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.