Do I need to change distribution block?

Went from 11" front drums to MOPAR discs last year, this on a '70 RT.

Have the Al. 15/16 manual MC that E-Berg recommended as a replacement when we were using the drums.

Pedal is hard, but not much increase in stopping power.
Is the problem with the drum-type dist. block or MC size/type?

Thanks, Joe

PS We have a front to rear bias adjuster plumbed in and adjusted for rear lock-up.

with the 4 wheel drums/drum splitter (they have no proportioning)/Eberg MC it was functioning well but you needed more braking & went to front discs? Sounds like a psi restriction. I would bypass/remove/open up to full open the bias adjuster & see what transpires.

Years ago I changed the front 11" power drum brakes on my 70 Charger R/T to disc brakes.All I did was put the spindles,calipers and rotors on,changed nothing else.I know it isn't the correct way but that car stopped great.It didn't pull or lock-up the rears.So it can be done,correct-no but function-yes.

You want 7/8" rear wheel cylinders and this: https://www.ebay.com/itm/NEW-Mopar-A-bod...E/173718540627?

As stated above the 4 wheel drum block is a distribution block not a proportioning valve and I would also take the rear adjusting valve out, if it was my car. The proportioning valve will bolt right into place on your 70 if the factory lines are still present.

If the pedal is still too hard for your taste you can play around with master cylinder bore size. maybe go up to a 1" or 1 1/32" bore.

So many people claim that their brakes worked fine without a combination valve.

If the brakes actually did work fine the factory would not have put them in.

There is a reason for them and the FSM does a decent job of explaining why.

The residual valves are suppose to hold 10+ Lbs. of line pressure on all 4 wheel drum brakes is not needed on disc brakes, hence the factory removing them from the factory disc brake master cylinders
No residual pressure on the disc brakes will extend brake puck life and slightly increase your fuel mileage also, not a bunch though Maybe .2 to .5 MPG
The idea of any car maker using parts not needed, like proportioning valves, is ridiculous
you need to correct those items and enjoy the results

I had a '66 dart with 4 wheel drums & I swapped the fronts to '76 A discs (2.75 bore) & kept the OE splitter & used a later (BBP) 7&1/4 rear which may have had slightly wider 10" shoes & it stopped flawless & no discernable pad wear after years of driving (but I concede that this was easy DD). It would lockup the rears if I stood on it (rarely needed if I kept my eyes off of the sidewalk & on the road). Joe bypass or open up the prop valve & report what happens.

Well said Cab.

Exactly, because drum shoes have return springs a residual valve keeps the wheel cylinder pistons close to the right position for best response upon application of brakes.


I'd say its none of those. Hard pedal, not much increase in stopping power is pretty much what should be expected when switching from drum to disk.
IMO the general problem is a misunderstanding of the differences in drum versus disks. That's not your fault. The advantage of going to front disks is generally more even effect of heat on the system and better removal of the heat under continuous or repeated hard use. Example would be long downhill on a steep mountain.

The maximum braking power is just less than what is needed to lock up the wheels. That can be accomplished with drums.

Drums on pretty much all American cars by the mid 60s used a duo-servo design in which the mechanism self-assists. That means if all othe things are equal, drum brakes are easier to apply. This is one of the reasons that most disk/drum mopars came with power assist.

A proportioning valve in the line to the rear is correct. It does the same thing as the combined distribution block/prop valve.

Try to set the prop valve so the front tires lock up first in the worst situation. Think wet downhill. If the rear lock up first, the car will do a 180 that is almost impossible to recover from. If the front wheels lock first, its possible to recover front traction by letting off the brakes.

To change the pedal force it takes to develop brake pressure, you can work in a few areas:
You can change the hydraulic relationship between the master and the caliper pistons. I'd start by looking at the factory masters used with those brakes in that car body.
Consider going to a power brake setup. This may change the linkage relationships between the pedal and the master. If the change in master cylinder diameter doesn't make sense, it may be because the mechanical leverage is different.
Pad and shoe linings can make a big difference. Some are more grabby than others. Some are more fade resistant, etc.
Rotor surface finish can make some difference. Finally, some linings are very particular about break-in procedure required to transfer a fine layer of material to the disk.

This 1970 Master Tech Book should get you up to speed. Back cover has the master cylinder p/n for that year by application.
http://www.imperialclub.org/Repair/Lit/Master/274/cover.htm

This chart, I think compiled by Brad's (aka mastershake) will help cross reference those to bore size.
http://www.moparts.org/Tech/Archive/brake/14.html


PS. Notice all but the a-body disks also got a metering valve. Slick idea if the car may ever see cold wet or icy conditions.

Mattax has good advice. B/E Body disc for 70 used a 1 1/8" Bore MC with power assist. Extra volume needed for the disc. As stated, the residual valve is removed for the front disc section of MC. Leave rear in for the rear drums. If you match the 70 service manual set up you will be happy. Certainly if you want more modern performance oriented, an after market system may better suit your needs, but I am happy with the 70 set up on my B body and it brakes fine. Just a cruise car.

All the OEM Mopar dual master cylinders I've seen have the rear of the M.C. the larger volume chamber for the front disc brakes, not the front chamber

I agree Cab. Probably just an mistake when he typed it and didn't think about that. Done that myself a couple times...

Thanks for all the input guys, it looks like the MC bore size may be my problem as the new front discs didn't improve the feel or performance.

I'll go with the larger bore size and new distribution block.
I hope I don't have to go with vacuum assist as I'm around 9" with my cam.

I assume I'll still use the proportioning valve to control rear lockup??
Will I have to change the actuating rod too?

Thanks, Joe

I would think that a larger (MC) bore would make the pedal effort harder rather than easier. On the rod: I would check/note the distance the rear piston is from the flange/depth of hole in piston/MC flange thickness, anything of that nature that would change the length of the pushrod needed (to keep the same brake pedal height).

You are correct on the pedal effort Robert.

that's correct. If making it easier for the foot is the goal..
Pressure = Force/area.
All other things equal, more area would be reduced pressure for the same force.
Therefore more force to onbtain the same system pressure.
And the distribution block should have nothing to do with it as explained above and in the Chrysler books.
maybe something is not working hydraulicaly.
maybe everything is fine and you just don't like the effort level.
maybe the linings aren't providing enough intial grab due to contamination or just that lining.

I'll check what I have now for a bore size and report back, the 15/16 I have in my mind may be wrong.

If it ever stops snowing here I'll get over to the car and check.

Thanks everyone...Joe

i'm thinking you may not have enough vacuum to run your booster. [as well as the master cylinder bore size, as mentioned previously].
somewhere in my foggy old mind, i remember reading something that the booster was designed to operate on 15lbs+ of vacuum. you may be wanting to add an electric booster pump. myself, i'm looking into a pump from a 2012 & up minivan, 300, or charger.
another thing to consider, might be to incorporate a brake pedal ratio of a manual brake pedal on your power setup. it would have a little more travel stroke, but give you more leverage. the attaching hole is closer to the pedal pivot point.
i did this mod to my ex-sister in law's 74 plymouth. she was a tiny thing, and even with power brakes, she didn't have enough umpff in her legs to stop safely. i can't recall how much closer to the pivot i drilled the hole, but it worked great for her.
my brother, however, HATED to drive that car after, because if you so much as breathed on the brake pedal, it would throw you through the windshield !

Well Joe it's still snowing here but as far as your brake question goes I have a tried a Dodge Diplomat master cylinder and it worked pretty good. Then I found a 15/16" Strange master cylinder and it works even better. I don't use any type of distribution block or pressure adjustment device. I have 73 Charger pin type calipers on the front and Wilwood 4 piston Dyna-Lite calipers and rotors on the rear. Now this deal works well for me and pulls down very smooth and easy at over 125 MPH in the quarter mile.
I was going to use the Wilwood adjustable block if I needed it but mine works good without it.

Gus

My bad, I missed that these were power brakes

If you are looking for a braking system that works with little effort you should consider a hydro-boost system. I converted my 92 to rear discs and installed a hydro-boost setup off a 98 truck and wow what a difference. It's really like night and day. I pieced mine together using factory parts and some fabrication but you can buy all the pieces you need for your car but it's not cheap. Check out Tallon Hydraulics