Disc Brake Conversion

[Caveman49]

I originally bought the residual check valves for my 59 Apache (sold the truck, never plumbed the brakes). These were used mostly for the 50s vehicles because of the MC being lower. From my understanding, the 10 lb valve plumbed for the rear keeps at least that much pressure in the line, thus it doesn’t take as much pedal to energize the rear brake shoes.
I may be wrong about this, but it makes sense to me.

[A/FX]

I originally bought the residual check valves for my 59 Apache (sold the truck, never plumbed the brakes). These were used mostly for the 50s vehicles because of the MC being lower. From my understanding, the 10 lb valve plumbed for the rear keeps at least that much pressure in the line, thus it doesn’t take as much pedal to energize the rear brake shoes.
I may be wrong about this, but it makes sense to me.

Yes, the valve keeps slight pressure on the shoes and in the case of a master cylinder mounted below the wheel cylinders the valve keeps the brake fluid from flowing back to the master. Many cars were built with the master cylinder mounted below the wheel cylinders and suffered from “backflow”, the residual check valve cures this issue.
Jim

[Lou's Comet]

If the Master Cylinder is higher than the calipers or wheel cylinders, what is the purpose of the residual valves?

If the master cylinder is higher than the calipers, for disc brakes you do not need a residual valve. For drum brakes the residual valve keeps the cups in the wheel cylinders expanded which gives you better pedal and stop possible seepage. You would need a residual valve for the drum brakes if it is not already in the master cylinder.
But a lot of master cylinders have the residual valves built into them for the drum brakes. For example if you have a master for a disc/drum setup the disc brake side of the master would not have a residual valve but the drum brake side would likely have a residual valve already built in to it.
A disc/disc master wouldn't have any residual valves so if you used a disc/disc master on a car with disc front and drum rear you would need to add a residual to the rear.

Lou

[Caveman49]

Thanks Jim and Lou. How can one tell if their MC has a residual valve in it without tearing into it, already on car. I’ve read on a couple sites that it won’t hurt to install one for the rear drums in any case. Thoughts?

From what I’ve been reading, it seems they stopped putting the valves in MCs after 74 timeframe, and a way to test one is with a paperclip or the like pressing in on the fluid outlet ports for slight resistance. The HAMB has some pretty good info on it.

[Lou's Comet]

Thanks Jim and Lou. How can one tell if their MC has a residual valve in it without tearing into it, already on car. I’ve read on a couple sites that it won’t hurt to install one for the rear drums in any case. Thoughts?

From what I’ve been reading, it seems they stopped putting the valves in MCs after 74 timeframe, and a way to test one is with a paperclip or the like pressing in on the fluid outlet ports for slight resistance. The HAMB has some pretty good info on it.

You would need to remove the rear brake line from master and check for the residual valve as you said. Or if you know what the MC is off or the part number you could probably research it online. No it won't hurt to install one for the rear drums if you already have one in the master cylinder.
When they stopped putting them in master cylinders they started using cup expanders in wheel cylinders and the residual valves were no longer needed.

Lou

[Caveman49]

Thanks Lou, as always, you and all the others here are very helpful.

[Rocket989]

A ton of good info/advice in this thread! I'll add my anecdotal experience(s):

-Disc brakes are better, but front disc/rear drum is plenty for 99.2% of what anyone is gonna do with our cars.
-Master Cylinder piston sizing is based on brake system volume. While the (semi-complicated) math is out there, unless you're running some BIG ole brakes on a Comet/Falcon, manual disc/drum = 7/8" to 15/16" piston. Manual 4-wheel disc = 1" to 1-1/8" piston. It's a trade-off between pedal stroke & pedal effort...
-Whether built-in or not, 10-lb. residual valve on rear circuit for disc/drum. For reasons I can't explain (frankly, have probably forgotten), I've had the best luck using a 2-lb. residual valve for the rears using 4-wheel discs.
-The later triangulated circuit MC's are the safest. If a circuit fails, you wind-up with wheels locked on a given side vs. front or rear. Though all these that I know of are the later clear polymer body design so may stick out like a sore thumb depending on what you're aiming for with your car.
-Bench-bleeding a MC is BS... Put in car, seal ports and get it done in <2-minutes with the pedal. If it doesn't hydrolock pretty quickly, it's junk. From there, a pressure bleeder for the four corners is your best friend.
-For the above referenced 99.2% of applications in our cars, the stock MC pushrod is what you want. Most aftermarket parts are too long to adjust & have fit properly (if at all?).

Most important of all is proper lockup. Get 'er all done and TEST!!! If you can't lay down even rubber stripes when you stand on the pedal - fix it!!