Why change the master cylinder when you swap to disc brakes?

[VooDoo]

The reason I ask that question is my neighbor came across this article in the June 2012 issue of Four Wheeler that gives a simple solution to converting a drum brake master cylinder to function with a disc brake conversion by simply removing the residual pressure valve or combination valve. Here it is:



In case it is difficult to read:

No Drag Disc Swap
“When swapping from drum brakes to discs it’s a good idea to remove the residual pressure valve from the master cylinder or combination valve. Simply thread a machine screw into the brass fitting inside the port and pull it out with a pair of pliers. Remove the spring and rubber check valve, reinsert the brass fitting, and then reattach the brake line to the master cylinder port. This will keep the disc brakes from dragging.”

My neighbor claims the only difference between a disc brake master and a drum brake master are these valves that keep residual pressure on the drum brake system that a disc brake system doesn’t have. Of course you still need an appropriate proportioning valve or an adjustable one in the system.

If correct, that is a BIG time and money $aver for people installing a disc brake conversion. This short article is suggesting that is all you need to do.

Thoughts???

[Doc Stewart]

Well, the volume of fluid necessary to activate the calipers is different from the volume of fluid necessary to activate wheel cylinders.

For my money, I would use the correct MC. Talk to Vanco Power Brakes for better info.

[Bill usn-1]

The residual valves are discussed in most disc brake conversion threads.
But most make an upgrade to the MC at the same time with something like the GM hydroboost system.
Lets face it...if your going to go the go BIG!!!

[Thomas]

Originally Posted by Doc Stewart

Well, the volume of fluid necessary to activate the calipers is different from the volume of fluid necessary to activate wheel cylinders.

For my money, I would use the correct MC. Talk to Vanco Power Brakes for better info.

---Exactly. The RPV is only part of the equation. If converting from drum/drum to disc/disc, the Inter-di,nk media article may hold some truth and while some*, I'd say, they'll never be what they were pre-mid-80s & you're better off not even picking up a magazine from the now owners of JD Power & As's (more homage to soliciting/sponsors/ads than tech/truth). Furthermore, the need for a dual reservoir might be void as well. I'd do as suggested by Doc & contact our pro Bros. They're not afraid to answer eMails or calls.

--- I didn't read the article, but am wondering if they had enough tech to mention inline or adjustable proportioning valves to balance braking ratio.

[VooDoo]

Originally Posted by Doc Stewart

Well, the volume of fluid necessary to activate the calipers is different from the volume of fluid necessary to activate wheel cylinders.

For my money, I would use the correct MC. Talk to Vanco Power Brakes for better info.

Good advice. Do you know which one requires MORE fluid, drum or disc? It may turn out that the correct MC is the same MC without the valves and/or different brass fittings to adjust the flow??? Guess that's a good question for a qualified brake shop expert.

That picture and paragraph is the entire article. The other comments were from my experienced automotive repair neighbor, but I doubt he is a "brake system expert" like a brake shop should be. You just always have to have your, "Our product is what you need regardless" guard up these days. They don't make money giving free advice unless that advice includes a purchase of their products or services. I went through this with the modern muffler shops these days.

It'd be interesting to peek inside my 68 or 72 Travelall drum brake dual master cylinder resevoirs to see what's in there and compare to a similar disc brake master cyclinder. My OEM setup worked perfectly and stopped on a dime on the power drum brake 72! Never drove the 68. The only reason I went 4-wheel disc is because I changed to 3/4-1 ton GM axles.

This quick and dirty removal of the residual pressure valve might be worth a try for someone that is willing to experiment. That just might be me after talking to the pros...

[Thomas]

---The caliper piston bore requires more fluid than a wheel cylinder, so the disc gets the larger reservoir. The fluid capacity of the bore is also why there is a bypass valve on the front of a proportioning/combination valve for the disc portion. The system will* detect a leak & block fluid off before air can be bled thoroughly. Finger pressure, pushing* the valve will delay the blocking off of the fluid & provide a more effective/efficient/thorough job.

[WRENCH MAN]

The large reservoir for the disk brake is to provide supply as the pads wear, the pistons stay out they don't retract like in a wheel cylinder with a mechanical adjustment for the shoes, period nothing more, nothing less, the bore dia's usually have more to do with manual vs power, for a disk/drum system to work properly you really should have the combination valve installed, some times you can get away with an adjustable valve, some times, usually if you don't have a valve of some sort you're skidding the tread off of your rear tires!

[Stan Lazeski]

I did the disc brake conversion on my 800B. I used a Scout II proportioning valve, the stock 800B master cylinder, GM caliper holders and calipers and Scout II rotors and hubs. I used the stock spindles and used the correct bearings and wheel seal. I left the residual valve in the master cylinder after reading Mitch Neals posting, and have driven it well over 6000 miles since last November and am very pleased with the conversion. I used the stock master cylinder because I had already purchased it new from NAPA. Stan

[VooDoo]

Originally Posted by Stan Lazeski

I did the disc brake conversion on my 800B. I used a Scout II proportioning valve, the stock 800B master cylinder, GM caliper holders and calipers and Scout II rotors and hubs. I used the stock spindles and used the correct bearings and wheel seal. I left the residual valve in the master cylinder after reading Mitch Neals posting, and have driven it well over 6000 miles since last November and am very pleased with the conversion. I used the stock master cylinder because I had already purchased it new from NAPA. Stan

Stan, will you direct me to the Mitch Neals posting you refer to? Thx.

[VooDoo]

Originally Posted by WRENCH MAN

The large reservoir for the disk brake is to provide supply as the pads wear, the pistons stay out they don't retract like in a wheel cylinder with a mechanical adjustment for the shoes, period nothing more, nothing less, the bore dia's usually have more to do with manual vs power, for a disk/drum system to work properly you really should have the combination valve installed, some times you can get away with an adjustable valve, some times, usually if you don't have a valve of some sort you're skidding the tread off of your rear tires!

I remember finding out about the way disc brake calipers retain more and more fluid in them as they wear the hard way. The first time I changed pads on a disc brake car I had topped off the MC reservoir and left the cap off. Then I removed the worn pads and pushed the pistons back in to install the new ones at each wheel of the car. Then I noticed a puddle of brake fluid on the ground under the firewall area. When I checked the MC it had overflowed when each caliper piston was pushed in and brake fluid was everywhere! And brake fluid will eat paint.

Since I've learned to crack the bleeder on the caliper before pushing each piston in to the new thicker pad location. In addition, I've been told this is very important on any new antilock braking system, because pushing the possibly dirty fluid in the capilers back into the antilocking brake module is not a good thing and can cause it to malfunction (so I'm told).

Sometimes learning the hard way is good. You generally don't forogt something learned the hard way.