Dual master cylinder update

[37 Cab]

I finally got around to addressing the brakes on my Cabriolet. It had the mustang master cylinder on it when I got it and the pedal has always been low. The stock ford master cylinder is a 1 1/16 bore where the mustang is 7/8. Since I already have the 39 pedals it seemed just a matter of finding a master cylinder with 1 1/16 bore and the outlets on the outboard side when installed since that is where they are already. A 1975 AMC Gremlin master cylinder fit the bill. It is a little taller so fitment is close but it fit. Also the reservoirs are reversed with the large reservoir on the front side where the mustang is on the rear. The fittings are bigger so a trip to the auto parts store for some purpose made master cylinder adapter fittings took care of that. There was no provision for a brake switch so I took it apart and drilled/tapped the end for the switch. Blead the system and now my brakes are great with plenty of pedal and 1 inch of free play at the top. Then for giggles I loosened a brake line and checked to see I still had brakes on the other axle.. perfect!... Hope this helps.

[V8COOPMAN]

Hey Tommy...….Now that is some valuable information. Seems like you were very descriptive both in the parts used, as well as your test procedures making sure you still had ENOUGH pedal travel should one side of the system develop a leak. Still a lot of folks that don't understand the importance of that step. Ya done good! DD

[hotrodA]

Your AMC m/cyl is 1-1/16"?

That's what I'm looking for. All I could find was 1", and it doesn't give me enough pedal.

Do you have a part number? THANKS!

Bill

[37 Cab]

Raybestos MC36456

[hotrodA]

Thanks, Tommy!!

[37 Cab]

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This master cylinder is taller than the mustang. Like I said, it fits but it is close. The bail is touching the floor pan. I plan to add a billet cover for a remote fill for a lower profile. The cover is the same as the mustang so it is available. Drove the car some more yesterday and my brakes are as good as they get with drum brakes. Pedal is great and I am sure it will lock up the wheels at 30 mph.

[rich b]

Good info here. Thanks.
Going to pick one of those masters up and see how it compares, height wise, to the 1-1/16" bore E-250 master I am using. Looked at one of the remote covers you mentioned; but couldn't figure out the venting, so I just made a domed floor board cover. Since then we had a clearance problem with an under dash master and had a friend mill out a shorter cover that was vented and still used the rubber bellows to isolate the fluid.

[37 Cab]

I found the best price at Rock Auto. 72.00 If I remember right.

[Talkwrench]

This is good info ..

[alanwoodieman]

could you tell me how you plumbed up this MC, which bowl did you use for which set of brakes front/rear? I did a similar installation and tried to use a MC out of a 75 model jeep but did not have enough pedal and it was suggested to me to reverse the outlets

[37 Cab]

Did it help? I really don`t see how it could make a difference since all four wheel cylinders are the same size and the M/C bore is the same front to rear. When there is no air present the M/C works evenly to both ports. No proportioning valve on a front and rear drum system. I used the large reservoir for the rear since my lines were already positioned there and not enough line to switch them. Adding line would only make for a messy looking install unless I cut them way back and made my flares while lines are on car. Just not worth the effort.

[alanwoodieman]

I did not see any difference with the line change, but I now suspect the current MC is bad and have already ordered one like yours from RA. I wondered if one of the MC pistons reacted first??? and yes it has helped a lot with one for a 40 Ford car. I have done several conversions using MC out of Ford pick up 68/72 drum brake F100 and F250 ( on a 41 1 1/2 ton) and these all work really good.

[V8 Bob]

Originally Posted by 37 Cab

Did it help?.....When there is no air present the M/C works evenly to both ports. No proportioning valve on a front and rear drum system. I used the large reservoir for the rear since my lines were already positioned there and not enough line to switch them. Adding line would only make for a messy looking install unless I cut them way back and made my flares while lines are on car. Just not worth the effort.

Most OE car/light truck dual drum masters produce equal pressure to each port, but not equal fluid volume, because of the larger front wheel cylinders. This is also true with many dual drum master cylinder fluid reservoirs that may look equal in size. You made a mistake by plumbing the rears to the larger reservoir intended for the fronts. Yes, it is worth the effort to correctly plumb the master cylinder.

[37 Cab]

If you research it you will find a typical dual reservoir M/C works like this.. Keep in mine the original system used only one line split to both front and rear.
When the brake pedal is depressed, it pushes on the first (primary piston) through a linkage. The Pressure builds in the cylinder and lines as the brake pedal is depressed further. The pressure between the primary and secondary piston forces the secondary piston to compress the fluid in its circuit. When the brakes are operating correctly, the pressure will be the same in both circuits.
In your comment you mentioned larger front wheel cylinders. I believe all 4 wheel cylinders are the same size. The only time a larger front wheel cylinder should be indicated is with front disc brakes necessitating a larger reservoir for the front brakes along with a proportioning valve to lower pressure to the rear brakes.

[37 Cab]

Originally Posted by alanwoodieman

I did not see any difference with the line change, but I now suspect the current MC is bad and have already ordered one like yours from RA. I wondered if one of the MC pistons reacted first??? and yes it has helped a lot with one for a 40 Ford car. I have done several conversions using MC out of Ford pick up 68/72 drum brake F100 and F250 ( on a 41 1 1/2 ton) and these all work really good.

The Ford P/U master cylinder is a 1" bore which is a significant improvement over the 7/8" mustang but still has the lines on the opposite side and I needed one with the lines facing the frame rail. Not to mention I wanted to match the factory 1 1/16" bore that the stock master cylinder would have had.

[V8 Bob]

37 Cab,
I am very familiar with dual master cylinder operation, mandated since '67. The output pressure will equalize but at different volumes of fluid. Re-read my previous post.
You are simply wrong about wheel cylinder sizes. The fronts are always larger than the rears on cars and light trucks, because the fronts do most of the work. That rule gradually reverses as vehicle weight increases in med/heavy trucks, RVs, etc.
The fact is your plumbing is wrong, and could be dangerous, because the loss of the rear circuit could result in marginal (or no) front braking due to insufficient fluid volume.

[alanwoodieman]

using what I believe is a 1" dual master cylinder, with it plumbed up as original, I can lock up tires but pedal is so low I am not comfortable with it, so I am going to switch to the bigger bore, as per V8 Bob suggested and 37 Cab found. I also was looking for a MC with the lines on the opposite side than the F100 and you found it 37 Cab, thanks, will let everyone know how this turns out.

[Mart]

Be careful, not all Gremlin master cylinders are 1-1/16.

I found a reference to 71-74 Gremlins with non power disc brakes having 1-1/16. The centric part listed by Rockauto for that model is listed (not on Rockauto) as 1-1/16.
the other part number is the Raybestos part number quoted above.

So be careful, make sure you order the correct one.

Mart.

[37 Cab]

Originally Posted by V8 Bob

37 Cab,
I am very familiar with dual master cylinder operation, mandated since '67. The output pressure will equalize but a different volumes of fluid. Re-read my previous post.
You are simply wrong about wheel cylinder sizes. The fronts are always larger than the rears on cars and light trucks, because the fronts do most of the work. That rule gradually reverses as vehicle weight increases in med/heavy trucks, RVs, etc.
The fact is your plumbing is wrong, and could be dangerous, because the loss of the rear circuit could result in marginal (or no) front braking due to insufficient fluid volume.

I will look into it and if necessary, switch the lines. Thanks Bob. Not trying to argue here, I do not claim to know it all and my experience has been with newer cars. Always thought drum systems used the same size wheel cylinders front and rear. As a side note.. I did loosen a rear brake line and had sufficient braking to stop the car.

[37 Cab]

Originally Posted by V8 Bob

37 Cab,
You are simply wrong about wheel cylinder sizes.

Thanks Bob.. Learned something new. Exactly why I joined this site in the first place... Shared knowledge. I will switch my lines. Even though they work the way they are and my little test showed it to have braking when one side was disabled. To be honest I did not have the car on my lift and was working on the floor so I decided to take the easy way out.

[Talkwrench]

Yes Tommy, the wheel cylinders are proportionally different front and rear and in some cases the cylinders themselves can have one side a different bore size also.

[37 Cab]

I knew about the different bore sizes in a cylinder but not front to rear differences.

[Chris Haynes]

Not knowing what I have can you identify this master cylinder?

[37 Cab]

https://www.speedwaymotors.com/1939-...Bore,1982.html

[Mart]

That's the standard fitment ford master cylinder. 1-1/16 bore.

I bought one from rockauto recently.

That is assuming it is the car/pickup application. Bigger trucks use a bigger bore master. You can fit a brake light switch in the end of the banjo bolt if you want to. Someone has plugged the hole.

Mart.

[Chris Haynes]

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Originally Posted by Mart

Bigger trucks use a bigger bore master.
Mart.

I see that the F2 has a 1 1/8 bore. Does this mean more braking power if used on a car?

[37 Cab]

No. A larger bore would give you less braking with the same amount of pressure on the pedal. You would probably not have the same stopping ability on an emergency situation.

[JSeery]

My guess is they can use the larger diameter master cylinder because the brake pedal is designed with more mechanical leverage. The larger the MC dia the lower the line pressure for a given pedal force (leg force). This can be overcome with a larger pedal ratio. This is somewhat difficult to do in a car because of space constraints, easier in a truck (more room). The only real advantage to a larger dia MC is it moves more fluid volume for a given amount of pedal travel.

[37 Cab]

According to Macs website the trucks used a larger diameter wheel cylinder so the master cylinder would have a larger bore as well.

[Mart]

The brakes are generally bigger all round on the size of truck that takes the bigger master. I only mentioned it because from the image showing the master cylinder, I could not identify what it was in.

Mart.

[37 Cab]

We need a "like" icon...

[alanwoodieman]

put the mc out of a gremlin on my 40 coupe project. lots of pedal and stops good, so hopefully cured the problem

[37 Cab]

I just had a member PM me saying he ordered the 36456 master cylinder from Rock Auto. It arrived and has a 1 inch bore. I checked and sure enough, it is now listed as a 1 inch bore. Thinking I made a mistake I went out and measured mine. It is 1 1/16. Couldn`t get a real good picture but it does measure 1 1/16. Also during my search I found several references to this being a 1 1/16 bore cylinder. Something changed?? (I ordered mine last fall) I have no idea.
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[alanwoodieman]

jeep cj5 has the same style mc but with a 1" bore, may that is the one he got. I have one in my collection, will measure to night

[Flathead Fever]

I did have an ASE Master Mechanic license and an ASE Heavy Truck Master Mechanic license until I retired. Both of those master licensees required separate brake licensing tests. One of the questions on those tests is what is the purpose of a residual pressure valve on drum brakes? Its something you would almost never need to know. They are built into a drum brake master cylinder where you never see them. Most people do not know they are there. If your replacing a master cylinder with another replacement master cylinder everything would work just fine. If you start swapping master cylinders you can create problems.

Something to think about with drum brakes is there is usually a built-in residual check valve in the master cylinder to hold a little pressure in the brake system. This prevents the wheel cylinder's seals from leaking when not being driven. The pressure held in the system forces the wheel cylinder cup lip out against the bore which makes the seal. It also eliminates some of the pedal travel because there is already some pressure in the system. Its just enough pressure to make the cups seal but not enough to overcome the pressure from the brake return springs. If you look at how the cups are made you will see that the outer edges taper and the back of them are recessed. The pressure pushes on that recess in back of the lip forcing the lip against the bore in the cylinder. If there was no pressure in the system when not in use the lip might not seal and fluid could seep out though there. If your ever rebuilding one and your not sure which way the cups go in just look at how they are designed. One direction would cause the seal to expand and seal. The other direction would push the edge of the lip away from the bore and it would leak (see photos)

A dual master cylinder for a vehicle with front disc brakes would not have the built-in residual check valve on the disc brake side. The larger of the two reservoirs is for the disc brakes. Because the disc brake pistons are so large they take a larger volume of fluid to move them, that is why the disc brake reservoirs are larger. You do not want a residual pressure valve on the disc brake side because that would cause the pads to always be dragging on the brake rotors. You want to find a dual master cylinder from a vehicle with four wheel drum brakes or install a residual pressure valve on the outlet port on the disc brake side of the master cylinders.

Scroll down through Wilwood valves and read what it says about residual brake valves. The correct valve will give you a better pedal with less travel on drum brakes.

https://www.wilwood.com/MasterCylind...CylinderValves

Also,here is a H.A.M.B. post on residual brake valves.

https://www.jalopyjournal.com/forum/...-valves.74978/

[richard crow]

the disk brake reservoirs are larger because as the pads wear the piston moves out. they don,t return all the way back like the rear cylinders. so as the pads wear more fluid is reqired to fill them . both front & read master cyl have the same dia you just need more fluid reserve on the disk chamber to fill the disk cylinders as the pads wear .

[37 Cab]

In line residual valves have been around for a long time for applications like this. A 10lb valve is recommended for drum brakes, and for disc brakes a 2lb valve is available for applications where the master cylinder is lower than the calipers.

[richard crow]

37 cab has it right