Front Disc Conversion Questions

I'm about ready to start gathering parts for a front disc conversion and was looking for a few answers. I'm a little confused on what master cylinder/proportioning valve I need to be using. The car currently has power 4-wheel drums on it right now (standard 72 issue) and will be going to wilwood front discs. Do I need to change the master and prop valve out, or just put an adjustable unit on the rear line? Am I good to stick with my stock drum hardlines as well?

Also, are there any good upgrades I can put on for the rear brakes while sticking with the drum setup?

 

Hey Logan,
I would suggest getting a new master cylinder and proportioning valve when going to disc.
My convertible has a stock type master and proportioning valve, and the red car has a whole different set up with adjustable proportioning valve and aluminum master cylinder, and no vacuum booster, all manual.

Both have stock rear drum brakes and the stock hard lines.

 

I think that the Wilwood brakes have a lot more stopping power, so if it were me, I'd upgrade the rears to keep the braking force ratio the same. I'm not sure what that upgrade would be, as I've never checked into it, as I rely on the stock brakes' adequacy. But you will for sure need a different proportioning valve. If you want to omit the booster, as Mark has mentioned, you just have to use a different master cylinder.

 

Wilwood brakes to me would mean Wildwood master and proportioning valve. Or at least have a phone conversation with them about what I can use.
Recently did a manual drum to manual disk front conversion on my 1970.
The lines, master and proportioning valve came from inline tube but it was a stock type swap.

 

Get ahold of "Underdog350"on this site.
I have met and dealt with steve a couple of times and results are/have been great.
He has a decent price on the products you will need and is knowledgeable on the conversion.

 

I'd go with an adjustable proportioning valve. If your master cylinder is not that old and in good condition keep it. If you don't know how old it is replace it. I just changed the master cylinder on my 65 Special, drum/drum and the application chart said it's the same one used on disk/drum systems. Drum/drum doesn't use a proportioning valve.

 

u can install willwood disc brake assemblies without changing the master cyl. I helped do that.

 

Thanks for the input guys. I'm thinking I'll start with an adjustable proportioning valve on the rear line and see how it goes with that. My drum/drum master is almost brand new, so I'd hate to replace it unless I have to. I'm having second thoughts on the wilwood d52's anyway and am wondering about the baer ss4 setup.

 

Logan,
You shouldn't need a residual check valve, those are only applicable for systems where the master is lower than the wheel cylinders. Think of old hot rods with frame mounted masters under the floor.
As far as a master and prop. valve, you can simply try your existing setup and use one of those prop. valves with the knurled adjusting knob. If you do a search for brake master cylinder size calculations, you can hopefully find the equations for that; maybe Wilwood or another brake company's website will have it. Long story short (I did it for a rod I was building using all manual discs, with 4 piston front Wilwoods and 2 piston stock Mustang rears, but it is the same for disc/drum setups really, as the use of the prop. valve compensates for the "power" discrepancies of the front and rear braking stroke), in the equations, master cylinder piston size(for example 1" or 1 & 1/8") and pedal ratio come into the mix as does the number of pistons and their diameter. It will help predict pounds per square inch of pressure from a given pedal push as a means to design an adequate system.
Here's what matters: Hook it all up, drive the car, see how the brakes work and adjust the prop. valve until the rears don't lock up. Power brakes are obviously a different animal from manual, but it all applies. Your pedal length from the pivot to the pedal divided by the pivot to rod distance is a constant and a multiplier of force(usually between about 4 or 5:1), all else can change through use of different size wheel cylinders, caliper pistons, quantity etc. Say you have a 14 inch pedal from pivot to pedal and the rod is 3 inches from the pivot. Your ratio is 4.67:1. Anyway, just look it up and do the math and you can at least get an idea where your system comes in as far as pounds/square inch. If it works well but could use a little more power, just sub a new master cylinder piston size in to get an idea of the change in order to predict which new master you may want to get to alter the characteristics of your system.
It's not really that complicated, but I just wanted to give you an idea of how to design a good setup. :Smarty:

 

Awesome advice, thanks for the breakdown on how to calculate some of the finer details. At this point I've at least narrowed it down to two different kits for the discs themselves from Baer. I like their ss4+ 11" front system as I know it will fit inside of my 17" torq thrust wheels and the cost is right. The other setup I like is the Baer track 4 system with 13" rotors, however the added expense and questionable fitment could be a problem. My concern with the 11"setup is that when I get ready to put discs on the rear I'd also like to use the SS4 rear setup with 12" rotors. I'm worried that having the bigger disc on the rear will lead to some potential bias issues with the braking system.

 

I have a 1970 GS car with factory power front discs and had factory drum rears. These cars have a hold-off valve (70 and older) to slow down the front disc action and a distribution block, not a prop valve.

I removed the hold-off and tried just putting the rear discs on. It absolutely could not produce enough pressure to keep the idling motor from turning the rear wheels when the fronts were locked solid.

The guys at Right Stuff told me the master had too little volume in the rear chamber to properly actuate the bigger pistons of discs. Swapped it for a disc/disc master and it was daylight and dark.

So, using a drum master with discs may not provide the proper amount of fluid, which would cause the rears to lock before the fronts do much.