Converting P15 drum/drum brakes to Dual-Chamber Master Cylinder

[Sam Buchanan]

When the '48 Special Deluxe sedan followed me home one of the first mods that came to mind was changing the standard master cylinder to a dual-chamber cylinder. Since the time in my very distant youth when my 1963 VW Beetle rolled into an intersection when a brake line let go I have been very wary of single-chamber master cylinders. I first checked with ECI to see if their master cylinder retro-kit was suitable for drum/drum brakes and was informed it was only for disks/drums. My hours spent researching the forum archives uncovered a photo of a retro-fit that formed the inspiration for this conversion. I don't recall who posted the photo....but Thank You. 

 

I'm including some in-process photos and will update as the conversion is completed. If anyone sees any gotchas feel free to wave a red flag. Here is the retro-fit as it stands this evening:

 

 

The new cylinder mounts behind the body of the old cylinder and is activated by a pushrod running from the brake pedal through the gutted old cylinder to the new cylinder. The pushrod was fabricated with a combination of 5/8" steel tube and the ends of the old pushrod:

 

 

It seems the original pushrod components are not easily replaced so this will allow them to be put back in service if a standard cylinder is used at a later time. A 7/16-20 nut was welded to one end of the tube and a 7/16-20 bolt to the other. This allows the pushrod length to be adjusted for proper pedal free-play. Below is the new master cylinder bracket, it is 0.090" steel and through-bolted to the frame rail:

 

 

 

 

Edited May 5, 2019 by Sam Buchanan

[Sam Buchanan]

Well....this is not the update I wanted to make. In spite of many attempts to bleed the brakes I was unable to get a satisfactory (hardly any) pedal with the dual-chamber master cylinder. I used to tell my science students that because an experiment didn't have the anticipated result that there was still knowledge to be gained. I suspect the cylinder I used didn't have sufficient displacement. It is a 1" bore and maybe a larger bore cylinder is needed since the standard cylinder is 1 1/8" bore. I reasoned that 1" was sufficient since each chamber was only pushing wheel cylinders on one end of the car but that must be faulty reasoning.

 

However, mechanically the mod worked flawlessly, the bracket and pushrod were an excellent setup. All that is needed is a master cylinder that is compatible with the P15 wheel cylinders. I wimped out and installed a new standard cylinder which bled as expected. I'll ponder this some more and may revisit the mod at a later date.

[plymouthcranbrook]

There was a company that sold Dual  masters for old Mopars but of course the link was lost in the latest computer crash.  I seem to remember that they were fairly expensive, maybe $275 or so.

[Cold Blue]

Very nice fab work Sam! Using the old MC as a "guide" for the tube was ingenious. I am surprised, as I am sure you were, that a 1" bore cylinder won't work. Does only 1/8" additional bore diameter make that much difference? Apparently so... I am thinking about putting a dual on my 48 coupe - your method is really cool. One question - I guess you would have to use a remote fluid fill as the filler hole on the new cylinder would now be behind the floor pan cover at the old MC?

[Sam Buchanan]

I don't really understand why the new MC didn't work either. First thought it was a bleeding issue but after running a quart or so of fluid through it never could get a pedal. The MC appeared to be pumping properly.....sort of a mystery. I even tried the front brakes without the residual valve but that didn't help. I suspect the problem is something simple but I couldn't nail it down. Since I have all the mechanicals done I may try again with a different cylinder when I feel energetic enough.... 

 

I cut a hole in the floor for access to the new MC. It would have required a cover with a slight dome shape but is mostly under the front edge of the seat so wouldn't be a problem. The geometry of the pushrod is such that it doesn't touch the old cylinder even at full travel.

Edited May 9, 2019 by Sam Buchanan

[vintage6t]

I believe a 1" master will move less fluid than a 1-1/8 master and it's not proportional either. A 1" will produce higher pressure though. Given that your wheel cylinders may not be extending all the way and giving proper pedal feel. You may be able to adjust the brakes out to get the pedal but the pads will probably be dragging hard on the drums when done. Even if that works the brakes might be very touchy.  

[Sam Buchanan]

14 minutes ago, vintage6t said:

I believe a 1" master will move less fluid than a 1-1/8 master and it's not proportional either. A 1" will produce higher pressure though. Given that your wheel cylinders may not be extending all the way and giving proper pedal feel. You may be able to adjust the brakes out to get the pedal but the pads will probably be dragging hard on the drums when done. Even if that works the brakes might be very touchy.  

 

Yes, the smaller cylinder will have less volume per given travel but yield greater pressure. I haven't explored the travel of each chamber of the new MC so don't know how displacement compares with the standard cylinder. The brakes were carefully adjusted for minimum shoe travel. Thank you for the reply, all thoughts are appreciated. I really want to figure out how to make this work.

 

 

[Cold Blue]

Nice home made brake gauge!!

[First P15]

I’m not an engineer by any means but here’s my observation regarding this issue. 1/8” reduction in diameter is approximately 12.5% less volume than the original mc. Since you’re now dividing the flow of fluid in half by sending it through two lines the reduction should now be 25%. If the new mc cylinder length is less than the original then you will lose volume there also. I think that in this case the volume of fluid required to move the wheel cylinder is more relevant than the psi exerted. Pressure will provide more stoping power but only after the Padres contact the drum

I’m curious about how this is solved since I am about to upgrade my 48 coupe brake system since the front brakes are the only ones that are functioning 

Edited May 11, 2019 by First P15