Brake master cylinder differences Drum-Drum / Drum-Disk

As in the title, what are the differences between a Drum-Drum Master cylinder and a Drum-Disk MC ? Or Disk-Disk

Thanks in advance
Luca

 

Hotrod-Streetrod...But we wont go there...

 

Gooday coke bottle ,we have a bit in common 38 ford coupe .Try to use a master cyl to match your brake system,if your using disc drum try getting a cyl off something with disc drum.

 

Any MC designed for drum brakes, be it front and rear or rear only, will have a residual pressure valve built in. This valve prevents the springs on the brake shoes from pushing the brake fluid back into the MC and thus increasing the distance between the shoe and drum. These valves can also be purchased and placed in systems that have a MC lower than the wheel cylinders and/or calipers.

 

The disc reservoir on a disc/drum master will be the larger of the two. A disc/disc master will usually have larger reservoirs than drum/drum masters, but are essentially the same, except for residual valves in the drum ports up until the early 70's.
Disc/disc masters can be used on drum/drum or disc/drum systems, and disc/drum masters can be used on drum/drum systems. Check for residuals in the drum ports, and add 10 lb valves if necessary for drums, and 2 lb valves to discs ONLY if the master is located below the floor.

 

Not entirely correct. Residual valves disappeared in the early 70's from most/all drum/drum and disc/drum masters because of wheel cylinder cup expanders, which keep air from entering past the cups during quick brake release, the main purpose of residuals. (they were never used on modern disc brakes) NO WAY can a 8-20 lb residual valve prevent 70-100 lb shoe return springs from fully returning the shoes to their anchors, along with returning fluid back to the master. With the shoes fully returned and properly adjusted, there should be about .025"-.030" total shoe-to-drum clearance.

 

Thanks for posting this Coke-bottle, I'm glad this subject came up. When I bought my '51 Ford F1 project it had been converted to 4 wheel discs with a dual master cylinder. I replaced the master cylinder with a like unit and now I'm wondering if I have the right kind. I believe its a disc/drum unit as the front reservoir is larger. I haven't messed with disc brakes much so any input would be appreciated. The brakes feel okay and the truck stops just fine but I want to be sure. Here's a crappy pic.

 

Thanks

 

But how can I ID a residual in the drum port? Now I have a vintage aluminium MC who have a larger reservoir, I think out from a Corvette but I can't see any particular parts in the reservoir

 

Residual(s) in 60's/70's dual master cylinders are located behind the tube seats. They are small rubber spring-loaded check valves, and can be felt when GENTLY inserting a paper clip or small drill bit into the port.

Keep in mind Corvettes never had disc/drum systems-they went from drum/drum to disc/disc in '65, dual system in '66, so there will be no residuals in any Corvette dual master, and the chamber sizes more than likely will be, or appear the same in all Corvette dual masters. (FYI, residual valves were never used on the disc circuits of US vehicles using modern spot disc brakes)

 

Post a picture of your m/c with the cover off, we can probably tell you pretty easily what type of system it was designed to be used with.

 

Why is the resevoir larger for disc? seems backward to what i thought i knew.

 

As the disc pads wear, the piston(s) in the caliper moves to take up the slack, and so it needs to have more fluid stored in the calipers. If you don't check the fluid often (typical driver) then the m/c will run out of fluid before the pads wear out....no brakes! It's just a safety thing.

 

I will post a pic asap

 

I am in the middle of replacing all the brake components on my 1950 Chevy Sport Coupe. It has the original Drum/Drum brakes and the stock single reservior master cylinder. I am aware that there is an internal residual valve in the master (which is being rebuilt) but do you recommend adding an additional residual pressure valve in the hard lines? The master is located under the floor with the hard lines and wheel cylinders located at the same level or above the master cylinder.

 

dmac620- I would suggest that you use a dual reservoir master so you have separate front / rear just in case the was a problem, you don't loose all brake capability. if that's the stock G.M. design then you shouldn't need additional residual valves.

 

Bore size of the M/C has been overlooked in this post and depending on who you ask, does matter. Smaller the bore, more pressure, mostly needed on disc/drum or D/D setups, drum/drum can be less. '67-75 Fords, be it Mustang , Granada, Pinto, LTD all have the same bolt pattern with a range of different bore and fitting sizes and locations, some left, some right. If you don't whata screw with a Wagner book or the kid at the parts house Speedway has the Ford style M/C now for $35 bucks, as well as the "Vette" style.
Also, lots of info in other threads on here about this if you do a search..............BobbyD