Mustang Dual Disc/Drum Master Cylinder


A Discussion on Mustang Disc/Drum Dual Master Cylinder

From: RB35 (Original Message) Sent: 4/12/2005 2:06 PM
I can't find the copy, so I'll ask again.
I'm installing the dual reservoir master cylinder and forgot which port goes to the front. It's mounted on a '39 brake ass'y, so it points backwards.
I think the reservoir connection nearest the mounting bolts goes to the back brakes and the other is to the front. I remember mocking up the brake lines and had to cross them to get it right.
Any help will be appreciated.

From: carlroseville Sent: 4/12/2005 2:17 PM
Largest reservoir to the front brakes........

From: ScottEmma2 Sent: 4/12/2005 4:25 PM
I have a master cylinder from a 72 or 73 Mustang for drum-drum brake setup I'm putting on my F-1. Both reserviors are the same size so I'm guessing it may not matter which goes to the front or back on that application? The fittings are different sizes coming out of the chambers for the brake line hookup, but I've put fittings on to reduce them to the same size brake line.


From: jacks39deluxecoupe Sent: 4/12/2005 5:04 PM
RB35, I am installing the 72 Mustang Dual Master on my 39. I plan to put on the 2LB, 10 Lb Residual pressure valves as I have Disc/Drums.. My plan also is to use the small round floorboard hole to service the master chambers....Your post helped my out because I never thought of the cylinder being in backwards... good post..You saved me a big mess... Thanks and maybe we could compare notes! jack

From: AlbuqF-1 Sent: 4/12/2005 8:46 PM
Is the Mustang master for drum/drum or disc/drum? Are you running stock (drum/drum) brakes? Sorry for the questions, but it matters. Several of the aftermarket brake companies have excellent websites explaining what to use and why.

Sent: 4/13/2005 4:25 PM
Just my 2 cents but the position of the M/C is irrelevent. The reservoir closest to the acctuating rod is for the the rear brakes as these are the ones that need to be energized first in order to help you keep going in a straight line during braking. Steve

From: V8Bob Sent: 4/13/2005 5:04 PM
I have to disagree with you on your master cylinder knowledge. The primary (nearest the mounting flange) resevoir can be the front OR rear system depending on vehicle design and whether it is a drum/drum or disc/drum setup. But the fronts ALWAYS go to the largest resevoir if they are not the same, primary or secondary. I have even come across different plumbing on the same vehicle family, usually police vs stock production. I have been in the brake field area for over 30 years and have seen every combination there is. This of course applies to vertical split (front/rear) systems and car/light truck only.

From: RB35 Sent: 4/14/2005 8:10 AM
I always forget the details. It's disk / drum, '39 pedal ass'y in a '35 Fordor, with the m/c pointing towards the rear. GM calipers front and '51 axle and drums out back. Adjustable proportioning valve in the back and the 2/10 lb residual valves up near the m/c. Hope that will clear it up.

From: V8Bob
Sent: 4/14/2005 8:22 AM
Your brake setup sounds great, but since you didn't mention the year or application of master cylinder you are using just be sure there isn't already a risidual valve in the rear (smaller chamber) outlet. This was common up until the late 60's.

From: RB35 Sent: 4/14/2005 12:22 PM
I'll check, but I'm sure I got mid 70's Mustang. More info tomorrow.

From: jacks39deluxecoupe Sent: 4/14/2005 11:08 PM
V8Bob,, I just started installing my updated Disc/Drum brakes and also using the 70's Mustang reservoir..Two questions: Why does the smaller chamber have the LARGER hookup fitting requirement than the larger chamber? Why did they not design the two outlet fitting holes the same size? (that one bugs me!)
How can you tell if the Master Cylinder has the built in residual valve?

From: V8Bob Sent: 4/15/2005 7:11 AM
The outlet size will vary from one style or manufacture to another. Sometimes the larger chamber has the larger port and visa versa. The main reason the ports are different is to prevent the primary and secondary lines from being connected to the wrong outlet, both in production and later in normal service.
You can check for a built in residual valve by gently pushing a small drill bit or paper clip into the outlet port-if you feel a resistance you have a built in valve. Of course this should be done with an empty master cylinder off the car. They are simply a small flapper valve located behind the tubing seat, and found only on the drum brake circuit(s). But these were pretty much gone by the late 60's/early 70's.

From: jacks39deluxecoupe Sent: 4/15/2005 7:28 AM
Bob, I will check it out.. thanks...jack

From: RB35
Sent: 4/15/2005 8:48 AM
Best I can come up with is it's for the Mustang, disk/drum setup with the larger reservoir closest to the mounting base. Larger outlet is on the smaller reservoir.
For some reason and I'll have to start looking, I remembered seeing this set up and since the m/c points towards the rear, noticed that the brake lines had to cross, meaning, in my case, small reservoir to front disk, large one (closest to mounting flange) to the rear drums. Sound right? When I find it, I'll post, but if anyone out there has this set up and I've got it backwards, I'd appreciate a heads up.

From: ohronnie1 Sent: 4/15/2005 9:14 AM
The largest chamber on the cylinder,regardless of fitting size goes to the front wheels. The disc brakes need more volume thats why a bigger chamber.

From: ScottEmma2 Sent: 4/15/2005 3:19 PM
So, if the master cylinder has the residual valves in them (I checked the one I had and it appears it does based on Bob's description) how do they come out? I can't figure out how it would unscrew out of there. I've also purchased the Wilwood residual valves, will it hurt if they are installed with the ones in the master cylinder as well?


From: V8Bob Sent: 4/15/2005 5:08 PM
Please re-read the posts as your question has been answered several times!

If you have one in the rear chamber outlet why take it out? But don't run two valves on the same circuit. And do not use a factory installed drum brake valve on the disc circuit as this is too much pressure and will cause a run-away brake. If you want to remove it you will have to remove the seat very carefully with an easy-out which will allow the valve to come out.

From: ScottEmma2 Sent: 4/15/2005 5:12 PM
I got the cylinder for a 72 mustang with drums front and rear at autozone, and that's what I have on my truck (drums front and rear), basically just upgrading the master cylinder to dual chamber. So from what I can tell I wouldn't even need the wilwood valves I purchased?


From: V8Bob Sent: 4/15/2005 6:17 PM
If your master has residuals built in then you are good to go-no need to run Wilwood's. One word of caution- if the master you bought is a rebuilt unit I would recomend you take it apart to be sure it is clean inside. I have found too many rebuilds full of trash and that's why I buy only new units (although I still take them apart!) as there is usually not a great difference in price, and this is a VERY important part of the vehicle. And the new one's have been clean so far.

From: jacks39deluxecoupe Sent: 4/17/2005 8:22 PM
V8Bob, On the 72 Mustang dual master cyl.. On the small chamber, I can draw air out of the outlet port but there is resistance forcing air in.
On the large chamber - Air can be drawn easily in, or out of the outlet port.. Is that telling me anything about possible residual valves being present or not?? What it might mean is that it would allow fluid back from the disc brakes in front but not allow fluid back from the rear drums.. Is this any indication of what residual valves (2 & 10) I need to install in the system?
Kinda want to get it right the first time - - -!! If you would be so kind to provide your expertise.... jack

From: V8Bob Sent: 4/17/2005 10:26 PM
It sounds like you have a valve in the rear port by the way you have checked it out, but to be sure gently insert a small drill bit, 1/16th or so into the port. If there is a valve you will feel resistance, if not the drill will go into the bore with ease and you'll need the 10 lb valve.
Keep in mind the use of a residual check valve in a disc brake circuit is ONLY necessary when the master cylinder is mounted lower on the vehicle than the calipers, and then only a 2 lb valve should be used. You don't want fluid to drain or siphon from the caliper back to the master.
I hope this answered your questions-thanks for asking.

From: jacks39deluxecoupe Sent: 4/17/2005 11:01 PM
Bob, I have the cylinder mounted so I was on a creeper with a mirror ..I can see a vertical "slim" object in the Small chamber port. If I go dead center with a small wire into the large chamber port, nothing can be felt or seen..
The cylinder is mounted beneath the floor same location as the stock 39. It would appear that i need to install the 2# valve in the large chamber and press on..
How would the rear brakes act if the 10 lb valve wasn't installed - in this situation? Plus I do plan on installing the proporsioning valve in the rear system...

From: RB35 Sent: 4/18/2005 8:24 AM
Bob and Ron,
Thanks for all of your help and advice. Now I can finish the line install.

Jack, if you want some pictures, let me know.

Thanks again,

From: V8Bob
Sent: 4/18/2005 8:25 AM
I would remove the master and check it out more accurately on the bench for a rear check valve, but it appears you may have one in that rear port.
Residuals were pretty much gone by the early 70's because of cup expanders in the rear drum brake wheel cylinders which prevented air from being drawn past the cups when the brakes were released. But with the master mounted low in the car I would use them; 2 lb for discs and stock/Wilwood 10 lb for drums. This will ensure a sound hydraulic system with a firmer pedal.
With a disc/drum system a proportioning valve is a must. If you use a Wilwood 10 lb valve in the rear locate it near the master outlet and the prop valve downstream from it to the brakes.

From: jacks39deluxecoupe Sent: 4/18/2005 10:17 PM
RB35, Does your Mustang Cylinder have either valve installed? I pulled my cylinder as Bob suggested and the small chamber port does appear to have a built in valve due to a light spring pressure noticed with the small drill bit test. Your "post" was helpful and prevented me from having had two valves in the rear brake system.. I would like to see your photos..... .. take care!

From: RB35 Sent: 4/19/2005 8:06 AM
Finishing the spring yardwork. If no rain, I'll try and get some pics tonite. After all of this discussion, seems I've got the lines mocked up pointing at the wrong reservoirs. So Thanks again to Bob and Ron for the expertise. But I did get the proportioning valve downstream from the residual valve. It will show in the pictures.

From: BillB Sent: Update 4/02/2006 7:38 AM

I have disc/drum. The Mustang M/C has the rear residual valve within and installed with Early V8 Garage's mounting bracket along with a Wilwood Proportioning valve. The braided stainless flex line to the rear drum is from a marine Merc I/O hydraulic that raises the outdrive. The 2 Lb front residual valve is a bit forward in the line.

It may look a bit vulnerable, but is well protected by the frame.

Brakes are great, firm pedal and will lock up on all 4's




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