Brake Residual Valves

[JSeery]

I agree, always thought they were referring to keeping pressure against the return springs. But there is no way 10 psi is going to move the return springs, at least not much! Still it is a strange way to word it.

[rich b]

You have to think about it in the other direction; not the springs retracting the wheel cylinders; but the fluid extending them.

When the brake is released the line pressure drops to zero, at that moment the cups relax and air can enter before the springs begin to retract the pistons. The residual valves prevent that zero pressure occurrence.

Drum brake residual valves have been around forever; just never very obvious when they were built into master cylinders; but the 10 PSI external valves are much more prevalent now for use with aftermarket dual masters that don't have them.

The 2 psi residuals for disc brakes serve a different purpose, they contradict the force of gravity on the fluid when the master is lower than the caliper.

Anyway, that's my take on residual valves; I'm sure others will know more.

[Bluebell]

Thanks V8 Bob and Rich.B. I can still learn.
I have here been alerted to a possible problem that I previously hadn't thought existed.
I have a situation where the master cylinder (Ford 91T - 1 1/4" bore) m/cyl is positioned about 20 inches below a single Ford 1 1/2" wheel cylinder.
There are two setups like this, 2 masters, each operating 1 wheel cyl.
I probably need some higher rated residual line pressure valves, for my two master cylinders.

[JSeery]

That is a very large master cylinder bore! I assume you realize the larger the master cylinder bore the lower the line pressure to the wheel cylinders.

[Bluebell]

J Seery, This is not something of my composition. It is a Bren Carrier built by New Zealand Railways in 1940. NZR. cast their own backing plates and drums and used Canadian Ford truck(Wagner Lockheed) brake components. It is the only one in existence and it is complicated because it ties the brakes and steering together. Tight turns are accomplished by individual brake application by the steering wheel There is no manual or parts book in existence that I have heard of.
It is a copy of an early British Bren Carrier, that due to shortages of supply during early WWII was locally modified at production to hydraulic brakes. Apart from the Australian first run of carriers, all previous carriers (and all after, T16 aside) had mechanical brakes. The rear axle assemblies were sourced from the USA.
Anyhow, Back to RLVs.

[JSeery]

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Sounds like it was engineered that way. The issue on a car would be the pedal leverage, and I'm sure they knew what they were doing in that department!! This should be an interesting project.

[V8 Bob]

Bluebell, Check for stock internal residual check valves inside the masters. If none present, a 10 lb inline valve for each master should work OK.

[Bluebell]

Thanks guys for the help.

[JSeery]

I never really answered your question, I just ask more questions!! I agree with V8 Bob.

[1948F-1Pickup]

Originally Posted by V8 Bob

1948F-1Pickup,
Do you really believe a 10-15 lb (approx. normal OE production range) residual valve can overcome shoe return springs of 50-100 lbs or more? The "keeps the shoes in close proximity to drums" makes no sense at all, (although is/was shared by some misinformed brake suppliers) as the normal shoe to drum total clearance is .025"-.030". That's .0125"-.015" per shoe. The shoes in a drum brake should always return solid to their anchors, by design.
My 35 year dynamic brake testing experience started in '71 at Bendix, when residual valves began a phase out because of cup expanders, better cup designs, and front disc brakes that do not need residuals.

It's all semantics. Nobody is really "saying" 10# in a line is going to overcome a drum system's return springs. The residual valve maintains
a pressure head in the line, so rather than allow unnecessary fluid
movement back into the master cylinder chamber and a resultant
"longer" pedal stroke, you get quicker take up. The end result is a
quicker reaction, "as if the shoes were closer to the drum."

I didn't know we were getting into an SAE discussion here.
I've got better things to do than this pissing contest.

[V8COOPMAN]

Originally Posted by 1948F-1Pickup

It's all semantics. I didn't know we were getting into an SAE discussion here.
I've got better things to do than this pissing contest.

This is a "discussion forum"! It's always interesting when some new "expert" comes along, huffin' 'n puffin' about how much and WHO he knows, yet hasn't got the time to open-mindedly chance learning something in the meantime. We don't do "pissing contests" here! We prefer to deal in "TRUE SCOOP"! DD