I've searched and seen all kinds of opinions/answers. I'm upgrading to power brakes on my '54 Customline. I'm staying with original drum brakes front and rear. Drivetrain is 5.0/AOD combo with stock rearend. Power brake booster is 8" dual diaphragm unit from Summit and I'm using this master for a '68 Mustang..... https://www.autozone.com/brakes-and-traction-control/master-cylinder-brake-system/remanufactured-master-cylinder-brake-system-m1488/715706_947841_2558_2877_3912 I thought maybe the master had built in residuals but I can't see any, stuck a small punch into the outlet holes and couldn't feel anything plus it's an AZ rebuilt unit so I'm not expecting much. So...with the power brakes, do I need aftermarket residual valves? If I need them, I was gonna use these..... https://www.speedwaymotors.com/Speedway-Drum-Brake-Residual-Valve-10-PSI-Purple,7502.html
I did something similar about 35 years ago - Late model dual master cylinder (not power) in a 1959 Ford F250 with stock drum brakes. It would suck in air in the front wheel cylinders and the truck would pull to the side on hard braking. Bleed the brakes, it would stop straight for a while. Installed a 10 PSI residual valve near the master cylinder and the problem went away.
You do need residuals for your older Bendix drums, either internally in the master, like factory, or aftermarket inline 10# valves. Power vs manual assist has no bearing on residuals. Be aware your pedal ratio should be changed from manual to power, or from around 6:1 to around 3:1. Power-assisted drums can be a real handful, so pedal ratio is important.
I'm using the stock brake pedal so pedal ratio should remain the same (it was a manual brake car to begin with). With the 8" dual diaphragm booster and stock pedal ratio, will the braking be too sensitive? Also, assuming there are residuals inside the master, would adding aftermarket residuals on top of that pose a problem?
V8 Bob was saying that you should change from the manual brake pedal ratio (6:1) to the power brake ratio (3:1) now that you have changed to power brakes.
My family's first new car, and my first car were '54 Customlines, but they were manual braking, so I don't know about any pedal ratio differences between manual and the remote style hydrovac booster used on the '54. If it measures near 6:1, it will most likely be way too sensitive with the dual 8" booster. Multiple residuals shouldn't cause any problems, but why not just be sure the master cylinder doesn't already have them instead of adding any???
On Chrysler products they changed the design of the wheel cylinders around 1976, newer ones don't require a residual pressure valve. They have a spring in the rubber cup to keep tension against the cylinder. I don't know if the rest of the industry did the same or what kind your car has.
According to a '74 Bendix spec manual I have, most of the US manufacturers (Ford, GM, Chrysler, AMC, IH) started to eliminate residuals in 1970, and most were done with them by '74, because of the cup expanders you mentioned along with common front discs.
How would I go about changing the pedal ratio from 6:1 to 3:1? Also, how do you determine if the master has residuals or not?
Here is a lesson on brake pedal ratio https://techtalk.mpbrakes.com/how-to-series/correctly-calculating-pedal-ratio Hope this helps. Phil
Using the tech tip on the MP brakes website, it looks like my pedal ratio is 5.1:1. Also, vacuum at idle is about 16" so I'm hoping the higher pedal ratio will compensate for the lower vac and put me pretty much where I need to be!
Sounds like you have a good starting point for pedal leverage. It looks to me like there are no residual valves in MC outlet ports. Phil
That has residual valves in it. The inverted flare seats are brass. The valves are under the brass caps. Master cylinders without them have flare seats that are made of the same material as the master cylinder body.
Should the brakes be too sensitive you can use smaller diameter wheel cylinders as an alternative to a pedal ratio change ..... pressure over area equals force. Smaller cylinders also take less volume to actuate/fill so the pedal travel should be less as well. They can also be different sizes front to back to achieve a good brake balance, like a proportioning valve does. Should you need a % change calculate the size required by area calculations not by the % of the cylinder diameter. In the early days of GM/Holden power front disc/drum brakes combinations without a proportioning valve here in Australia the rear wheel cylinders were 9/16" diameter. https://www.kingswoodcountry.com.au...e-drum-wheel-cylinders-x2-ej-eh-hd/kP5573x2-a
Having done a drum/drum conversion with an 8 inch booster. We had to go to a 9 inch booster to get it to panic stop. It was on a 64 Merc. Monterey . It would stop , but not panic stop as soon as we went to the larger booster it stopped fine.
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