 70% of the braking power is in the front. 30% in the rear. Doohhhh. Dang monkey wrench in the works. Is this only applied to systems with Disk brakes in the front, and Shoes in the rear?

I've worked on those old 2 cylinder in the front systems in the past. They're basically two half cylinders. Can't remember bore size though front to rear. Dandy Dave!

Edited March 11, 2022 by Dandy Dave (see edit history)

[pfloro]

Hi Matt, 

 

Might "Brake and Equipment Warehouse" be the place to send your poorly rebuilt single reservoir MC for a proper rebuild?  I'm sure it won't be cheap but it will be done properly...

 

Good luck,

 

Paul

 

Edited March 16, 2022 by pfloro (see edit history)

[Matt Harwood]

Well its almost two years later and we still have mushy brakes on the Chrysler. Pedal goes almost to the floor, then grabs. I've taken everything apart and adjusted it as well as I can given the ridiculousness of the system. Nothing has changed. I continue to believe it's a volume problem at the master cylinder.

 

Today I had a new idea that might solve the problem. We installed a 1.125-inch dual master cylinder, which--in theory--should have provided identical performance to the original single master which was also 1.125. HOWEVER, maybe there's insufficient volume despite the same diameter piston. We plumbed the front and rear brakes separately, as you would with a dual master cylinder. But perhaps by doing that, there's not enough fluid volume going to the front brakes, which uses two wheel cylinders per side. I am going to try to T the two lines coming out of the dual master cylinder into the single line that used to feed front and rear brakes through a distribution block. That way, ALL the fluid from the master cylinder is feeding the whole system rather than half going forward and half going to the rear (or whatever proportion it might be).

 

I understand this will negate any benefit of having a dual master cylinder, but it's the only thing I can think of at this point that might have any effect on the soft brakes. Any thoughts?

[61polara]

Matt I'm late to the game here, but have you adjusted the brakes based on the service manual with the correct adjusting tools?  In my experience, a low pedal is an adjustment problem, not air in the system or another fluid problem.  The shoes should be adjusted correctly before you bleed the system.   I had a problem with failure on several 1965 Thunderbird (single MC) new master cylinders.  We were bench bleeding them.  The last one I purchased had a warning note inside the master cylinder should not be bench bleed.  Apparently the bore is not machined all the way in and if you bench bleed, you will push the seal across the raw area and damage the seal.  After I bled it on the car, I had no more master cylinder failures.

[R Walling]

Matt Harwood,

 

56 Chrysler Master cylinders are available from Andy Berbaum  for about $80.

 

 Andy Bernbaum Auto Parts | Newton MA

Edited January 24 by R Walling (see edit history)