The Stovebolt Site Forums The Shop Area The HiPo Shop Single to Dual Master Cylinder Question

I am in the process of converting my 65 GMC Single MC to a dual MC. I will be using a 68 GMC dual MC because it mounts perfectly. I have been reading through the forums and still have a few questions. I plan on leaving the four wheel drums due to the fact that i will be driving the truck less than 2K miles a year. In some forums people recommend a proportioning valve and in others some recommend a distribution block. What is the best way to go for my setup? And what is the difference between the two? Thanks for any information.

68 gmc was 4 wheel drum. You are already using the 68 MC, use whatever else the 68 gmc used.

If he originally had a single circuit MC then the question is what new valve or distribution block if any should he now use with a dual circuit MC?

I say find a proportioning valve from another GM truck with drum/drum brakes and use it with your new dual circuit MC.
What did the 68 GMC use (your MC donor)?

But I don't have any technical reasons why or what you should do.

Grigg

you should be able to add the extra line and go. on a upgrade to dual mc from single and both are for drum setup the 68' mc should have built in portioning valve

I agree with 10 micron. You don't need a proportioning valve with the drum/drum set up.

After checking around, I found out that I need a distribution block to keep the rear brakes from locking up. (The Dual MC does not automatically do it). All of the parts houses around Nashville told me to go to the dealer to get one. Fortunately I found a fellow parting out a 68 gmc, and he was willing to part with the needed piece. Now to plumb in new lines, and I will be back on the road again!!

I still think you didn't need it.

Since you found the "part" on a '68, I'd say go with what Chevy did. My books on the '68 don't show it, so are you it is not just a splitter? The books show a splitter on the front cross member.

best wishes,

Les

Just a thought.......unless you're putting your truck back into showroom stock condition, where authenticity seems to over-ride current technology, why not add a relatively simple little device that helps you control your brake bias?

Having tried the "everything new but stock" configuration with my trucks brakes, I soon discovered that "anticipating stops and laying back" were just prolonging the inevitable. Even at their very best, a panic stop was going to require some major pressure on the pedal and room to control the inevitable rear wheel lockup. Back in the day, when everyone was running drum brakes and traffic was a whole lot lighter, you were on a more level playing field in relationship to bringing the beasts to a halt. But with todays upgraded braking technology on every vehicle, it became apparent that if I was interested in having a safe driver I'd better make some upgrades.

Now I'm not advocating that every Bolt be shod with discs....if you're putzing along on country roads at stock truck speeds and focused on what you're doing you'll probably be okay. However, it only takes one panic stop that wipes out an $80K Beemer to ruin your day. So to me, maximizing your braking POTENTIAL is paramount whether it be drum/drum, disc/drum or disc/disc.

I'm a FIRM believer in using an ADJUSTABLE proportioning valve in all of the options above and also residual valves where applicable. Drum/drum masters are "supposed" to have built-in valves but who knows for sure on a repop? Any issues I've encountered with setting up an upgraded brake system, have come from trying to use a generic proportioning valve. Especially those supplied in the so-called upgrade kits. Incorporating a $40.00 adjustable proportioning valve in even your drum/drum brake system is going to let you better CONTROL rear wheel lockup. Being the obsessive compulsive person I am (my own admission) I extend the adjustment knob up through the floorboard so I can fine-tune the brake bias according to my load and the road conditions. My own take on ABS. (sorta)

When you're upgrading your brakes with the intention of driving your truck on the modern highway, I'd seriously consider adding a relatively inexpensive adjustable proportioning valve instead of a junkyard proportioning/metering valve. Once you get it dialed in you'll enjoy even more, putting your "Bolt" out on the road.

Dave

Are there any "approved" adjustable proportioning valves?
I used one on a truck with disc front and rear, and it worked fine but technically was for "race only" or something like that, wasn't supposed to be legal on the road. Actually once I got it adjusted the first time I never had a need or desire to play with it...

I've also used a "junkyard proportioning valve", a GM one from a vehicle with the same disc brakes front and rear as I have, again no problems, and I'd do it again without a second thought.

And the first time I re-did/upgraded a brake system I used a "generic" proportioning valve that looked just like a GM one, and also worked just fine but cost more that I think it is worth.

What is the great advantage to an adjustable one over the proper factory one or a suitable one form a junk yard?

Grigg