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post #1 of 22 (permalink) Old 01-22-2007, 04:47 PM Thread Starter
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drum/drum vs drum/disc MC

Is the only difference the size of the bowl for the front discs?

Just wondering if I really need to swap out the MC after converting to disc brakes.
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post #2 of 22 (permalink) Old 01-22-2007, 06:10 PM
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drum/drum vs drum/disc MC

Reservoir size and lack of a residual valve are the two key
elements. Bore size can be an issue as well, because calipers
in general require not only more m/c reservoir capacity but
the ability to move more fluid as well.
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post #3 of 22 (permalink) Old 01-22-2007, 06:10 PM
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drum/drum vs drum/disc MC

Well it depends on the bore size more than anything. Most drum/drum M/C bores are larger than disc/disc M/C bores...and disc/drum M/C bores are always different sizes internally, which gives the drums more pressure than discs. The smaller bore gives more pressure, which is why most of your M/C's have the rear brake line attatched to the front of the M/C....Discs require more volume but less pressure so you can get away with a slightly larger bore size and have good pedal. Drums require more pressure but don't need as much volume.

Generally what I've always run into was that most drum/drum MC bores are small, like 7/8 or 15/16 (manual brakes) to give good pedal feel and still provide enough pressure to get the job done. When using the same MC on disc/drum or disc/disc, the pedal gets "mushy" because it simply cannot supply enough volume to work the discs properly. That is where you have to increase the bore size up to 1" or 1 1/8"..sometimes more depending on the setup.

It's been my experience that the '79-'93 Dodge 1/2 ton pickups (doesn't matter whether reg cab or ext cab...4x2 only) does the job quite well on most applications, including disc/drum and disc/disc. Same MC as the one Wilwood sells, but it can be had at the parts houses cheaper than wilwood. I believe it is a 1" bore, IIRC. Been a while since I measured the one I have.

Doing these brake swaps is one of those things that if not thought out correctly, you will be pulling your hair out for weeks. I've been through this and luckily I had this site to help me through the headaches. I learned a LOT!

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[ This Message was edited by: mavman on 1/23/07 9:12am ]
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post #4 of 22 (permalink) Old 01-22-2007, 06:19 PM
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drum/drum vs drum/disc MC

Not only do discs require more volume, but higher line pressures as well. Thus, you'll want a larger bore to keep pedal effort reasonable.
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post #5 of 22 (permalink) Old 01-22-2007, 06:29 PM
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drum/drum vs drum/disc MC

Swap out your old MC with an MC that matches your brake set-up and you will be fine.

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post #6 of 22 (permalink) Old 01-23-2007, 04:17 AM
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drum/drum vs drum/disc MC

Drum brakes have a little umbrella type residual valve found underneath the brass seat were the brake tubing is attached. Spring pressure from the brakes overcome this valve and allow fluid to return to the MC when the pedal is released. You can see this seat when you remove the seat using a screw/ bolt of the appropriate size. This valve is included in MC rebuild kits for drum brakes and there will be 1 under each seat that is served by drum brakes.

Disc brakes do not use this valve as there is no inherent pressure to return the fluid to the MC when the pedal is released.

You can convert a drum brake reservoir to make it work for disc simply by removing the umbrella valve and reinstalling the brass seat. Of course the MC fluid reservoir may be too undersized to make this a safe modification.

I once tried to use a 65' single reservoir drum MC on a car that I converted to front disc brakes. You could push the pedal to stop the car, but you had to wait 30 seconds for the fluid to return to the MC so you could move the car again.


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[ This Message was edited by: dennis111 on 1/23/07 7:20pm ]
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post #7 of 22 (permalink) Old 01-23-2007, 04:43 AM
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drum/drum vs drum/disc MC

Quote:
On 2007-01-22 18:19, thekingofazle wrote:
Not only do discs require more volume, but higher line pressures as well. Thus, you'll want a larger bore to keep pedal effort reasonable.
Sorry, backwards. To increase line pressure, you need a SMALLER master bore. For a disk/drum setup, 7/8~ 1" seems to be the sweet spot.

Yes, you need a disk/drum MC. If you run with the drum MC, there should be 10 lb residual valve in the master - that will keep the pads against the disks and generate heat and excessive wear. Some are getting by with modified drum MC - I prefer to use the correct parts.





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post #8 of 22 (permalink) Old 01-23-2007, 04:51 AM Thread Starter
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drum/drum vs drum/disc MC

Cool, thanks for the info. I have a new Disc/drum MC from SSBC, I'll go ahead and put that one in.
Thanks again for the info!
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post #9 of 22 (permalink) Old 01-23-2007, 05:31 PM
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drum/drum vs drum/disc MC

Quote:
On 2007-01-23 04:17, dennis111 wrote:
Drum brakes have a little umbrella type residual valve found underneath the brass seat were the brake tubing is attached. Spring pressure from the brakes overcome this valve and allow fluid to return to the MC when the pedal is released. You can see this seat when you remove the seat using a screw/ bolt of the appropriate size. This valve is included in MC rebuild kits for drum brakes and there will be 1 under each seat that is served by drum brakes.

Disc brakes do not use this valve as there is no inherent pressure to return the fluid to the MC when the pedal is released.
Might be splitting hairs, but....
Residual valve in a drum m/c is there to keep some fluid
pressure in the lines. You don't want much fluid bleeding
back into the m/c. The cup seals in the wheel cylinders
being solidly up against the wheel cylinder walls is the main
reason and yet another usually overlooked concern is
that if a bunch of fluid bleeds back, the shoes will be too
far away from the drum. If this happens, you will have some
braking "issues" - that I will guarantee. Diagnostically, it
feels similiar to a bypassing master cylinder that must be
"pumped up" to get a pedal.
Disc caliper system doesn't have these concerns, you can
let all the pressure bleed back to the m/c. On some GM
products they use a special caliper seal and m/c to improve
fluid return to the m/c after braking- that way the caliper
piston will retract even more, the pad won't be dragging
against the rotor as much and fuel economy is improved.
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post #10 of 22 (permalink) Old 01-23-2007, 05:34 PM Thread Starter
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drum/drum vs drum/disc MC

Thanks again! Always appreciate the info!
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post #11 of 22 (permalink) Old 01-23-2007, 06:03 PM
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drum/drum vs drum/disc MC

i got 4-wheel disc on my truck and had to go with the biggest bore i could get.. a chrysler master cylinder with 1 1/8" bore
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post #12 of 22 (permalink) Old 01-23-2007, 08:41 PM
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drum/drum vs drum/disc MC

I ended up using an aluminum 1 1/8 bore MC also.

I have a dual disc setup (granada's and wilwood's) and the pedal is very firm, but travel and feel are perfect. You don't want squishy brakes, and if you're any sort of person who enjoys driving your car, you want pretty firm ones.



[ This Message was edited by: thekingofazle on 1/24/07 11:41am ]
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post #13 of 22 (permalink) Old 01-24-2007, 03:22 AM
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drum/drum vs drum/disc MC

Quote:
On 2007-01-23 17:31, 66GT289 wrote:
Quote:
On 2007-01-23 04:17, dennis111 wrote:
Drum brakes have a little umbrella type residual valve found underneath the brass seat were the brake tubing is attached. Spring pressure from the brakes overcome this valve and allow fluid to return to the MC when the pedal is released. You can see this seat when you remove the seat using a screw/ bolt of the appropriate size. This valve is included in MC rebuild kits for drum brakes and there will be 1 under each seat that is served by drum brakes.

Disc brakes do not use this valve as there is no inherent pressure to return the fluid to the MC when the pedal is released.
Might be splitting hairs, but....
Residual valve in a drum m/c is there to keep some fluid
pressure in the lines. You don't want much fluid bleeding
back into the m/c. The cup seals in the wheel cylinders
being solidly up against the wheel cylinder walls is the main
reason and yet another usually overlooked concern is
that if a bunch of fluid bleeds back, the shoes will be too
far away from the drum. If this happens, you will have some
braking "issues" - that I will guarantee. Diagnostically, it
feels similiar to a bypassing master cylinder that must be
"pumped up" to get a pedal.
Disc caliper system doesn't have these concerns, you can
let all the pressure bleed back to the m/c. On some GM
products they use a special caliper seal and m/c to improve
fluid return to the m/c after braking- that way the caliper
piston will retract even more, the pad won't be dragging
against the rotor as much and fuel economy is improved.
Not splitting hairs at all. Your info is indeed correct. You eloquently expanded on what I was trying (so poorly) to say.

Good job!!!!

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393W NA, 2.78 Toploader 4 Spd, 9" 3:70 Posi
Pro Comp Heads, Vic Jr. Intake, 750dp, 236/561 Solid Cam, 9.6CR, 1 3/4" Headers, Lakewood, Subframes, Caltracs




[ This Message was edited by: dennis111 on 1/24/07 6:27pm ]
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post #14 of 22 (permalink) Old 01-24-2007, 06:56 AM
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drum/drum vs drum/disc MC

Quote:
On 2007-01-23 20:41, thekingofazle wrote:
I ended up using an aluminum 1 1/8 bore MC also.

I have a dual disc setup (granada's and wilwood's) and the pedal is very firm, but travel and feel are perfect. You don't want squishy brakes, and if you're any sort of person who enjoys driving your car, you want pretty firm ones.
ited by: thekingofazle on 1/24/07 11:41am ]</font>
That's "correct" - the larger the MC bore, the harder the pedal and the lower the line pressure. Don't be fooled that a hard pedal equals good brakes. If it stops good for you, good enough. Just note that if someone with less leg strength drives the car, they may not experience the same braking performance.





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post #15 of 22 (permalink) Old 01-24-2007, 07:04 AM
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drum/drum vs drum/disc MC

Quote:
On 2007-01-24 06:56, ckelly wrote:
Quote:
On 2007-01-23 20:41, thekingofazle wrote:
I ended up using an aluminum 1 1/8 bore MC also.

I have a dual disc setup (granada's and wilwood's) and the pedal is very firm, but travel and feel are perfect. You don't want squishy brakes, and if you're any sort of person who enjoys driving your car, you want pretty firm ones.
ited by: thekingofazle on 1/24/07 11:41am ]</font>
That's "correct" - the larger the MC bore, the harder the pedal and the lower the line pressure. Don't be fooled that a hard pedal equals good brakes. If it stops good for you, good enough. Just note that if someone with less leg strength drives the car, they may not experience the same braking performance.
It stops straight, no muss, no fuss from triple digit speeds.....like you threw a boat anchor off the back. As far as pedal pressure, it's not as easy as "power brakes" But an arthritic midget would have no trouble panic stoping the car.
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