Anyone know how to figure a change in rear wheel cylinder diameter might cause issues? I'm working on a 1939 Plymouth Pickup (same as 39-47 dodge) which I installed a rear axle from a 86 jeep and its drum brakes. It originally had Lockeed brakes, 10" drums with each wheel cylinders having two bores (1-1/4 and 1-1/8" diameters) front and rear. The transplanted Bendix rear brakes use a 7/8" diameter wheel cylinders, also with 10"drums. I suspect this may affect braking but not sure how to calculate? I also plan to upgrade to a dual circuit master cylinder. Would like to find one that has a internal residual pressure valves. Can these still be purchased? Any advice would be appreciated.
About 30% less..You check and you may be able to find wheel cyl with bigger bore; what did the bendix come from?
Brake cylinder pressure(force) is a function of the cylinder area. A 7/8 inch cylinder will have approximately half the pressure (force) of a 1-1/4 inch cylinder, for any given input pressure.
Also be aware that a change in the Wheel cylinder bore will require a change in the master cylinder. ie. If you increase the diameter by 30%, you also increase the volume by the same 30%. This means that if the master does not change to respond to the change in the wheel cylinder, your pedal travel will cause the wheel cylinders to move 30% less. In order to maintain a constant pedal you need to replace the master with something that produces the same 30% more volume. The 30% more volume will change the pedal feel. To gain the pedal feel back you will need to change the pedal geometry.
You may want to review the formula for the volume of a cylinder. For the same stroke length in the master cylinder, you will discover that a 30% increase in diameter will result in a 69% increase in volume.
My head is hurting! Lols! This truck's brakes worked perfectly until I started messing with them. The original single outlet M.C. had a 1-3/8" dia. bore. with an internal residual pressure valve. The truck originally had a drum style emergency brake mounted to the back of the trans. I installed a T5 trans. and my logic at the time was to use along with a donor Jeep's 3.55 differential and housing, the rear brakes which gained me emergency brakes and self adjusters. Never once did I consider wheel cylinder differences. (Stupid is what stupid does) Anyhow, I also installed the master cylinder from same Jeep. Figuring on gaining a split system for safety. It has a .9375 bore and no residual pressure valve. Brakes are doing strange things, mostly pedal has way too much travel. Was considering installing front discs, but not really liking the available kits for these trucks. Mounting plates seem flimsy, spindle modifications, offset changes, Aluminum hubs? After my recent attempt at backyard engineering, I'm feeling a bit incompetent. The more I read about brakes the more confused I get. Any logical suggestions to get this truck to stopping correctly again? Oh yeah, I was running DOT 5 in this truck for over 25 years with no issue, but I'm considering going back to DOT 3 or 4. Since my modifications it also seemed that the pedal travel and feel would change with heat?
My understanding is that you have the same brakes on the front as before, the Jeep dual master cylinder, and the Jeep rear axle with much smaller bore wheel cylinders, correct? The basic issue is that you are not moving enough fluid with that small bore master cylinder to make the front brakes effective. To move more fluid takes either a bigger bore master cylinder, or a different pedal ratio to move the plunger further in the master cylinder. Bigger bore master cylinder is a better solution than changing the pedal ratio, as you may end up bottoming out the master cylinder and still not have effective braking. You also likely need to use bigger bore cylinders in the rear. Also, is the rod from the pedal going into the master cylinder adjusted so there is a minimum of free play? The master cylinder you are using was from a vacuum assisted power brake setup, and now you're using it in a non-boosted configuration, correct? The engineering of a boosted system is totally different than that of a "manual" setup, because the boosted systems give a human leg the power of a buffalo.
Keep in mind the very different rear drum brake designs; your original Lockheed NON-servo drums vs the more efficient Bendix servo drum brakes. Really cannot compare wheel cylinder sizes because of brake design and lining effectiveness differences. Best to get the truck on the road to test whether the cylinders are going to perform OK. Most dual drum/drum master cylinders had internal residuals up until the mid '70s, but check the master you purchase to make sure, and add external 10# valves if necessary. A front brake upgrade should be in your future.
I shouldn’t bother but....increasing master bore size also reduces pressure given the same force on the pedal, also when the system is bled there is very little volume change or flow...less than a teaspoon typically. The fluid essentially acts as a solid. Sent from my iPhone using H.A.M.B.
Thanks very much for responding! Yes, you are correct. The 1986 Jeep did have a booster with front disc brakes. My 39 Plymouth is non boosted having oem pedal ratio of 7.5:1. Yes, the push rod has minimum free play and around 1.125" available travel. There are some manual brake dual outlet m.c.'s available with larger (1.0, 1.031. 1.125) diameter bores that could be adapted. Still not as large as the original single outlet 1.375. I'm uncertain if I can find them with the internal Residual Pressure Valves which I've read are important on all drum brakes? It seems logical that the smaller bore would work better if I switched to discs due to higher pressure needed? My original thinking was to convert to front disc, but not sure that's the best route? The Jeep Comanche's and Cherokees were of similar weight and wheelbase as the 39 Plymouth truck with same bolt pattern, drum size, and shoes on the rear. Maybe front discs with similar caliper bore size as the jeep (2.6"dia.) and the jeep proportioning valve would be the best route? But I'm really not sure at all? I don't wish to use a booster (Not enough room without major alterations). Might end up with longer stopping distances than with the original braking system? The original brake system performed very well. You're probably sorry you responded now. Lols!
Gee thanks Bob! Just what I need more variables. Lols! I'll have to read about servo and non servos. Perhaps you could explain further? Thanks for reply! When you say "upgrade" I suspect you mean front disc?
As I was reading through the posts I was wondering about the same point that V8 Bob brought up. I have no knowledge of the '39 Plymouth brakes but later brakes are self-energizing. I guess that's the right term? Many older brakes relied on sheer brute force to apply the brake shoes, but later brakes utilize the rotation of the wheel to "help" apply the brakes more effectively with the same, or possibly less hydraulic pressure. Before making a bunch of changes I think V8 Bob's suggestion to get the truck on the road to assess how the brakes are going to work and then make changes if necessary is good advice. Lynn
Thanks Lynn, I agree and that's sort of my plan. Seems like the master cylinder is the place to start to get her on the road. It does not work correctly with the current one. I do not yet understand the difference between power and non power brake master cylinders? Seems to me other than bore diameters and stroke lengths they all work the same? The ones designed for disc brakes I believe just have a larger reservoirs? I imagine there's more to it, but I haven't figured it out yet. Installing the M.C. is a bit of a project on this truck. Limited space and requires fabricating an adapter. Not to mention re-plumbing. I'm thinking perhaps I can find a m.c. with a slightly larger bore, say 1.125 and a large enough reservoir for front disc if I should decide to later? My dilemma continues. Lols!
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