What if I don't change my Master Cylinder on a disc conversion?

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equium

equium

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It goes without saying that on a disc conversion, you have to change the MC if you have a single MC reservoir.
I'm about to finish up a Scarebird conversion on a 70 Valiant. The PO installed a new MC before I bought it. It's the MC for a 4-drum system with 2 reservoirs, but they're the same size.
What happens if I don't change the MC? Anyone try this?
I'm not trying to be cheap, just curious.
thanks
 
The drum brake masters typically do not have enough volume of fluid to work the disc properly (they will run out of fluid before you brake properly). You should change the master to one that will work.


I recommend The Ram Man. He hooked me up with the proper manual brake master cylinder that I can use with my discs (I converted a 66 Valiant to Kelsey Hayes discs and a manual dual reservoir master cylinder with a larger front reservoir for the discs). He's real easy to talk to, friendly, and will explain to you what you need to do to make the system work (ie. master cylinder, prop valve, etc.). He rebuilds brake parts and sells them, and will probably have what you need to make your system work properly. His customer service is very good and he knows brakes very well. I've had people thank me for recommending him.


THE RAM MAN
7200 Winters St
Fort Worth, TX 76120
Telephone: 817.691.5996


http://www.theramman.com/
 
The fluid reservoir is typically too small, empties the fluid into the caliper then sucks air. There is also a residual valve that holds pressure that should not be there for disc brake calipers. Don't cheap out on your brakes, they are the second most important things on your car next to tires.
 
While the M/C doesn't have enuff fluid capacity to fill the void created as the disc brake pads wear,, it certainly has enuff capacity to work the disc brakes on a daily basis,, as long as you "top up" the M/C when you do your regular maintainance, to keep up with the pad wear...

To remove the residual pressure valve from that M/C,, simply remove the rear most reservoir's valve as shown in the vid,, (only leaving the M/C on the vehicle)..

http://www.bing.com/videos/search?q...&mid=FBA26E668FEAF993E5F9FBA26E668FEAF993E5F9

Brake fluid is excellent paint remover,, so if any gets on the paint, rinse with lotsa water immediately..

If you find the pedal overly hard to push,, changing to a smaller bore M/C will give an easier pedal, with better "feel"..

Rock Auto has them..

RAYBESTOS Part # MC36406 More Info {Professional Grade; Bore Size=15/16" Number of Ports=2 Primary Outlet Size=9/16x20 Secondary Outlet Size=1/2x20}
Front Disc Brakes; Rear Drum Brakes;
http://www.rockauto.com/catalog/more...117&cc=1088760

hope it helps
 
The size of the reservoir is not an issue. Once the system is bled, and the fluid topped up, the reservoir level will drop very little when the pedal is depressed (as soon as the pedal begins to move the piston the seal passes the reservoir port isolating the fluid in the reservoir from the fluid in the lines, if it didn't you would not be able to build pressure in the lines). Also, pre-73 disk equipped cars generally had same sized reservoirs from the factory, the larger front brake reservoir is there for convenience and safety. As the pads wear the level will drop. Having a larger reservoir means you don't have to top off the fluid as often.

You should be more concerned with residual pressure valves in a master cylinder that was originally on a drum/drum car. The valve (if present) can be easily defeated with appropriately sized drill bit.

A commonly available 15/16" bore master cylinder should work fine. Most of the new or rebuilt master cylinders out there are based on the large/small reservoirs. So unless you want an equal reservoir master cylinder for visual appearance, it's probably easiest to swap to a rebuilt disk/drum part.
 
C130 Chief said:
The size of the reservoir is not an issue. Once the system is bled, and the fluid topped up, the reservoir level will drop very little when the pedal is depressed (as soon as the pedal begins to move the piston the seal passes the reservoir port isolating the fluid in the reservoir from the fluid in the lines, if it didn't you would not be able to build pressure in the lines). Also, pre-73 disk equipped cars generally had same sized reservoirs from the factory, the larger front brake reservoir is there for convenience and safety. As the pads wear the level will drop. Having a larger reservoir means you don't have to top off the fluid as often.

You should be more concerned with residual pressure valves in a master cylinder that was originally on a drum/drum car. The valve (if present) can be easily defeated with appropriately sized drill bit.

A commonly available 15/16" bore master cylinder should work fine. Most of the new or rebuilt master cylinders out there are based on the large/small reservoirs. So unless you want an equal reservoir master cylinder for visual appearance, it's probably easiest to swap to a rebuilt disk/drum part.
Click to expand...

Thanks Chief. So when I get a new MC from a vehicle with disc/drum setup, will I have to worry about the residual valve? I'm looking at the Raybestos MC36406 that was suggested on another post.
thanks
 
Why half *** it? Master cylinders are cheap enough. Like stated brakes are pretty important.
 
Hey Joe, I'm planning on changing the MC, I was posting the question out of curiosity and perhaps it others had the same question, too. This car is for my son, so I don't mind the extra expense of the MC.
(I like the mustang. Always been a fan of the fox bodies)
 
If you use a m/c from a disk/drum car there will not be a residual pressure valve in the front circuit. Therefore a bolt in swap that solves possible problems.
 
DDodger said:
you xplain why?
Click to expand...

Not any less safe than it was before, just that the dual master is safer as you have two kicks at the cat if you blow a line.

I did the disc swap on my 65 Stang and left the single master, never had a problem with fluid volume but having said that, I'm planning on going to a dual master during the current never ending restoration.
 
DDodger said:
you xplain why?
Click to expand...

krazykuda said:
The drum brake master cylinders do not have enough "***" (fluid capacity) for the disc and will empty...


Or in laymen's terms - You'll crash....
Click to expand...

The large reservoir is supposed to contain enuff fluid to last thru the life cycle of a pad set,, approx 50,000k (+ -), on a daily driver, or aprrox 5 yrs driving by the average driver..

The "smaller" reservoir would only contain enuff fluid for about 1/2 the life cycle of the pad,, or about 25,000 miles..

So, were you to drive it daily for over 2 yrs without checking the brake fluid,, you may start to get a progressively softer pedal..

I don't know about you,, but most folks service, or have serviced their vehicle annually anyway.. ( I would hope),,

So if you think you might drive the car 25k without checking the fluid levels,, you may get what you deserve...

jmo
 
abodyjoe said:
Why half *** it? Master cylinders are cheap enough. Like stated brakes are pretty important.
Click to expand...

The OP mention in the original post it was a new M/C,, so removing the residual valve is quite viable,, as you prob know,,. both man/disc M/Cs are prob the same casting with or without the valve.. jmo
 
It takes more fluid to fill/move the caliper in the discs than it does for the drums. You need a larger reservoir or the front one will run dry.

If you want to take a chance of your brakes not working, it's your call. The possible consequences are you can crash and get injured up to and including death. It's not worth the risk to wreck your car or hurt yourself and/or others. If you want to save money on your build, do it somewhere else, not the brakes....

I say get the proper master cylinder and prop valve for your application. I would not put my son's life at risk by trying to save money on brakes or recommend someone to do that to their kid. It just cost $12,000 to bury my mom a few months back, you can buy alot of master cylinders for that.....
 
Mopar disc/drum and drum/drum MCs never used the same castings. Disc/drum MCs always had one large and one small reservoir. I would surmise that there is a good reason for this. 45+ years of engineering practice and field experience probably mean something.
 
krazykuda said:
It takes more fluid to fill/move the caliper in the discs than it does for the drums. You need a larger reservoir or the front one will run dry.
Click to expand...

Are you under the impression that the whole volume of the M/C is used on each application of the brakes,, the tube size alone would restict that.. and in fact poorly adjusted rear brakes can take more volume of fluid than the front discs..

When you bleed the front brakes,, does the reservoir empty on every stroke,, .??

In fact only a few cc's of fluid a used to apply pressure on the front bakes, the calipers merely relax off the disc,, counting on the rubber seal to relax/withdraw the piston..

Granted, the small M/C isn't ideal,, I merely answered the OP original question,, and how to do it the way many others have... cheers

hope it helps..
 
inertia said:
Granted, the small M/C isn't ideal,, I merely answered the OP original question,, and how to do it the way many others have... cheers

hope it helps..
Click to expand...


Just because it will allow the brakes to work a little, doesn't mean that it is right. It takes more fluid to move the pistons in the discs than in the wheel cylinders. The drum brake master cylinder may get the brakes to actuate a little, but not fully.


Just because it fits, doesn't mean it's right....

View attachment Fat guy speedo A04.jpg

View attachment Fat guy speedo A01.jpg

View attachment Stars Stripes.jpg
 
inertia said:
are you under the impression that the whole volume of the m/c is used on each application of the brakes,, the tube size alone would restict that.. And in fact poorly adjusted rear brakes can take more volume of fluid than the front discs..

When you bleed the front brakes,, does the reservoir empty on every stroke,, .??

In fact only a few cc's of fluid a used to apply pressure on the front bakes, the calipers merely relax off the disc,, counting on the rubber seal to relax/withdraw the piston..

Granted, the small m/c isn't ideal,, i merely answered the op original question,, and how to do it the way many others have... Cheers

hope it helps..
Click to expand...

View attachment PS_0772W_CAN_EXPLAIN_t.jpg
 
krazykuda said:
Just because it will allow the brakes to work a little, doesn't mean that it is right. It takes more fluid to move the pistons in the discs than in the wheel cylinders. The drum brake master cylinder may get the brakes to actuate a little, but not fully.


Just because it fits, doesn't mean it's right....
Click to expand...

Balderdash ! !...

nice pix,, but you're avoiding answering my question,, can you empty the M/C reservoir in a single stroke,, 6 or 8 maybe..???
 
C130 Chief said:
Also, pre-73 disk equipped cars generally had same sized reservoirs from the factory,
Click to expand...

Sorry but this is not true. Every disc brake car I've owned back to 1966 models, which is a lot, all have larger reservoirs for the disc. Never seen a factory disc brake car of any make that didn't have a lager reservoir of a common reservoir.
 
krazykuda said:
View attachment 1714766202
Click to expand...

niiice.. but let me help you to a logical conclussion,,

With one stroke of the pedal with an open bleeder,, it certainly doesn't empty the reservoir,, not even close..

Were you to have marked the fluid level before the stroke,, and then take note after,, there may be as much as 1/4inch of fluid loss in a single 4(?) inch stroke,,,

ergo,, with well adjusted brakes, regular pedal tavel would be 1 -2 inches..

Wellll,, if in a 4 inch stoke you move a 1/4 incha res. fluid,, hmmm with 2 inch stroke... you got 1/8 inch of res. fluid moving back and forth, back and forth,, in the M/C,, how many cc's you think that is..??

Oh never mind, ..lol ... cheers

PS Have you had this discussion with Del,.. lol.. How'd that end for ya..?? cheers
 
Alright now. I didn't mean to cause a stir. I've read so much about brakes and bore and stroke in the past, I just wanted a little education. I was going to upgrade the MC anyway. Posting a picture of me in my swimsuit was a bit low, but does motivate me to lose weight. Seriously, though, thanks for all the posts and education.
 
equium said:
Alright now. I didn't mean to cause a stir. I've read so much about brakes and bore and stroke in the past, I just wanted a little education. I was going to upgrade the MC anyway. Posting a picture of me in my swimsuit was a bit low, but does motivate me to lose weight. Seriously, though, thanks for all the posts and education.
Click to expand...

Lol.. you're welcome..

hope it helped.. lol
 
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