Brake Wheel Cylinder Matching

[keithb7]

Hi folks, bit a of a mix up here with my brake cylinder sizes. Perhaps someone who is brushed up on hydraulic force calculations can chirp in here:

I had 1 left rear leaking wheel cylinder after going through and reassembling my brakes. I looked around for new replacement options. 

 

I found various parts listings for wheel cylinders. I was unsure, I'd get the right part. So I contacted my chosen supplier. I emailed them to ask if they could run to the shelf with a slide caliper and measure the front and rear bores of the LR brake cylinder, that they list for my 1938 Plymouth P6. I stated that being as the car is 82 years old who knows if my cylinders are stock? I wanted to attempt to buy a match. I waited and waited and they never responded.  I took a chance and  went ahead and placed an order. Two weeks later it arrived.  Looking at it, it seemed a little big to me. A week later, this morning, I finally get things apart. Yup different cylinder bores than the one I removed.

 

The cylinder I removed is 1" bore front end. 1 ⅛" bore rear end. The cylinder they sent 1 ¼" same on both ends.

 

So my thoughts are, I only bought 1 new cylinder. I likely should not utilize it, and have two different sized cylinders on the rear wheels. Correct? My options are  to order another new matching RR  1 1/4" cylinder so they are the same; or seek out a new stepped cylinder to match the old one that I removed.  So for now I will polish up the old stepped cylinder and re-install it again to get the car back up. (I enjoy driving it too much to leaved parked for two weeks, waiting for another new cylinder)

 

Does anyone know what stock rear cylinder bores should be? My car was built in Canada. So maybe a different size was used up here, when built new? If I go with 2 of the new 1 ¼" cylinders, what effect does upping the bore size do? More braking force? Thanks.

 

 

Edited May 9, 2020 by keithb7

[Dodgeb4ya]

Original Factory P5/P6 cylinders were...

Front wheel cylinder bore sizes...

*Front bore..1-1/4"

*Rear bore...1-3/8"

 

Rear wheel cylinder bore sizes...

*Front bore..1-1/8"

*Rear bore...1-1/4"

I now days do not accept what the parts suppliers have to offer as usually they are wrong and can only offer what they have or is available that might work.

As for 1-1/4" straight bore ...yes it will work but hydraulic forces to the primary front shoe and secondary rear shoe will not be what the factory designed and engineered.

Bigger w/cyl bore more pressure on the shoe.

[Jerry Roberts]

In a pinch you can have your existing cylinders sleeved although I think that it would be more expensive than replacement . And time consuming . I guess that the replacement cylinder fits the backing plate ? 

[desoto1939]

OK according to my EIS Brake catalog here are the wheel cylinder numbers that are the correct one for your 38 Plymouth and also 37 Plymouth

 

Right Front EW3596

Left Front  EW3595

Repair Kit S

 

Right rear EW3598

Left Rear EW3597

Repair Kit T

 

Get NOS parts not the new junk

 

Rich HArtung

desoto1939@aol.com

[keithb7]

Thanks Richard. I will see what I can do with those numbers. 
By chance is there bore sizes listed in your book for these part numbers you supplied?

 

I also stopped in to my local Napa store today. The oldest guy working there dragged out the oldest brake parts book he had. Blew the dust off.  
 

Under 1938 it said...N/A.  Boo...Hiss. 

[Tooljunkie]

It was my understanding they did away with the step bores after a few years. 
replace them in pairs to keep braking forces equal. 
 

napa/eis cylinders arent quite right for the front of my fargo, had to slot the mounting holes. 
nos? 
better off to replace the cups in what you have,if you can find them or complete kits. All these replacement parts for 70-80 year old vehicles are a crapshoot. 

[squirebill]

As stated: larger diameter means larger area and greater force for a given pressure. Also, Larger diameter means more volume for a given travel and will affect the travel of the brake pedal.

[Bobb Horn]

You may notice on our 38s, that the shoes are the same length, where as more modern shoes have a shorter shoe for one side.      I have been running the same length brake shoes for several years now with no problem.   My shoe linings are from a Ford product.    I believe one will wear quicker with my step type wheel cylinders, not much wear difference so far.