You say you're using a "stock style" master cylinder. Is it a disk/disk master or disk/drum?

I got the MC/booster combo from O'reilly's and I ordered it for disc/drum.

Yep - looks like that's what I am missing !

That's your problem. You will never get the brakes working properly if you're using a master cylinder designed for rear drum brakes when you're actually using rear disks. You NEED a Disk/Disk master cylinder. That will solve your problem.

Nope

The only difference is reservoir capacity, just check it when doing an oil change, top off as needed and the residual pressure valve in the rear brake (front port) outlet, which he's already been told to remove.

Even removing the built in residual valve won't make it safe. There is no way I would use a disk/drum master cylinder on a disk/disk system. As the pads wear the pistons have to make up the difference and need larger amounts of fluid. That's why the reservoirs are larger on the disk side. It is ALWAYS best to use a master cylinder designed for the type system you're using. Jury rigging brakes is never a good idea.

then suggest one that will fit his booster rather than pontificate.

Other than the reservoir capacity and rpv there is no difference and if you can't be bothered to check fluids at each oil change then maybe you should be using Uber.

I agree with supercuda, just top offfluid as needed and ditch the rpv.

You can also use a disc disc master for drum drum car.

This doesn't take into account bore sizes but that's a different discussion.

A quick search turned up disk/disk masters for '73 Barracuda at Summit, and I'm sure a quick trip to Napa or AutoZone would turn up more. My point is.. you can use a disk/drum in this situation but when an engineered disk/disk version is available why jury rig the system. Makes no sense to me but then apparently, I'm no expert since you guys seem to think barbed wire and bubble gum is a better idea.

that would be a hell of a trick since a 73 Barracuda never had 4 wheel discs from the factory. Care to link to it, because whatever you think it is, it's not stock and it's will be a jury rig. Surprised you don't seem to have heartburn with the OP's setup since it is all a jury rig.

The OP didn't choose the best conversion kit for sure and that is the main problem. And Summit's parts aside, there are other Mopar disk/disk master cylinders that will fit and work on his power brake booster. They may not be listed for a Barracuda, but given he's using GM style rear calipers, going with a non Barracuda Mopar master cylinder seems trivial.

Provide a link, the OP has an issue, you say there is a correct fix for it, but will not help out with a link. Why?

So what you are saying is there are no Mopar master cylinders for disk/disk that will bolt up to a Mopar power brake booster? Yea, right.

I'm done arguing with someone who is bound to give redneck advice. Have a nice day.

Several have already suggested to remove the residual valve under the brass seat in the rear brake port of the master cylinder. Here's how to do it.

Using a wood screw, screw into brass seat just a few turns. Then use a claw hammer or wire cutters to grab hold of the screw and leverage the brass seat out of the master.

Remove the residual valve and spring, and reinstall the brass seat.

This should resolve the rear brake drag assuming the brake pedal linkage is properly adjusted.

I used a Strange 15/16" master cylinder with the line for the rears going straight back to the T-fitting and the line for the front brakes going straight to the line lock with no blocks or valves in the system. It stops straight and steady from over 120 with no lock up or pedal issues. I used one of Doctordiff's adapters and pedal pushrod. I am still using the original manual drum brake pedal assy.
I was going to use the Wilwood adjustable block but my car stops great now so why waste my money

Gus

In 74 IIRC the Chrysler imperial used rear disc brakes which should have been a bolt on including the master cylinder.. Probably take someone willing to look through the books rather than the normal parts jockey that needs to know if you have A/C to find you a brake light bulb..

No, I believe I was saying you are FOS as evidenced by your repeated inability to provide the link for the part you say will do the job. That is because it does not exist. The is no "disk/disk" Mopar master cylinder that will just bolt up to his existing booster.

does anyone have a picture of a wagner # F84054 [1 1/16"] master ? that is the number for the 74-75 imperial with the four wheel discs. caliper sizes were 3 3/32 front, and 2 19/32 rear. info from my wagner master catalog printed in november of 95.

I dug thru the 2 FSM on the Imp 4 wheel disc setup and really couldn't find anything specific about the MC, other than it's unobtanium.

But it sure had a lot about the Sure Stop setup (ABS)

I am still working on getting the math right for this brake system. I hope you guys can help me finish figuring this out. I may try to use the OE style stock replacement master disc/drum (and remove the residual valve for the rear) along with the stock style booster and see if I get enough pressure going into the gm style rear calipers. I am not opposed to changing the MC but I may as well try the one I have since I've already bought it.

Since this will be a "driver" I want to stay with power brakes if I can.

Front:
Single piston caliper with 2 3/4" bore on 11" rotor

Rear:
Single piston caliper with 2 1/2" bore on 11" rotor
*have manual valve inline to adjust bias

The best I can calculate, it seems ideally I would need an MC with 7/8" bore. I am not sure what the bore is on the stock style MC.

The calculations show I will need pedal travel with 8:1 ratio.

Any help will be much appreciated!

thanks guys!