Well said Cab.
The residual valves are suppose to hold 10+ Lbs. of line pressure on all 4 wheel drum brakes
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.
<snip> '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?
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.
PS We have a front to rear bias adjuster plumbed in and adjusted for rear lock-up.
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.htmThis 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.htmlPS. 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.
