I need help!
Maybe some of the more experienced brake techs can lend a hand with my crappy situation. My brakes in the Duster just went from bad to worse. I had a factory power disc brake master cylinder and booster in the car, but the low speed braking was always bad because of the lumpy '508 cam in the 360 motor. I have 10 1/2" rear discs along with the 73-76 Duster 11" front discs. Before with the power setup, the brakes were decent at speeds over 20 mph. Firm pedal with reasonable effort. I switched to a new non power master cylinder that is supposed to be for 4 wheel disc brakes, since it came with the rear brake kit. The pedal feel is great: I bled them yesterday and the pedal firms up within 2". When I drove the car today, the brakes are so bad I could barely even get them to skid on dirt! They now feel bad at all speeds.
The car has a drum brake combination valve. It may be the original one that came in the car from the factory. I have several more combination valves to try. I have one from a front disc brake 74 Duster and 3 from 4 wheel drum Valiants. My thinking before was that a drum combo valve would work in a 4 wheel disc application better than the front disc combo valve.
Sorry if all of this seems confusing or if this post jumps all over the place. I'm just wigged out about all this.
Jeff

On important variable is piston diameter on the master. Without power assist you generally want a smaller diameter master to keep pedal effort reasonable. I'm far from an expert, but I believe 1" or even 15/16" diameter is best with a manual setup.

Do you know the diameter of your current master? Also, if you really stand on the brakes can you get decent braking?

The non power master has what looks like a one inch bore. I have some basic dial calipers, but they are not profesional grade. I also have 3 or 4 other master cylinders that I can try, but they are all for front disc/rear drum cars/trucks. One is a 1 1/16" and the others are one inch.
Slamming on the pedal barely gets the front tires to skid in the dirt in my backyard. I blew compressed air through the passages of all my spare combination valves and they all seem clear and free flowing. I think that the one in the car should be okay too, since each corner moved a fair amount of fluid when I bled the brakes yesterday.
I checked a website: www.imperialclub.com/repair/lit/films/274/index.html and while it is very old fashioned, there were some helpul tips there.
Jeff

More info:

#1: The master cylinder I have on the car has a smaller reservior toward the firewall and the larger reservior toward the radiator. This seemed odd. The outlet fittings are also reversed. Almost every other M/C I have looked at has a large reservior at the firewall with a 7/16 fitting and a smaller reservior in front with a 1/2" fitting. My master is opposite of that. My friend says that his Ramcharger M/C is the same as my wierdo M/C. His forward reservior line goes to the front brakes! This has me wondering.
#2: To fit my brake lines to the odd M/C, I had to install adapter fittings since my front line was too small and the rear line was too big. Now I think that I might flip the lines and see if it makes any difference. I guess there are no absolutes with master cylinders, huh?

The large reservoir usually goes with the front brakes. If the reservoir is reversed and F/R is reversed the bias would be messed up and could cause your problem.

Its worth switching ot trying another of your masters at least as a diagnostic. Still, I'd suggest getting in touch with Dr Diff or Master Power Brakes to get a small bore master set up for your application.

If I am reading this right, you simply changed a master cylinder?

Or did you convert to manual brakes? Because in a manual setup the factory used more leverage than a power setup. If you do not duplicate this extra leverage then you will have a firm pedal with no braking.

I pulled the entire booster/master cylinder off the car and put in a simple non power master cylinder. The brake pedal lever has only one hole. The power setup has the bellcrank in it to reduce pedal leverage.
Jeff

you sure the bell crank wasn't to INCREASE leverage? i.e. more pedal travel for less master cylinder stroke? that would be an increase in leverage.

if you had this multiplication of effort before, but removed it and ended up with less leverage, AND went to a manual master, it could be your issue.

can you add the bellcrank back in if it indeed added leverage?

Sounds like you have some mis-matched parts. The brake parts need to be matched together just like engine guys match cam, compression and heads.

You didn't tell us anything about the parts so it is impossible to help you, but my guess is that your rear calipers aren't the correct size to work with your front calipers. And/or, nothing is correctly sized to work with your master cylinder.

The brake system is a hydraulic system. Piston sizes need to match properly at all four corners to give you the correct brake force at each tire for your vehicle's weight distribution. If you mess up the brake force by using calipers that are too small or too big, then the car doesn't stop very well.

I want to thank those that have lent a "brain" with my car problems. I agree that there are/were a series of mismatched components on this car. I'll give a history on the car.

*Originally a 4 wheel non power drum brake car.
*I added the 11" a body front discs, Upper contol arms and power brakes including booster. This gave me the big bolt pattern.
*I added a B body 8 3/4 axle with a Master Power 10.5" disc brake kit, bought from a swap meet.
*The car had decent but not great braking at speeds over 20 mph. The low engine vacuum made stop and go driving unsafe, so I decided to try using the new master cylinder that came with the axle and brakes. The brakes bled out okay and the pedal was firm, but the stopping ability was terrible at any speed.
*The fitting sizes and reserviors on the MP M/C are reversed from most other Mopar M/Cs I have seen. Usually the rearmost reservior serves the front brakes and has a smaller outlet fitting. Not so with the odd Master Power unit.
*I switched the brake lines on the master back to front today. The braking improved a small amount, but the effort was tremendous to do it.
*I pulled the master off and REmeasured. The bore looks to be 1 5/32" which I understand is HUGE for a non power unit. Also, there is a raised round nub at the mount, 1 1/2" in diameter similar to what a power brake M/C looks like. The M/C was in a bag and box when I bought the axle and brakes, but now I'm thinking that it could have been a switcheroo.
*I cleaned and installed a non power disc master cylinder from a 75 Valiant. It has a bore size of one inch. The car stops better than it has since I have owned it. It isn't as good as a new car, but its good enough for now. Thank you all once more,
Jeff

See if you can find out what the piston size is in the rear calipers.

The rear brakes should apply about 1/2 of the force of the front brakes. So in a 4 wheel disc brake application, that means that the rear calipers need to half 1/2 the piston area of the front calipers.

This rule of thumb should work fine for most street driven Mopar muscle cars.

The piston size in the rear calipers is very important. If they are too big, or too small, the car will never stop correctly. You can't fix it with a prop valve either. The front to back bias has to be correct before you tune anything else.

The master cylinder is too big. Put a small-piston master on it, 7/8 would be good, and that will fix it. You might want to check the brake pads and put new ones on if they are glazed, in which case they will not stop well even with tremendous force. That's all you need to do.

I bet you have a "Corvette" 1 1/8" bore master cylinder. That would explain the reversed ports, hard pedal, low line pressure and decreased braking performance.

All else being equal, a smaller bore master cylinder increases your mechanical advantage at the pedal. The result is a longer, lighter pedal stroke and increased clamping force at the calipers.

BTW, post '72 power brake A-bodies were originally equipped with a 15/16" bore master.

Thank you Andy, Steve and Cass. Your words are respected on this board and I appreciate what you and everyone else has written here. I think that eventually I'll probably go with the 7/8" bore.
Dr, I think that you sparked something in my grey matter: The guy that sold me the axle and brakes DID say that this Master Cylinder was a "Corvette" unit. I thought that he was just bragging, like when people call an 18 spline transmission a "Hemi" 4 speed.
Jeff