Drum/Drum dual master question

I am in the process of installing a new dual drum/drum master for my 59’ chevy truck. I have it mounted up in the stock location under the floor and just need to know which port goes to the front and which to the back? There are 2 ports, 1/2"-20 thread and 9/16"-20 thread. The 9/16<SUP>th</SUP> port is on the plunger/mounting end of the master. Both chambers are the same side and I have verified that it is a drum/drum master. I would think that the mounting end with the 9/16<SUP>th</SUP> thread would be the port that I’d run to the front brakes since you want them to engage first, correct? I called the company I purchased this from and they said it really didn’t matter since it’s a drum/drum configuration. I believe whoever manufactured this part made those port threads different to indicate that one side is for front and the other for the back. I also am planning on installing 10lb residual pressure valves and using 1/4" lines for both front and rear.fficeffice" /><O></O>

 

All drum/drum masters are not 50/50 in operation even when the fluid reservoirs may look the same, because the fronts always have larger wheel cylinders (in cars/lt.trucks), so it's best (and sometimes critical) to have the fronts plumbed to the correct port of the master designed for the front brakes.
The port thread/size really has no relevance to front or rear brakes, but does prevent the lines from being switched during service or replacement.
I recommend using a late '60s/'80s disc/drum master on drum/drum systems because the larger front fluid chamber is obvious, and you're all set for a future front disc up grade.
Check your master for internal residual valves located behind the tube seats, as all drum brake circuits had them until the '70s. </o

 

I have driven cars and swapped the lines, never could tell any difference. The master is the same bore for both reservoirs and the brake bias is taken care of at the wheel cylinders

 

The bore may be the same, but the two pistons don't always travel the same distance, and the reason front brakes should be connected to the correct port.

 

So this master fits 1967-76 Corvette and 69-70 Chevelle and I did find some instructions. I believe that the "front" of this master is the round end and the rear is the end you'd mount to the firewall. So I believe that the front port goes to the rear, and the rear port goes to the front. Correct?

 

All Corvette dual masters are disc/disc, and '69'70 Chevelle had either drum/drum or disc/drum. Corvette used both 1" and 1 1/8" masters while '69-'70 Chevelles used 1" for drum/drum and 1 1/8" for disc/drum. Chevelle did use the Corvette part # 1 1/8" master in '69 for disc/drum, according my brake books.
You might want to find out more about your master, bore size and what port is intended for the fronts.

 

I believe this is correct.

 

Every dual reservoir master that I have seen has one brake pedal pushing one one rod against one piston. What master cylinder has two pistons and how do they get these two pistons to move different lengths?

 

Front always plumbs to the port closest to the pedal.

 

^^^^^^^^^^^^^God I hope your wrong^^^^^^^^^^^^^^^^^

manyoldcars, all the vette and chevelle M/C I have had to tear apart had one break pedal pushing one rod pushing two pistons WITH diff. rate "springs" between the two pistons..............................

 

cool. Learn something all the time,. I've never messed with chevelle or vette masters
but I do have a complete C-4 suspension to put under my 50 Ford so I will watch for the master

 

Obvious you've never had one apart, but All dual master cylinder have two pistons, operating two separate systems, with the primary piston pushing the secondary piston with a column of fluid, hence the name dual. The design and layout of the fluid ports allows one piston to move more fluid than the other in most masters, and the reason they have to be plumbed correctly.

 

I don't think you can compare a Vette C4 M/C to a '70s Vette/Chevelle M/C.
I have a C4 here to check against but that won't help the OP any.............

 

^^^^^^^ Although.... my experience comes from medium and heavy trucks

 

The primary (closest to the mounting flange) master cylinder piston is not always plumbed to the front axle, as I've worked with several over the years. Some '80s/'90s masters with screw-in (rear axle) prop valves had to be in the primary section of the master to function properly. Some with traction-control also had the rears plumbed to the primary, while some OEs simply wanted the rears in the primary circuit.

Bottom line, know how the master you are going to use is supposed to be plumbed, because most (vertical split) are not 50/50 in fluid volume output, and you always want the larger volume going to the fronts.

 

I don't know about Vettes, but most the GM master cylinders I've worked on had the rear port closest to firewall plumbed to the front brakes. I've done the same when using them on other applications and they worked great.

 

Is that on a disc/drum setup? That would make sense. The op is talking about a drum/drum application I thought. If the master is set up for his application, the primary should be at the flange. I've never seen rear primaries unless its air brakes, but I have never worked on cars professionally, only rigs. Very interesting. Why did they want rear primaries?

 

It's been over 40 years since I've worked on OE drum/drum hydraulic systems, and don't remember details on why some were plumbed with the primary going to the rears. But as I wrote before, know how the master you are using is designed to be plumbed, as they are not all 50/50 in operation, including drum/drum and disc/disc that have fluid reservoirs that look the same.

 

Find out what vehicle that master cylinder was originally for. Find that vehicle in a junkyard or a schematic online and duplicate how it's plumbed for your system.

The fellow who found the ideal temperature for egg incubators was uneducated and simply stuck a thermometer under a chicken's ass. This proved to be the ideal temp.

Sometimes you got to get the info straight from the horses mouth or like the example above, from under the chicken's ass.

 

They do not have two separate pistons. There is one that has two pistons on it. What makes one engage sooner is the placement of the bypass holes in the cylinder. The rears will bleed off while the fronts get pressure. Then when you push the pedal farther the rears pass the bleed off point and begin applying pressure.
They did and may still do make some that have the fronts at the far end of the master cylinder. Had a customer who had his 36 Ford repaired and someone installed the newer style in place of the older, and the brakes didn't work every well. I had to replumb the system.
You can check it by cracking the fittings and have someone gently apply pressure. watch to see which one squirts first.

 

Give ECI, www.ecihotrodbrakes.com, 860-872-7046, or Master Power Brakes, www.mpbrakes.com, 888-251-2353 a call, all they do is brakes. MPB has a great catalog with many diagrams and facts.

 

IF you ever took a dual master cylinder apart, you would find (surprise!) two separate pistons.

 

When I say two separate pistons I mean two pistons that could be held in each hand. There "may" be some like that but the ones, and I mean many,that I have taken apart are two pistons machined on one piece of metal. All of the later GM type and the ones I have bought from Master Power Brakes are built that way.

 

You just didn't completely disassemble the masters. Below is a typical late '60s-early '70s drum/drum dual master. All dual masters have two separate pistons.


Below is a later disc/drum GM style.

 

i used a 70 chevelle master drum drum on my ride, i also put residual valve front and back, if my master has built in residual valve what will it do bad if any.

 

Nothing, it's just redundant........................

 

If this is for your '59, that is not the correct M/C. BTW, when you do buy the correct version, you can get a Chinese one, or a USA made one for $40 more - get the US one...

That is soo not the correct MC for a 59 pickup