67 Master Cylinder and Cobra Brakes

[mnstr65]

Going to do a Cobra brake install on my 65 FB. PO installed a big 67 MC for drum brakes. Car brakes good as is but I wanted a little more and to fill in the 17 rims.
Question is would it be wrong if I used the same dual master with a Prop valve to the back drums with the cobra clipers up front? It seems to me that the master would provide enough fluid and pressure compared to a 67 diskbrake master or 2006 mustang MC.

[GT289]

Originally Posted by Monster

Going to do a Cobra brake install on my 65 FB. PO installed a big 67 MC for drum brakes. Car brakes good as is but I wanted a little more and to fill in the 17 rims.
Question is would it be wrong if I used the same dual master with a Prop valve to the back drums with the cobra clipers up front? It seems to me that the master would provide enough fluid and pressure compared to a 67 diskbrake master or 2006 mustang MC.

A 67 drum master cyl is a 1" bore.... pretty standard fare on conversion to a big caliper setup. A 1" bore will work all the way up to and including a 1.95" four piston caliper. I'll go out on a limb here and guess that the Cobra caliper pistons aren't that large. (I'm at home and don't have a reference book) You should be fine with a 1" bore master.
Your problem will be in the proportioning to the rear. Even with the prop valve knob all the way "open" you won't get enough pressure to the rear of the vehicle- the car will tend to lock the fronts well before the rears.
A secondary problem will be the reservoir capacity of the '67 master- It'll do the job but there will not be a lot of extra fluid capacity.
Oh, and BTW, is that rear line on your master cylinder for REAL? It's huge. Was that a PO thing- I can't believe it came stock? A smaller line will provide you more responsive brakes, a 3/16" would be ideal. That 1/4" is more at home on a truck.

Akebono Brake Corporation
Brake Systems Engineering Center
Farmington Hills, MI

[Grabber70Mach]

If I remember correctly the 94-98 Cobra's use two 38mm pistons and the 99-04 Cobra's use twin 42mm pistons.

[GT289]

Cool. It would work fine either way.....
The smaller are basically 1.5" and the larger
are 1.65" dia pistons. You can get away with a
15/16" bore master for the smaller caliper
pistons & enjoy an easier to press brake pedal.
I'd use the 1" bore master for the larger piston
Cobra calipers.

[mnstr65]

So it "looks" ok to try right?
I posted this late and maybe would have thought about it a little more before but what was making me wonder was looking at the other MCs, is that some after market units are basically the same thing or look like they work the same. 1" bore and look equal on both sides. The 67 MC like on my coupe has a smaller bowl going to the rear brakes and I remember dialing the prop valve all the way open. In my mind I was thinking that if I use this MC and use the prop valve with it closed (50% less pressure), it may be around the same balance. If that makes any sense??

The current set up is all PO- funny thing is that it works After buying the car I thought, it stops really good for a all drum car. So I looked more close. 67 MC? big line to the back 9" diff from a 67. Front big drums- bigger than my coupe ever had. I figured the big line to the back was to somehow balance the system giving more to the fronts.(?)
Well, should I try it? What would be safest configuration with this mc? libility being all mine, you bet after any of my mods I test in a safe area...
Calipers are 87-93 Cobra

[caveeagle]

Don't forget that disk brakes need a MC without a residual pressure valve.
Also, shouldn't the rear (closest to the firewall) line be running to the front brakes? IIRC, the front reservoir should be running back to your rear brakes. You may want to double check your POs work here. That larger brake line looks strange to me.

[GT289]

Originally Posted by Monster

So it "looks" ok to try right?
I posted this late and maybe would have thought about it a little more before but what was making me wonder was looking at the other MCs, is that some after market units are basically the same thing or look like they work the same. 1" bore and look equal on both sides. The 67 MC like on my coupe has a smaller bowl going to the rear brakes and I remember dialing the prop valve all the way open. In my mind I was thinking that if I use this MC and use the prop valve with it closed (50% less pressure), it may be around the same balance. If that makes any sense??

The current set up is all PO- funny thing is that it works After buying the car I thought, it stops really good for a all drum car. So I looked more close. 67 MC? big line to the back 9" diff from a 67. Front big drums- bigger than my coupe ever had. I figured the big line to the back was to somehow balance the system giving more to the fronts.(?)
Well, should I try it? What would be safest configuration with this mc? libility being all mine, you bet after any of my mods I test in a safe area...
Calipers are 87-93 Cobra

Should be OK, but you threw another monkey wrench in with the calipers again..... It's an unknown again as far as piston size. My books at home don't get that specific and the Ford Motorsport catalogs from that era (and FMS typically) don't get specific about piston size in the calipers.
As far as the bowls in the master, it doesn't really mean diddly squat (in general) whether the front bowl does the rear or vice versa. There is more to it than that. (I'm NOT getting into MC design in this forum) The 67/68 drum master cyl will work with its 1" bore, unless you get back about your caliper piston size and it's significantly different than 1.5 or 1.6".
If you have a master with a larger reservoir, it does the disc side of the system. And the port on the larger reservoir shouldn't have a residual check valve under the fitting seat. On the 67/68 drum master, both ports from the factory have residual valves. You would need to remove the rear port's residual valve for use with front disc or make sure it didn't have a valve.
My concern is still the proportioning on your application, but it may work just fine.
You can always try it without a prop valve and plumb it in if the rear locks up too easily.

[mnstr65]

Should be OK, but you threw another monkey wrench in with the calipers again..... It's an unknown again as far as piston size. My books at home don't get that specific and the Ford Motorsport catalogs from that era (and FMS typically) don't get specific about piston size in the calipers.
As far as the bowls in the master, it doesn't really mean diddly squat (in general) whether the front bowl does the rear or vice versa. There is more to it than that. (I'm NOT getting into MC design in this forum) The 67/68 drum master cyl will work with its 1" bore, unless you get back about your caliper piston size and it's significantly different than 1.5 or 1.6".
If you have a master with a larger reservoir, it does the disc side of the system. And the port on the larger reservoir shouldn't have a residual check valve under the fitting seat. On the 67/68 drum master, both ports from the factory have residual valves. You would need to remove the rear port's residual valve for use with front disc or make sure it didn't have a valve.

Well I think I was mistaken on the Calipers- they are the same as used in the 5 lug kit for 87-93. They are the black calipers and I think are the later larger size.
But you got me with this one..."residual check valve under the fitting seat" I read the tech pages at mpbrakes and yes you are right. The ports should have valves in them. That would make the disk brakes drag- correct?
And now I understand why the bigger port goes to the disk brake- the pads wear faster and need more volume as they go down using up the reserve. If I can get rid of the valve on the one side I may give it a go, but its looking like I have to shop for a MC. Im like a kid wanting to do this upgrade in one day and go drive....
Thanks for your insight and expertise!

Mark you also caught the res valve thanks. I will trace that line back and see what I find- was hoping not having to replumb that entire line. I know that sticks out like a sore thumb.

[GT289]

Now you're cooking!