Need help understanding plumbing of Disc Conversion

[mrd5_psu]

Hi All,

I'm finally getting around to doing the disc brake conversion on my 1970 Newyorker (originally power drum/drum). I sourced the parts needed for the swap from a '69 300. The disc brake sticky page is a phenomenal resource, so a BIG thank you to folks that have contributed there!! At this point I've installed the spindles/rotors/calipers/etc and now I'm starting the plumbing but I need a little help getting educated on how the factory disc/drum plumbing worked. The factory original power drum/drum plumbing on my Newyorker included the brake safety switch distro block which, as I understand, manages an even split front to back unless a leak causes the internal safety switch piston to seal up one side. The plumbing from my donor disc/drum set up includes a similar brake safety switch plus an additional "pressure metering" block inline to the front discs, which according to the FSM, blocks flow to the front discs until line pressure exceeds 135psi.

I gather that the intent of blocking flow to the front until 135psi is to ensure the rear drums engage first, correct? So here is where I'm getting confused... In other posts (including the "ramman" video link that BigJohn posted) it sounds like discs need a much higher line pressure than drums. If that's the case, won't the rear drums inherently engage (and potentially lock up) before the discs do anyway?... why the need for the 135psi metering block for the front?

Based on suggestions in the disc swap sticky page I bought a new adjustable proportioning valve which I assumed was needed for the lines headed to the rear, but I'm wondering if the factory 135psi "pressure metering" block should be used for the front as well. Also, is the brake safety switch distribution block from a '69 disc/drum the same as my '70 drum/drum setup or are there internal differences? (I'd like to retain that safety switch feature).

Thank you in advance!!
-Mike

 

[Fury1969WI]

this is a bit out of date but still mostly relevant; https://www.allpar.com/threads/disc-o-tech-stop-on-a-dime.237038/

Excerpt from article:
"... Next, you'll need a rear brake proportioning valve. The valve may be of the stock non-adjustable variety (which, in some years, was built into the brake tee assembly.) Ideally, though, for ease of installation and, more importantly, optimization of front/rear braking balance, the Mopar Performance/Wilwood "add-on" adjustable valve, Mopar #P5249088 (or similar) is highly recommended, and, therefore, was used in our swap. The standard, nonadjustable valve was, of necessity, a compromise. The same valve was used on wagons and 2-door coupes, slant sixes and Hemicars, meaning that most cars rolled out of the assembly plant with less-than-ideal proportioning, characterized by a tendency toward early rear-wheel lockup.

There are other ways to alter front/rear balance. One way, which reduces rear lockup, is to use a 7/8-inch rear wheel cylinder in place of the standard 15/16-inch unit. Try Raybestos part number WC37236 for this. (Original application: late '70-early '80s light-duty Dodge trucks w/10-in. rear drums).

The second variable is caliper piston size. The slider-type calipers used on 1973-'75 A-bodies used a smaller piston: 2.60" as opposed to the more common 2.75". Using these can be helpful if your car has a "reverse" proportioning problem and tends to lock the fronts first, usually only a problem with no-option, slant-six cars or lightened drag cars. ..."


I followed a lot of this but used the 1973 spindles (for the big unicast rotors available at the time) on my 69 Fury. In any case you do NOT want the rear brakes locking first. Otherwise don't overthink it and just get to the wrenches. I love the updated braking and (to me) ease of maintenance and service. Good luck!

 

[CBODY67]

Whzt Chrysler terms "metering valve" is otherwise referred to as "proportioning valve". Same function, to account for the fluid pressure differentials needed between the front and rear brakes for optimum stopping capabilities. With drum brakes, that valve was not needed as the "hydraulic leverage" was controlled by the relationship of the diameters of the pistons in the master cyl and in the wheel cylinders.

GM's power disc brake systems used a "combination valve", which combined the functions of the brake warning light (if one circuit loses pressure) and the proportioning valve functions in one physical piece. Chrysler (as seemed to be common back then) took a different route to achieve the same end result. Just that the GM orientation took less tubing ($$$) to happen, with less assy line complexity ($$$). As time progresses, it might be easier to find a one-piece GM combination valve than a Chrysler metering valve, I suspect. Even at the salvage yards for older cars. FWIW

In earlier times, it was common for the rear wheel(s) to lock-up first in a hard stop. In the earlier 1980s, the then-new fwd brake systems were biased to lock-up the front wheels first. Which is where the adjustable proportioning valve can come in, to manually adjust the front/rear braking pressure bias. Rather than have it "designed into" an OEM-production valve which can fit many vehicles body styles.

The "default mode" is to plumb the system like it came off of the donor vehicle. Plain and simple. No understanding required.

Hope this might help,
CBODY67