Better Stopping

[Gospel Runner]

I have a 70 Roadrunner with drum brakes in the rear and aftermarket disc’s in the front. It still won’t stop. 1.25” Master cylinder, aftermarket power booster, with a separate vacuum pump and extra vacuum canister. It still won’t stop. What are you using to stop your hot rods? Any help will do!

 

[68 Sport Satellite]

What MC are you using? Is it a dual reservoir MC spec'd for disc/drum?

 

[Kern Dog]

If that wasn't a typo, an inch and a quarter bore master cylinder is gigantic. Holy crap....who chose that size???
The 75 Dart had a 15/16" master cylinder with power front disc and that car isn't that much lighter than yours. As bore size increases, travel decreases and the pedal gets stiffer. Even an 1 1/8" bore is likely too big.

 

[diesel_lv]

I'm using 15/16" on my manual disc/drum on my '67 Coronet 4dr and it stops on a dime. My 110 pound daughter can stop it easily w no leg strain.

 

[slepr1]

I agree with Kern, your MC is way too big. I run stock disks and drums, stock MC with several big cams over the years and never any vac/brake issues.

 

[70rcode]

What Brand/model # Front brakes are you using ?

 

[33 IMP]

As said above, I'd bet your master is the problem. I'd go for the smallest bore you can find. And I'd get rid of the power brakes, I hate them, especially if you don't have enough vacuum for them.

 

[Gospel Runner]

I suspected that master cylinder was the problem. Yes it is a dual reservoir. Should I take out the booster and just attach the master to the fire wall? Can I use a master from a modern car, or so I have to stick with the old school style?

 

[Gospel Runner]

I suspected that master cylinder was the problem. Yes it is a dual reservoir. Should I take out the booster and just attach the master to the fire wall? Can I use a master from a modern car, or so I have to stick with the old school style?

 

[Gospel Runner]

The brand name is SSBC disc brake conversion kit.

 

[33 IMP]

Isn't SSBC belly up?

 

[diesel_lv]

I'm not sure if you need the adapter plate going from power to manual. I'd get a modern 15/16 bore master cylinder that you can get from any parts store. If you need the adapter plate, Dr Diff sells them. You already have a power brake pushrod. The difference w Dr Diff is his are machined out to accept a manual pushrod.

 

[Budnicks]

Isn't SSBC belly up?

It lives still, it's under a different ownership
it did shut down in 2019, like a lot of businesses did
Covenant Harbor Lights Holdings, is the new or was the new owners

I saw/recently still listings for them on Summit
(I have their front brakes)
sort of long lead times to get products now

don't know much at all about the new owners

 

[Gospel Runner]

I will call Dr. Diff Thursday.

 

[oldkimmer]

Cass will have everything 4 u. Ask his opinion. Kim

 

[Gospel Runner]

He told me that I need to buy his stage two front brake kit. He said it has a larger fluid capacity, and a larger rotor at 11.75”. I need to ask him if it is a two or four piston caliper, and what if any difference that makes. He has not answered my question as to whether I still need my power booster, vacuum pump, and canister, with this new set up?

 

[Geoff 2]

[1] Removing the booster will make it even harder to stop.
[2] Contact the disc brake supplier & find out what size m/c they recommend for use with boosted brakes.
[3] If it is a smaller m/c, you will need a proportioning valve for the rear brakes because pressure will be excessive with a smaller m/c.

 

[Tex013]

I have a 70 Roadrunner with drum brakes in the rear and aftermarket disc’s in the front. It still won’t stop. 1.25” Master cylinder, aftermarket power booster, with a separate vacuum pump and extra vacuum canister. It still won’t stop. What are you using to stop your hot rods? Any help will do!

go to a smaller master cylinder , that will give more output pressure though brake pedal will be lower and softer . I removed my booster /pump combo due to it being a pain in the arse . Up to 127mph disc/10"drum was ok . After that not so much . I now have disc rear , GM type , pulls up pretty good from 131mph . This is a street strip 68 Satellite @ 3790lb race ready . Drives/stops great on the street and highway .
I have a 1 1/16 or 1/8 master
OH yeah , you will need a different firewall plate and pushrod if removing booster


Tex

 

[Geoff 2]

You cannot just go to smaller m/c willy-nilly. The OP has fitted aftermarket front discs. These may have a larger piston or multiple pistons. That means more fluid displacement. A smaller m/c has less fluid displacement & without consulting the disc manufacturer about m/c size, a smaller m/c could bottom out....& not apply the brakes.

 

[Tex013]

i know Geoff , why i noted a lower softer pedal . Most mopar m/cys are 1" or 1 1/8"
here is a list from Mancini .

Master Cylinders

www.manciniracing.com

You can also look @ Rockauto for sizes

https://board.moparts.org/ubbthreads/ubbthreads.php/ubb/download/Number/206344/filename/mastercylchart.jpg

And yes Cass is very knowledgable and helpful .
As i noted different pushrod setup for manual , this also changes pedal ratio as per factory
Tex