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

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You could do the math. Typically the pistons retract maybe .003 inch, maybe .005 if the discs have a little run-out. And typically the 73-up pistons are 2 inchers. So; using the pi.r sqd to find area and then multiplying by the travel, .005inch and the doubling it for both sides, you get .0314 cubic inch. to convert to CCs multiply by 2.54 cubed. And that, my friends, gets you about 1/2 CC, total. I love math. Its a non-issue. just remove the residual, and check the level now and then.
- And if you do the math on the rears, with typically poorly adjusted mechanisms;lets say a 7/8 bore and at least 1/8 inch of travel per side(probably more) you get about 2.5 CCs total.
-I love math.
-Now, if you have the KH 4-piston units; I cant remember the piston size, so Imma gonna guess about 1.125 inchers. And there are 8 all together. And they typically retract maybe .004. Doing the math gets you, again, about 1/2 CC total.
-Did I mention I love math?
-Not another word about running out of fluid on the first application!
-Somebody want to do the math on how many CCs are in the MC? Im gonna estimate this one;LxWxHx2.54cubed= about 140 plus CCs per small chamber. So if she starts sucking air at 80% empty, that would be a lotta strokes.
-Bear in mind, that if you remove the calipers from their brackets, and stroke the pedal so that the entire chamber in the M/C is flushed out to the pistons, well thats some new math, which wouldnt really be relevent, so Im not going to bother.
 
You guys build your cars your way, I'll build mine my way...


If they didn't need a different master cylinder for disc and drums, then they wouldn't have made them....
 
It will be fine, just remove the residual pressure valve
and check the fluid at every oil change.

Might need the adapter plate to do a change over if you
go that direction.
Use the small bore M/C.


Might up your life and accident insurance and
keep a lawyer on retainer just incase you run
into a fear monger that's sue happy.
 
krazykuda said:
You guys build your cars your way, I'll build mine my way...


If they didn't need a different master cylinder for disc and drums, then they wouldn't have made them....
Click to expand...

Just give your 2 cents and let it go man. Not worth the aggravation.
 
AJ/FormS said:
You could do the math. Typically the pistons retract maybe .003 inch, maybe .005 if the discs have a little run-out. And typically the 73-up pistons are 2 inchers. So; using the pi.r sqd to find area and then multiplying by the travel, .005inch and the doubling it for both sides, you get .0314 cubic inch. to convert to CCs multiply by 2.54 cubed. And that, my friends, gets you about 1/2 CC, total. I love math. Its a non-issue. just remove the residual, and check the level now and then.
- And if you do the math on the rears, with typically poorly adjusted mechanisms;lets say a 7/8 bore and at least 1/8 inch of travel per side(probably more) you get about 2.5 CCs total.
-I love math.
-Now, if you have the KH 4-piston units; I cant remember the piston size, so Imma gonna guess about 1.125 inchers. And there are 8 all together. And they typically retract maybe .004. Doing the math gets you, again, about 1/2 CC total.
-Did I mention I love math?
-Not another word about running out of fluid on the first application!
-Somebody want to do the math on how many CCs are in the MC? Im gonna estimate this one;LxWxHx2.54cubed= about 140 plus CCs per small chamber. So if she starts sucking air at 80% empty, that would be a lotta strokes.
-Bear in mind, that if you remove the calipers from their brackets, and stroke the pedal so that the entire chamber in the M/C is flushed out to the pistons, well thats some new math, which wouldnt really be relevent, so Im not going to bother.
Click to expand...

Unfortunately you are not solving the right problem. I'm not sure what Scarebird calipers are specified, but it does not really matter. The disc master cylinder volume of brake fluid is the {area of all the pistons x [thickness of the brake pads + (max rotor thickness - the minimum rotor thickness)]} X about 1.5 factor of safety. I've seen rotors that have been chewed into the vents and still have fluid in the master cylinder. Maybe your average Canadian is smarter than the people we have to design for in the US. I'm also sure this is a simplification of the problem. You can do the math, I'll trust the engineers at Chrysler to have done the right thing.
 
AJ/FormS is correct, and Physics works the same here in the U.S.

66fs poses a totally different problem:
If a clueless customer buys a new car and doesn't take it a brake shop until hearing metal scaping metal when braking, how big a reservoir is required so the inital fill of front brake fluid doesn't run out?

Of course, later cars added brake fluid level sensors, so they shouldn't reach such limits.

Actually, the flip-side is probably more important:
If during a brake job, the shop presses the caliper pistons back in, how large should an almost empty reservoir be so the brake fluid doesn't over-flow and ruin the firewall paint?
 
common sense.
6 to 1 pedal and a 1" bore master
move the pedal 3 inches and the master cylinder piston only moves 1/2 inch
just remove the residual in the disc outlet...you will be fine
 
BillGrissom said:
AJ/FormS is correct, and Physics works the same here in the U.S.

66fs poses a totally different problem:
If a clueless customer buys a new car and doesn't take it a brake shop until hearing metal scaping metal when braking, how big a reservoir is required so the inital fill of front brake fluid doesn't run out?

Of course, later cars added brake fluid level sensors, so they shouldn't reach such limits.

Actually, the flip-side is probably more important:
If during a brake job, the shop presses the caliper pistons back in, how large should an almost empty reservoir be so the brake fluid doesn't over-flow and ruin the firewall paint?
Click to expand...
I like math too. But in actuality, AJ/S has made some incorrect assumptions that threw off his calculations from reality and led to the wrong conclusions. I just actually measured the drum-drum MC that the OP has for his '70 Valiant; the reservoir for the front brakes has a USABLE fluid capacity of approximately 50 cc's (NOT 140 cc's). That usable capacity starts with the fluid about 3/16" from the top of the MC reservoir down to where there is about 1/4" left above the ports in the MC bottom. That only gives you about 0.6" of fluid drop in the the drum-drum MC. (And the front brake's reservoir dimensions on this drum-drum MC are 2.5" long (average) and 2" wide if you want to check the numbers.)

What many are missing is that the drum-drum MC is MUCH shorter than the later disc-drum MC, so both of the reservoirs' fluid capacity are not all that ample. It is fine for drums where the adjusters take out the shoe wear, but not for caliper pad wear where the fluid has to fill in the long-term piston movement.

With a total pad wear of 0.7" of material per caliper (that's the total for both pads in a caliper), and assuming a 2" piston diameter (I don't have one to measure), the front calipers will suck out a bit over 70 cc's of fluid from new to worn down (where the pads are still NOT eating into the rotors). So yeah, it is pretty clear that the drum-drum MC will indeed run out of fluid in the front brake's reservoir well before the pads are worn out when it is used for front discs.

The design of the taller disc-drum MC and the longer front reservior when disc were introduced was done for good reason, as 66fs correctly posted.

The OP should change the MC IMO when going to discs, or be ready to add fluid regularly and then remember to slowly push back caliper pistons and suck out the excess fluid when renewing pads.
 
Running drum brake dual master cylinder & portioning valve on 71 Dart, with 73 disc.brakes, works fine. Have lots of friends that have done it, works fine. Have done this same conversion several times on other A bodies. Works fine.
 
I swapped my front drums for disc and changed nothing else. Ran it that way for 8 years with zero safety issues. It wont empty the reservoir at all and the car will stop fine...just not as well as you would expect a disc brake car to stop. That being said, mc's are cheap and its a simple bolt on swap so buy the correct one, ditch the prop valve and it's good to go.
 
Ditch the P-valve? You better have a good reason for flat out recommending that!
-Disc brakes are not self-energizing, as are the drums. So by design they require more pedal pressure to operate.Thats why the factory db cars usually have vacuum boosters on them.So now,with all this hydraulic pressure available,what do you imagine will happen when you get on the brakes hard? Thats right, the rears will lock up early.And what happens to a vehicle when the rear brakes are locked up? Thats right, they tend to swap ends. So if if you like that kind of thing,then by all means ditch the valve. But I wouldnt be recommending it to others.
-Furthermore,That valve is actually a combination valve.In addition to the proportioning function, it also has ,built in, the brake failure warning switch.On cars with the dash-mounted brake warning lite,(cars with split braking systems)ditching the valve will defeat the purpose of the lite. And no the primary purpose of that lite is not to remind you to disengage the park brake.Its there to tell you one side of your split system has failed, and your car is unsafe to drive.
-On the other hand;If your car has non-factory tire sizes on it, particularly, big and littles,Then it may be possible to ditch the proportioning side of it.
-When I fitted 295s out back, I was able to gut that valve,leaving the switch in it, and gained a goodly amount of rear braking capacity.
-Also; getting air into the front brake lines in a split brake system, is not an immediate catastrophic failure. Remember the rear brakes are still functioning 100%. And the first time air enters the line, you hardly even notice it. After a bit, the pedal seems soft, but people continue to drive.Its not until theres a real bubble in there that people search out a mechanic.I used to hear it all the time.They will say something like,"yeah,the pedal hasnt seemed right for a while now....."

- And to nm9;Not only did i make assumptions, I took liberties. And I completely forgot that those drum/drum MCs were that much smaller.Thanks for the clarification.
-But even at 50 CCs,theres still a lot of wiggle room,for those who regularly check the fluid level. Also I was responding more to the posters who were imagining brake failure on the first application,than to those who never check the fluid levels.
 
CJP4130 said:
I swapped my front drums for disc and changed nothing else. Ran it that way for 8 years with zero safety issues. It wont empty the reservoir at all and the car will stop fine...just not as well as you would expect a disc brake car to stop. That being said, mc's are cheap and its a simple bolt on swap so buy the correct one, ditch the prop valve and it's good to go.
Click to expand...

then they are not operating properly.
 
AJ/FormS said:
- And to nm9;Not only did i make assumptions, I took liberties. And I completely forgot that those drum/drum MCs were that much smaller.Thanks for the clarification.
-But even at 50 CCs,theres still a lot of wiggle room,for those who regularly check the fluid level. Also I was responding more to the posters who were imagining brake failure on the first application,than to those who never check the fluid levels.
Click to expand...
OK, liberties are fine LOL! And I recognized that you were answering the questions of "will it work at all?" Of course it will; the diameters are are right and so forth. It is only a question of how much reservoir capacity there is and how will it effect long term operation.

I just wanted to keep all aspects of this straight, so people considering this will have all the data points for their own best decisions. And we might have some guy asking someday " Why do I have to add fluid to my drum MC with disc calpers? Is this a problem? "

Regards!
 
CJP4130 said:
I swapped my front drums for disc and changed nothing else. Ran it that way for 8 years with zero safety issues. It wont empty the reservoir at all and the car will stop fine...just not as well as you would expect a disc brake car to stop. That being said, mc's are cheap and its a simple bolt on swap so buy the correct one, ditch the prop valve and it's good to go.
Click to expand...
Do you mean 'ditch the prop valve' or 'ditch the residual (check) valve'?
 
nm9stheham said:
Do you mean 'ditch the prop valve' or 'ditch the residual (check) valve'?
Click to expand...
I was just about to ask that. According to my FSM, since my vehicle was originally drum/drum, I have a residual valve (which I plan on leaving in). I have already ordered a prop valve.
 
Sorry guys I got ahead of myself. Pitched the factory prop valve and installed an adjustable prop valve when I swapped mc's. It really did work just fine though for 8 years the other way, no empty mc, no soft pedal, no locking rear brake issues. Although it's not optimal for good disc brakes, it will work fine and the emptying mc thing is just bull. If you can empty your mc in one stroke of the pedal, your brake system has real issues... If that were the case don't you think it would have happened to me at least once in 8 years? Like I said, I had zero safety issues, just didn't brake as weel as expected
 
CJP4130 said:
Sorry guys I got ahead of myself. Pitched the factory prop valve and installed an adjustable prop valve when I swapped mc's. It really did work just fine though for 8 years the other way, no empty mc, no soft pedal, no locking rear brake issues. Although it's not optimal for good disc brakes, it will work fine and the emptying mc thing is just bull. If you can empty your mc in one stroke of the pedal, your brake system has real issues... If that were the case don't you think it would have happened to me at least once in 8 years? Like I said, I had zero safety issues, just didn't brake as weel as expected
Click to expand...

No one said you would empty your master cylinder in one stroke. It is over time as components wear and in all conditions. It is a safety issue not to be taken lightly. Years have nothing to do with it. As others have said, if you keep it topped off the only problem is when you press the pistons back in and it pukes brake fluid down your firewall. I would never recommend it.
 
not sure why there is even a debate about this..


do it right the first time and be done with it..

use the proper master cyl and either an adjustable prop valve or the original style for the system. its not like the parts are stupid expensive or anything. this way everything will work the way its supposed to..
 
we are all chasing the perfect system stopping effortlessly with full control and no skidding.
...think killing the power on your outboard.
the 60's and 70's factory "right way" always fell short
 
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