Modification to a 4 drum master cylinder for use on 4 wheel discsv. ?

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Brooks James

Brooks James

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Current set up, factory KelseyHays front discs, drum rear

Plan , rear discs kit from pirate jack, I already have the kit.

I have been unable to find a new factory OE MC for the factory disc/drum set up.

Picture here is a new MC from NAPA
That was listed for 70 Dart disc or drum

I remember a post that to use the 4 drum MC shown here, saw it could be modified to use when adding an aftermarket rear disc kit. I can't remember clearly , something about removing a " restrictor " or a "valve" just before the bosses to install the lines. And also using an adjustable proportioning valve for the rear ?? .

I am trying to keep all my speed and stock parts "era correct". I know about Dr Diffs MC kit with firewall mounting adapter, which, being aluminum and plastic is definitely
Not era correct.

Does anyone remember the post, or know what to do ??

PXL_20230521_151400208.jpg


PXL_20230521_153636167.jpg
 
In the master cylinder there are brass fittings (front and rear line hookup).
remove them by screwing in a sheet metal screw and pull them out.
Now I'm not sure,been awhile but the residual check valves are apart of the brass fitting or behind.That I can't remember.
 
What's wrong with how it stops with drums on the rear?
 
A disc drum master will still be wrong for your application. Yes you can remove a residual check valve in the rear circuit of a disc drum master but the reservoir is still too small for your rear disc brakes.

buy the attached link below master cylinder and you’ll be golden.

Mopar Aluminum Master Cylinder Kit

rear disc brakes aren’t era correct either. And disc drum master will have too small of a reservoir to support disc brakes. Not enough volume..

safety is more important than trying to keep your master looking correct, especially if the rust right away and look like crap like most do.
 
RustyRatRod said:
What's wrong with how it stops with drums on the rear?
Click to expand...
Nothing, when I get finished with my Dart, I am anticipating 11.70s in street trim. I have plenty of power to get the car going but, Im thinking you cant have to much braking power.
Its a safety issue.
 
Brooks James said:
Nothing, when I get finished with my Dart, I am anticipating 11.70s in street trim. I have plenty of power to get the car going but, Im thinking you cant have to much braking power.
Its a safety issue.
Click to expand...
My 65 Valiant ran 11.40s in the same trim I drove it 12 miles one way to work everyday for about four years. KH disc brakes on the front and 10" drums on the rear. It stopped very well. I slowed down fine to get off on the first return road. I like the look of the engine. Looks like you've made some progress!
 
That pictured master doesn’t hold enough fluid for disc brakes. Disc pistons need lots more fluid that wheel cylinders.

Oh I’m sure several have done it and it “works fine”. But it’s not the best master to use.

use the aluminum one pictured.
 
Brooks James said:
Current set up, factory KelseyHays front discs, drum rear

Plan , rear discs kit from pirate jack, I already have the kit.

I have been unable to find a new factory OE MC for the factory disc/drum set up.

Picture here is a new MC from NAPA
That was listed for 70 Dart disc or drum

I remember a post that to use the 4 drum MC shown here, saw it could be modified to use when adding an aftermarket rear disc kit. I can't remember clearly , something about removing a " restrictor " or a "valve" just before the bosses to install the lines. And also using an adjustable proportioning valve for the rear ?? .

I am trying to keep all my speed and stock parts "era correct". I know about Dr Diffs MC kit with firewall mounting adapter, which, being aluminum and plastic is definitely
Not era correct.

Does anyone remember the post, or know what to do ??

View attachment 1716092565

View attachment 1716092567
Click to expand...
DON'T use a drum/drum MC with disc brakes. The (rear) reservoir for the front disc brakes will not be large enough for the front calipers. Use a disc/drum master (Rock Auto has them listed- 1972 DODGE DART 5.6L 340cid V8 Master Cylinder | RockAuto ) and remove the residual valve from the (front) reservoir for the rear brakes; just run a sheet metal screw into the brass seat and pop it out- the residual valve is right behind it, then reinstall the seat. The (front) drum reservoir is plenty big for 99% of the rear disc conversion kits out there, rear discs have a relatively small caliper volume since they do comparatively little braking.
The drum/drum part that you're probably thinking of is to use the drum distribution/proportioning valve since the bias remains fairly accurate for a four wheel disc system- more so than the disc/drum unit; although with the Pirate Jack rear disc system you'll probably need to run a separate proportioning valve in your rear brake line anyhow.
 
RustyRatRod said:
My 65 Valiant ran 11.40s in the same trim I drove it 12 miles one way to work everyday for about four years. KH disc brakes on the front and 10" drums on the rear. It stopped very well. I slowed down fine to get off on the first return road. I like the look of the engine. Looks like you've made some progress!
Click to expand...
Thanks !!
 
Professor Fate said:
DON'T use a drum/drum MC with disc brakes. The (rear) reservoir for the front disc brakes will not be large enough for the front calipers. Use a disc/drum master (Rock Auto has them listed- 1972 DODGE DART 5.6L 340cid V8 Master Cylinder | RockAuto ) and remove the residual valve from the (front) reservoir for the rear brakes; just run a sheet metal screw into the brass seat and pop it out- the residual valve is right behind it, then reinstall the seat. The (front) drum reservoir is plenty big for 99% of the rear disc conversion kits out there, rear discs have a relatively small caliper volume since they do comparatively little braking.
The drum/drum part that you're probably thinking of is to use the drum distribution/proportioning valve since the bias remains fairly accurate for a four wheel disc system- more so than the disc/drum unit; although with the Pirate Jack rear disc system you'll probably need to run a separate proportioning valve in your rear brake line anyhow.
Click to expand...
Professor Fate said:
DON'T use a drum/drum MC with disc brakes. The (rear) reservoir for the front disc brakes will not be large enough for the front calipers. Use a disc/drum master (Rock Auto has them listed- 1972 DODGE DART 5.6L 340cid V8 Master Cylinder | RockAuto ) and remove the residual valve from the (front) reservoir for the rear brakes; just run a sheet metal screw into the brass seat and pop it out- the residual valve is right behind it, then reinstall the seat. The (front) drum reservoir is plenty big for 99% of the rear disc conversion kits out there, rear discs have a relatively small caliper volume since they do comparatively little braking.
The drum/drum part that you're probably thinking of is to use the drum distribution/proportioning valve since the bias remains fairly accurate for a four wheel disc system- more so than the disc/drum unit; although with the Pirate Jack rear disc system you'll probably need to run a separate proportioning valve in your rear brake line anyhow.
Click to expand...
Are you talking about the Centric 13063015 , which looks remarkably like the original one I had ??
 
Thanks guys, there's no way I could have put the car together without info, help, and encouragement from you guys
 
Brooks James said:
Are you talking about the Centric 13063015 , which looks remarkably like the original one I had ??
Click to expand...
That's a drum brake (9") master.
You need a disc master, like the Centric 13063019... bore size will depend on your particular taste for pedal feel, as well as whether you're running a power booster.
For manual brakes people seem to like a 15/16 bore, like the Dorman M80266.
For power brakes it's more in the 1.031 range., lots of suppliers for those (like the Centric unit I listed above).
To keep your preferred "period correct" look, browse through all the listings for Mopar RWD master cylinders through about 1977 to find the bore size you want.
I'm using Rock Auto's listings just for reference.
 
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Unless I missed it, an important piece of info has not been mentioned.
[1] Disc brakes require about 50% more line pressure than drum brakes.
[2] With drum/drum brakes, car makers 'proportioned' the extra braking force to the front wheels by juggling the wheel cyl size.
[3] With disc/drum, the proportioning of the line pressure was done by a ....proportioning valve. Not normally fitted to drum/drum cars. But needed with disc/drum.
 
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