If the brakes are stock go with the same size bore as stock, changing the bore size changes both pedal travel and effort needed to stop the truck. If you are adding power assist … you on your own.
For my 46 1/2 ton with 54 (bendix) ‘non power assisted’ brakes 1” is the answer for stock and the new m/c. Using dot approved steel braided flex lines the pedal is solid, travel is right and effort right. Larger bore than stock means less travel but more leg effort.
It is about psi, it’s the per square inch matters… the math gets long but it starts here 1” bore area is .79sq in, 1-1/8” area is .994. The 1” bore multiplies force, 1-1/8 is a almost neutral multiplier
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
For anyone who has done a drum/drum manual conversion to dual master….. (AD truck, MC under the floor)
I’ve seen some stuff that suggests a residual valve is needed? But the kit Jim Carter sells leads me to think you would just plumb it straight up without any such valve.
1 inch on the master for a drum drum. On the pressure residual valve, the original single pot master had a pressure residual valve built in. The pressure residual valve kept pressure on the wheel cylinder pistons and rubber cups, and to a lesser extent the brake shoe springs, to keep the wheel cylinders and shoes from fully retracting and causing excessive brake pedal travel, and to keep a little pressure against the cups to prevent leaks.
When the dual pot masters came on the scene, the original GM units has removable brass seats on each port. Behind those brass seats were a spring a rubber "duck bill" pressure residual valve. One each for drum drum masters, one in the drum portion of a disc drum master. The rebuilders for years would install the duck bill residual valves as part of the rebuild, and then they didn't. And so the rebuilders, presumably because its cheaper have not installed the valves for some time now. But they can be retrofitted. The last time I purchased a rebuild kit, a Raybestos No. 472, the valves and new brass seats were included.
The new manufactured masters do not have removable seats, all cast integrally with the master so no retrofitting.
You may also see a lot of "proportioning" valves on a lot of installations. Its not exactly an accurate term. A common master cylinder to use for a non-power conversion is a 67-69 Camaro master. For drum drum applications, GM used a "distribution" valve at the master. The distribution valve was used to trigger the dash brake failure warning light.
A "proportioning" valve was used on certain Camaros, not many, to delay pressure to the rear drum brakes to allow the fronts to take on the heavy braking so that the rears did not lock up prematurely. Only certain Camaros had these, AC cars, cars with 12 bolt rears.
But the "proportioning" valve everyone refers to, the brass type, is more accurately called a combination valve and is used in disc/drum applications and handles the warning light feature, distribution to two separate circuits, and a hold off or proportioning valve so that the disc brakes hit first before the rear brakes hit. There is no pressure residual function.
So if you want to replicate the pressure residual function on your dual master conversion, you will want to consider adding two 10 pound red Wilwood inline pressure residual valves. One each side, close to the Master.
Thanks, Mike. That clears up a bunch but also raises a couple questions….
1) As for the “may want to consider” a couple residual valves…..
What would be the risk or effect of NOT having the residual valves on a modern dual MC that doesn’t have the built-in style?
2) Converting to dual MC drum/drum MIGHT be a baby step toward front discs in my case. Starting to venture out on some longer trips. I like the idea of dual master. Not scared of drum brakes but also appreciate the practicality of discs.
Is there harm in using what the kits call a “proportioning” valve (the disc/drum kits) in a drum/drum setup? I don’t see what the downside would be in a bias toward the front brakes. But there’s a lot I don’t know.
Rule of thumb is to keep residual pressure valves as close to master as possible and low as possible. After bench bleeding using a Motive pressure bleeder makes it an easy bleed when working alone. 1” or 1-1/8? Better get it right…. I thought it would be 1” also, but not my dog, not my hunt Here is a photo from under my 46 With master under floor residual valves are not optional, required either as part of proportioning valve setup or stand alone. Going to discs later would mean switching to different residual psi valve for front, one takes 2 psi, other take 10 psi and maybe a proportioning valve for the rears for balance. Chuck
Last edited by Hanks custodian; 10/20/20254:45 PM.
One potential risk, too much pedal travel before the brakes engage. Leakage past the cups.
Keep in mind, that the 1 inch master is not what you want for a disc/drum set up. You will need the larger bore master for the disc/drum set up.
Using a combination valve, I don't know what that might do in a drum drum set up as its not needed. But my gut tells me that it would not be on the positive side of the equation. So little changes adding up to larger outcomes. Skipping the residual valves and then using a combination valve i would think would be a negative outcome. Maybe not a huge negative outcome but negative just the same.
Btw, if you want some good brake shoes, contact Muscle Car Brakes and see if they have something for you in their offerings. Their brake shoes are a ceramic matrix friction material as opposed to what's on the market today in terms of organic friction material. Organic friction material is not as good as the asbestos based friction material used when these vehicles were new or for year s after and the organic material just does not do the job. Not inexpensive but they work great. I have them on two of my vintage vehicles.
Right, even if I had to replace the master again later….it would not be hard or expensive to do. The replumbing and bracket mounting would already be done.
You might be putting the cart before the horse, maybe. Install the drum drum manual master. Use two Wilwood red 10 pound pressure residual valves. Find the brake line junction block to the back of the truck. Disconnect the line to the rear and install a flare plug. Get a roll of 3/16 NiCoop and some fittings, a roll of stainless brake line wrap (both available from Inline Tube). Remove the original line from the master to the front junction block. Run a new NiCopp line to that junction block, make a coil or two near the master, install the stainless wrap to make it look pretty, and install it in the front port of the new dual master. You have isolated the rear from the front.
Run a new NiCopp line from the rear port of the master, a coil or two, following the original lines over to the rear junction block, stainless spring wrap for a professional look. Connect the new line to the original rear line with a flared union. You have now isolated the rear. So, a new Master, two residual valves, a coil of NiCopp, some fittings and some spring wrap. In terms of $$$, pretty inexpensive relatively speaking.
Now, give it a try. If you decide that you want the discs later on, then order the correct master, the proportioning valve and a bracket. Everything else you have, including a loop or two of brake line near the master that you need only uncoil, cut and fit to the proportioning valve. The NiCopp is very easy to work with, bend, fit, flare and install.
On the other hand, if you are wavering, you might consider biting the bullet and just put the disc brakes on. If you do decide to go with disc brakes, consider using some Hawk Street 5.0 pads, for the same reason as the Muscle Car Brakes brake shoes, the over the counter organics disc pads are just OK.
No not at all, unless you want the bracket to serve another purpose like securing the lines to the residual valve. Here is one I have waiting to go on one of my vehicles. I had to modify the bracket for what I wanted but this is for a drum drum set up, with a line lock and two residual valves.
Yours would be simpler and easier without the line loc. Sometimes, easy is good.
And yes, lol, I sprayed the metalflake, and yes, I got carried away but no I am not repainting it. I will have to just live with my embarrassment, lol.
