The Stovebolt Site Forums The Shop Area Driveline dual master cyl.

which dual master cyl. is
better for drum-drum brakes
i am working on 1954 1/2 ton chevy
and what to do a dual master
for safety.
also is the cpp bracket a good
choice to mount the master cyl?

i just talked to cpp
they say i need bracket,
master cyl, and two 10 lbs.
pressure valves to do the job
anyone use their products?
let me know how they worked?

I have used their bracket and had to cut it apart and re weld it to get it to my liking.

before

after

another view before

after showing tweaks (blue) and new bends (red)

final showing bracket tweak.

This was over 10 years ago so they may have made amends. (yeah, right)

I ordered my MC and bracket from classicparts. When I got them, they were actually from CPP.
MC: Dorman M89160
Bracket: worked perfectly with no modification
and I got two 10# pressure valves from Summit

My son and I just finished installing the dual MC from classicparts. Didn't have an issue with the bracket.

I was unaware of the need for 10# pressure valves. Can you provide more info on this?

Hy The Hill Boys, if the master cylinder used was designed for a disc/disc, or disc/drum system but actually being used on a drum/drum system, then the circuits that were intended for disc brakes would need a 10 p.s.i. residual pressure valve installed. Residual pressure valves maintain a pressure in the lines of the drum brakes which helps to keep the piston cups against the wheel cylinder walls and that pressure also helps counteract the action of the brake shoe return springs, without the residual pressure valves the brake shoe return spring will force the wheel cylinder pistons together in the wheel cylinder, this action will cause the brake pedal to feel low on the initial brake application and then feel correct on subsequent applications. This action will occur every time the vehicle is allowed to sit unused for a time, the other issue that occurs without residual valves is air can be drawn past the wheel cylinder cups which can contaminate an alcohol based brake fluid with moisture causing rust to occur in the system and a possible spongy feeling brake pedal, hope that helps.

Thank you! Here is what we bought. Didn't see anything specific about application...

http://www.classicparts.com/1947-59-Dual-Master-Cylinder/productinfo/71-201/#.VRof7I6Slec

I agree with most of what 3B says about the need for the residual pressure valves, except for the fact that they're only needed with a MC on drum brakes that was mean for a disc/drum, disc/disc circuit.
Regardless of what the MC was made for (dr/dr, di/dr, etc), IF the MC is below the floor, or below the wheel cylinders, it needs to have 10# residual pressure valves. If its below the wheel cylinders, then theres a chance that fluid will drain away from the wheel cylinders... resulting in what 3B described above.
In my opinion its a necessary safety precaution

Thanks for the info gentlemen. Looks like I'm not done with the brakes after all.

Are you drum/drum. ?

It seems to me that the factory never put out a system that didn't include residual valves, no matter where the MC was. The theory that it is OK with no residual valves if the MC is above the wheel cylinders, doesn't make any sense. If that explanation was correct, the truck couldn't be parked on a steep hill if it caused the MC to be below a wheel cylinder. Those valves are there to retain a positive pressure in the system and prevent air infiltration.

Hy GMJager and guys, my comments on residual valves were aimed at the question being asked, I did not intend them to be a tutorial on all the possible permutations of brake systems. I never said that residual valves were only required for drum brake systems. As GMJager said if the master cylinder is mounted closer to the ground than the wheel cylinders or brake calipers then resisdual valves should be used to prevent fluid drain back to the master cylinder, 10 p.s.i. valves for drum brakes and 2 p.s.i. valves for disc brakes. I always try to answer the question being asked with only the information needed to answer that question, so as not to confuse the person asking the original question. Perhaps a Tech Tip on brakes would be a good idea. Hope that clarifies.

I should comment on my statement above. The idea of not using the residual valves is being pushed by the suppliers of these parts and I find that quite disturbing. I respect the opinions and advice of our members here.

Yes.

So I'm a little confused. If you are running a drum/drum system (and won't be upgrading to disc/drum disc/disc in the future), is the MC that is ordered from CP ready to use WITHOUT the residual valves?

http://www.classicparts.com/1947-59-Dual-Master-Cylinder/productinfo/71-201/#.VRwHBvzRJ8E

Hy GMTK and guys, most of the conversion kits I have seen advertised use master cylinders designed for Corvettes, Corvettes were full disc brake cars. Drum brakes need residual valves (10 p.s.i.) to function correctly. I assume that the aftermarket suppliers decided to stock/sell only disc brake master cylinders and also sell the residual valves to those installing their kits on drum/drum or disc/drum equipped vehicles. The problems arise when the purchaser of the kit doesn't realize that the residual valves are required for their particular installation and the person taking the order doesn't know what's required, or thinks to ask what type of braking system this kit is being installed on. Other than the non stock bracket required to mount the dual master cylinder in place of the single master cylinder, the parts to convert should be available through your Friendly Local Auto Parts Store (F.L.A.P.S.). If you have a good F.L.A.P.S. you should be able to come up with a system that will be adequate for your application and not require separate residual valves because those valves will be built into the correctly spec'd master cylinder, hope that helps.

Thanks 3B for the clarification, makes sense now. Seems the local stores are also cheaper than ordering online.

That's the one we bought/installed. Nothing was supplied indicating application or if residual valves were needed. I have sent them emails requesting info on other things and they have never replied to me. I wouldn't bother asking them in this case.

i am going with drum-drum
so i am going with cpp bracket
and 10 lbs. pressure valve
and a mc from my local
advanced auto parts store
hope that will work?
thanks guys with all the info

Hy Keithfielder, you should be able to get your F.L.A.P.S. to come up with a master cylinder for a drum/drum application, if they can then no additional residual valves should be required, hope that helps.

have you checked for a 70 Malibu master cylinder, that should be drum/drum, and already have residual valves built into bottom of master and be very reasonably priced, I'd guess?

my auto parts store is getting me a
master for drum brakes
so i do not need pressure
valves from cpp?

You have to decide whether you want to use the valves. Personally I would not go without them. You must also find out if they are built into the new master cylinder.

i am useing a mc from a 1970s
and i do not need valves
and i just order bracket from cpp
i just have to get all new
brake lines and hoses
thanks guys for all the info.