Disc brake/MC/general brake questions - JeepForum.com
 
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post #1 of 5 Old 01-20-2012, 06:57 PM Thread Starter
AFRd_CJ5
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1976 CJ5 
 
Join Date: Jan 2011
Location: Carson City
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Disc brake/MC/general brake questions

Ok, so I swapped in a DTS built 9" with disc brakes this winter. My drum/disc setup has always seemed "spongy". Took it to 2 shops to look at it over the years, and it's never been right in my opinion.
Questions...
Do I need to change proportioning valves going to disc brakes all around?
Should I use a different master cylinder?
I don't want to "get by" with what I have if there's a much better way to do it. My current setup works, but is at times a little "touchy" trying to get it to slow down enough at a couple of the sand tracks. I want to go faster this year, so I really want the brakes to work!
Opinions?


'76 CJ5
430ci E85 SBC/single plate system
th400/Coan converter/DTS 9" rear
Family grocery getter
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post #2 of 5 Old 01-21-2012, 08:17 AM Thread Starter
AFRd_CJ5
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1976 CJ5 
 
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Location: Carson City
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No brake people here?

'76 CJ5
430ci E85 SBC/single plate system
th400/Coan converter/DTS 9" rear
Family grocery getter
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post #3 of 5 Old 01-21-2012, 08:35 AM
JeepHammer
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1973 CJ5 
 
Join Date: Aug 2007
Location: South West Indiana
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You will find disc brakes on all 4 corners seem a little 'Spongy' take a lot of input to get the brake process started.
Once it does start, you can lock all 4 wheels with no problems.

The MASTER CYLINDER needs to be set up for 4 wheel discs.
Discs take VOLUME while drums take PRESSURE.

The standard Disc/Drum master cylinders usually have a 2 pound back pressure valve in them for discs,
And a 10 Pound back pressure valve for Drums, so you usually have to pull the back pressure valve out of the 'Drum' end of the master cylinder when you put in rear discs.
It's right inside the brake line connection fitting and has the inverted flair taper built into it.

A 'Combination Valve' for discs (also called a 'Proportioning Valve' by laymen) is usually required.
Since that combination valve delays the front braking so the rears can grab first and stop you in a straight line,
The valving is usually a little 'Off' for 4 wheel discs.

DRUMS stop GREAT ONCE.
Then they fade.
Friction heat expands the drums, and they don't stop so good after that.
Friction is applied to the extreme outside of the brake circle, so drums get a LOT of leverage on the tire/wheel.

Discs stop 'So-So', but they do it every time.
The close you get to the spindle, or center of the wheel, the more leverage the tire/wheel have on the brake and when the tire/wheel have leverage (mechanical advantage) over the disc, it takes longer to stop.
You have to apply more fluid pressure to the calipers to get the same stopping power...

Less small parts to fail in Disc brakes, and disc brakes will take more heat before they need service than drum brakes.

----------------

Normally, you want the rears to apply first, but not as hard as the fronts.
The rears apply first, but don't lock, that will help keep the vehicle in a straight line when you brake around corners.

You want the fronts to apply pretty hard, since the weight has transferred to about 80% front, they NEED to pull harder than the rear to stop the vehicle.
So a proportioning valve you can adjust is a REALLY good idea...

When I'm doing 4 wheel disc for the race cars, I prefer a proportioning valve that has a brake failure switch in it.
Always good to know when a brake system is giving up on you, and that dash light is a good idea...

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post #4 of 5 Old 01-21-2012, 09:51 AM Thread Starter
AFRd_CJ5
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Hmmm, ok, that's what I was figuring.
Now....is there a particular master cylinder that will work better than another for this particular setup?
Same question for the proportioning valve? Any particular one that you recommend?

'76 CJ5
430ci E85 SBC/single plate system
th400/Coan converter/DTS 9" rear
Family grocery getter
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post #5 of 5 Old 01-21-2012, 10:42 AM
JeepHammer
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Location: South West Indiana
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If you are looking for a bolt on replacement, I can't help you.
The proportioning valve (Combo safety valve) off anything with disc brakes all the way around will work, but you might have to monkey with fitting sizes to get some of them to work.

When I did my 4 wheel disc set up on the dedicated trail crawler, I used a '80 Corvette master cylinder,
But that took a GM booster to work and I had to build the bracket to hang it, the 'Jeep' bracket didn't fit.

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