Dual Chamber Power Brakes - 50 Desoto

[vintage6t]

I'm in the middle of a project to re-power my 50 Desoto with a Mopar 383 B Block/727 Trans combo. The upgrades include four wheel disk brakes. I'm using an explore 8.8 disk rear outback.  This is going to be a nice cruiser, not a race car but being a big heavy car I thought having power brakes would be a nice upgrade as well. I'm posting this thread as a twist on using the existing single chamber master cylinder as a basis  for a dual chamber upgrade. Similar to Sam Buchanan's thread on mounting a Wilwood dual master but In this case for a 7" power booster and GM style master cylinder.

 

My goal was to use the original brake pedal and keep the master under the floor. I originally was just going to make a complete pedal and booster bracket from scratch. The master cylinder of course mounts on the back of the booster, so the only consideration there is having enough space/length under the floor to mount pedal, boost, and master. I started by making a bracket out of wood to make sure of fit, including re-use of the original cylinder mounting holes on the frame.

[vintage6t]

One of the worries of creating a bracket from scratch was the mounting boss to hold the shaft that the pedal rides on. It needs to be strong and to capture the shaft tightly. After thinking about this for a while I decided I could simplify the entire thing by trying to  use the boss from the old master cylinder. To do that I first sectioned the old master to hold the mounting plate for the booster. I  kept all the original mounting holes intact, so it would bolt right up to the frame as the original.

Edited July 2, 2022 by vintage6t

[vintage6t]

Next was to mount the booster plate to the sectioned master. I used the original push rod/cylinder hole in the old master as one mounting point. The other mounting point is a tab welded to the booster plate that is bolted to the top of the master via new drilled and tapped hole in the top of the master.  

Edited July 2, 2022 by vintage6t

[vintage6t]

The entire assembly bolted together. One thing I knew from building the original wood pattern was to clear the floor pan the entire assembly has to be dropped 5/8". Had a built everything from scratch, I could have build that into the bracket. Since, I can't move the exiting mounting holes on the old master by 5/8", I'm going to have to move the original mounting bracket on the frame down 5/8" instead.  This will drop the brake pedal down by that amount as well but that will be fine.

Edited July 2, 2022 by vintage6t

[Sniper]

Ok, I see what you did.  Since you are running an automatic you pressed the pin over and mounted the brake pedal where the clutch pedal would be.

 

Interesting.

[vintage6t]

23 minutes ago, Sniper said:

Ok, I see what you did.  Since you are running an automatic you pressed the pin over and mounted the brake pedal where the clutch pedal would be.

 

Interesting.

It would appear that way, but no. The pin is in the exact original location in the master boss. The clutch pedal never was part of this assembly. The clutch on this car has its own mount of the outside of the frame. This brake assembly mounts on the inside of the frame and the brake pedal and pin sit exactly as original. 

 

What has changed is that the original push rod mounted directly to the small boss at the bottom left of the pedal and ran straight through the master. I changed that to use a long pin through the original brake pedal hole and then offset through the booster  rod.

Edited July 2, 2022 by vintage6t

[Sniper]

How are you going to address the lever ratio going from manual to power assisted?  Likely to be touchy if you keep the manual ratio.

[James_Douglas]

I ran across these old notes from the 1949 desoto disc conversion. I used a stock MC with a midland ross remote booster. It is to touchy. Below are some measurements I took of non-power and power pedals.

 

                                                                            Long Side Pedal                             Short Side Pedal                                        Ratio
            
            
Chrysler Power Brake Pedal                            12.00                                               1.80                                                               6.67
                                                                           10.00                                               1.80                                                               5.56
            
Desoto Pedal                                                    12.00                                                1.45                                                               8.28
                                                                          10.00                                                1.45                                                               6.90

 

Basically it looks like the changed the upper long side. Also they used a different shaped push-rod that also affected the final ratio. See attached. I would look around to find the lowest ratio pedal I could get.  I would be curious as to the long side and short side measurements on the 1950 to see if it stayed the same.

 

James 

 

 

 

 

[vintage6t]

25 minutes ago, Sniper said:

How are you going to address the lever ratio going from manual to power assisted?  Likely to be touchy if you keep the manual ratio.

Good point. Admittedly it's an experiment and it will have to be assembled to see.  I'd say first of all a 7" booster isn't vey big, many people say they just give marginal boost so it may be fine. If not the choices are 1. Move down to a 6" booster. 2. Shorten the brake pedal itself, 3. Drop the booster and move to a manual master, this would only require a new set of mounting holes in the current booster plate and a little replumbing of brake lines.

Edited July 2, 2022 by vintage6t

[Sniper]

Be interesting to see how it works out, that's for sure.

[James_Douglas]

I can share that the 4 wheel disc set up, as I said above is a bit much, is using a midland ross C-490-K vacuum booster. Same as on the T-Birds in the 1950's and the Chrysler 300 RAM cars in the early 1960's.

 

Its specs are:

At 19.5 to 20.5 HG Vacuum if 100 lbs. of brake pressure is applied from master cylinder you get 185 to 285 lbs. out. At 250 lbs. you get 635 to 750 lbs. out.

 

That is with a vacuum booster bore of .8110 to .8135  and a 1 inch (I think although I may have used a 1-1/8 inch) master cylinder feeding the midland ross unit.  With this info you may able to run the numbers against what you are proposing to get a feeling of what it will do.

 

If it was winter and I had the car up at the house on the lift...I would take a direct at the wheel pressure reading and tell you what that is and then you could shoot for something a little less.

 

Attached is a file I managed to get some years ago. It is very similar to the unit I am using.  In the top right of the print it has the typical curve of pressure in and pressure out for the midland ross (as their successor) units.

 

James

 

c468l_v.pdf

Edited July 2, 2022 by James_Douglas

[vintage6t]

21 hours ago, James_Douglas said:

I ran across these old notes from the 1949 desoto disc conversion. I used a stock MC with a midland ross remote booster. It is to touchy. Below are some measurements I took of non-power and power pedals.

 

                                                                            Long Side Pedal                             Short Side Pedal                                        Ratio
            
            
Chrysler Power Brake Pedal                            12.00                                               1.80                                                               6.67
                                                                           10.00                                               1.80                                                               5.56
            
Desoto Pedal                                                    12.00                                                1.45                                                               8.28
                                                                          10.00                                                1.45                                                               6.90

 

Basically it looks like the changed the upper long side. Also they used a different shaped push-rod that also affected the final ratio. See attached. I would look around to find the lowest ratio pedal I could get.  I would be curious as to the long side and short side measurements on the 1950 to see if it stayed the same.

 

Thanks for all the info, it's very helpful. I measured my 50 Desoto manual pedal and got 10" long side and 1.49 short, so a 6.7 ratio.  I think where to measure is obvious but I can verify apples to apples if you can provide your short and long measuring points.

 

So here's where I think I stand at least on paper. Disk brakes need 1000 lbs at the wheels. I'm using only a 7" booster. That is a very marginal size in terms of boost and to be honest it was selected really for space considerations more than anything. I'm using a 1-1/8 dual disk brake master cylinder. I bought the master some time ago but if I remember correctly that was the closest size to the original metric master cylinder on the Explorer that the 8.8 rear came out of.  Either that or I had determined that was just the proper size master for disks in general. I really don't remember.

 

Regardless  7" booster with 1-1/8 master combo with a 7:1 pedal ratio will give 1059 lbs at the wheels. See the attached chart. That's 150 lbs less than a 8" booster.  Given that my pedal ratio is  lower than 7:1 at 6.7:1,  I get 1013 lbs to the wheels. That should be right in the serviceable range.  

[Sniper]

I wouldn't bet on it working out.  Do you have 23" of vacuum?  I get around 20-21" out of my engine.

 

Nice thing is, if it doesn't work out you can drop the M/C bore size to increase pressure.

[vintage6t]

7 minutes ago, Sniper said:

I wouldn't bet on it working out.  Do you have 23" of vacuum?  I get around 20-21" out of my engine.

 

Nice thing is, if it doesn't work out you can drop the M/C bore size to increase pressure.

I think boosters are designed to work at a minimum of 15" of vacuum. Unless the engine has a more radical cam, your 20-21 is typical. I agree that the master bore size can be decreased if the pedal is too hard. Unfortunately I won't know how it works for some time b/c it's a project car with much more to do until it's drivable again.

[Sniper]

Les us know how it works

[nonstop]

Might want to go with the dual diaphragm booster.  I started with a dual 7 inch and eventually ended up with an 8 inch dual with 1 1/16 master.  I am not sure of pedal ratio, but I am happy with the performance and pedal feel.