master cylinder question - drum/drum to disc/drum???

[Chuck78]

As you may have read elsewhere on here, I am doing my brake upgrades in phases with my 62 that has the combo brake/clutch master cylinder. For now, I am divorcing the clutch and brake cylinders, mounting a new clutch cylinder in the stock linkage location, and adding a power booster with custom brackets that will space it off the firewall far enough to clear a clutch master, and the at the same time, adding a dual circuit brake master. For now, I will stick with a drum/drum master, as swapping the crossmember for discs will take a lot more time than I have at the moment. I was planning on using this drum/drum master:

1967 GMC C35/C3500 PICKUP 5.4L 327cid V8
DORMAN M66787 1-1/8" bore Master Cylinder

I figured at the very least, when I swap to discs, I could get one of these old style power brake disc/drum masters (still has the old deep bore pushrod hole, works with same booster type as I will already have)

1972 GMC C15/C1500 SUBURBAN 5.7L 350cid V8 (E) Master Cylinder
DORMAN Part # M80568 More Info {First Stop #3491500, 3821715, 4089212, 436148C92, 6273936}w/Manual or Power Brakes; Bore = 1-1/8"; Bendix



NOW, what I was wondering, was first off, I think the master that I have on order for drum/drum is going to have residual valves internally like my 62 master does for the brake portion. If not, and a 67 had them inline or in a metering block/prop valve, someone speak up please! I was thinking, that since the bores are going to be the same diameter for drum/drum over to disc/drum, could I just keep this master cylinder and disassemble it and remove one of the two residual check valves internal to the master, and use it as a disc/drum master by just simply adding a disc/drum proportioning valve? Is there something that I don't know that will keep this from working well?


Two other lesser questions, is it the norm for a hydraulic system to still use the same proportions for master cylinder bore vs caliper/wheel cylinder bore diameters, regardless of power or manual brakes????? I figured since my manual brakes and clutch both use a 1-1/8" bore master, that power brakes and same clutch would still both need 1-1/8"

Also, our trucks stock (my 62) has no metering block or proportioning valve. I assume they never came with them, and that I also won't need one when switching to a dual circuit with drum/drum, correct? It would really make sense that in any system, you would want less fluid/power to the rear vs the fronts, but it seems that it's only a real life issue when discussing drum/disc or disc/disc setups. Should I add a proportioning valve when I swap to a dual circuit setup?

[Captainfab]

What I did on my Suburban is use a master and the proportioning valve from a '67 C10 with power brakes. This is for drum/drum brakes.

[slepysal]

Originally Posted by Chuck78

drum master:





Also, our trucks stock (my 62) has no metering block or proportioning valve. I assume they never came with them, and that I also won't need one when switching to a dual circuit with drum/drum, correct? It would really make sense that in any system, you would want less fluid/power to the rear vs the fronts, but it seems that it's only a real life issue when discussing drum/disc or disc/disc setups. Should I add a proportioning valve when I swap to a dual circuit setup?

The OE master was as you know a sigle circuit, so no prop valve or meetering block was or is needed. For the drum/drum dual master none is need , but you are correct the disc/drum set up requires a prop valve.
I am going to go with a 72 c10, and a prop valve from CPP. ( i have both already)
I like you am doing mine in stages, I already installed a seprete clutch master and hope to add the dual drum/drum master and split lines soon.
I have the spindles and disc kit , but will wait on doing that latter

[Chuck78]

I thought about adding a Wilwood adjustable proportioning valve in the lines now, but didn't really want the billet look under the hood. Kelsey-Hayes used to make a more factory looking adjustable proportioning valve that some stock car places used to sell, but I don't know if they are available anymore.

I am assuming that, like my 62, the 1967 drum/drum master will have the residual check valves built into the master. On that note, I am still wondering if I will be perfectly fine to just continue using that master when I swap to front discs, and just remove the proportioning valve for the disc portion. All that I can see it will hurt is that I will have a much larger reservoir for the rear brakes than a specific disc/drum master would. The bore and stroke will remain the same.
I did read an article on Pirate Jack's FAQ that said that basically if you have a 1-1/8" manual master, that you will benefit by switching to 1-1/4" when adding a power booster, and not changing the wheel cylinders or calipers, because the extra power you will have now will allow you to do so. If still using manual, and stock was 1-1/8" and you ran a 1-1/4" master, then you would stop in a very short amount of pedal travel, and the pedal would be incredibly stiff. If you had too small of a master bore, you would actually exert more force on the brakes, but your pedal would feel sort of spongy and the travel would be so long that it might bottom out the piston in the master. So I assume that with drum/drum, I might experience a bit of a soft pedal, but really good stopping power with a 1-1/8", but the front disc calipers I eventually plan to run are made for a 1-1/8" power brake master, so I'll probably just stick with that and maybe run smaller rear wheel cylinders or add an adjustable proportioning valve.

I was hoping to not have to do any work to the front drums until I replaced my crossmember with discs, but I just blew a cracked front brake hose shuffling cars around out of the driveway today! Dual MC would have been nice! ft whl cylinder rebuild kit is here, new hoses in the mail with the dual master tomorrow... Haven't gotten my power booster yet. Almost ordered last night, but might go with an 85-89 Astro Van 8" dual diaphragm unit if I feel like trying to knock it out tomorrow! A few S10's had dual diaphragms in the yards, but EVERY Chevy/GMC Astro/Safari had an 8" dual diaphragm booster! They accept the large rear bore quick takeup masters, but I'm trying to figure out if there is a problem at all bolting a 1" to 1-1/4" bore old style master to one of the boosters made for the bigger aluminum/plastic masters.