Disc Brake Conversion - JeepForum.com
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post #1 of 10 Old 02-26-2020, 05:31 PM Thread Starter
VTCJ76
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Disc Brake Conversion

New to forum but have read many great posts so with my new project I wanted to be more active.

My history with jeeps. 85 Cj7 new from my father that rusted completely out in northeast buy 1994. 61 willys pick up, 97 TJ and my current jeep 1976 CJ7 shipped from Arizona to the northeast. Levi's edition with a 360 installed, otherwise original.

I have converted the front drum to disc using a conversion kit and changed master cylinder but cannot get hydraulic pressure to breaks the way I feel it should be (pedal almost to floor) brakes have been bled,they are clocked correctly but I have struggled to find way I cant get the pedal more reactive now that I have disc on front.

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post #2 of 10 Old 02-26-2020, 07:38 PM
Fourtrail
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Did you plumb the master correctly? Rear large reservoir to the front discs and smaller front reservoir to the rear drums?
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post #3 of 10 Old 02-26-2020, 09:00 PM
BagusJeep
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Did you use a disc/drum proportioning valve? They are different.
Are the calipers on the correct side? The bleed nipple needs to be at the top of the cavity behind the piston, not the bottom.
Are the rear drums correctly adjusted? Have you got 5lb pressure valves for the rear (one way valves that keep the shoes near the drum)
Did you use new flexible hoses?
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post #4 of 10 Old 02-27-2020, 05:01 AM Thread Starter
VTCJ76
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Thanks for the quick responses. I will look at all those items this weekend.

I did not change the proportioning valve, that was on my list of potential issues but previously I had been told it was ok to use the original.

You guys are a great resource thank you.
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post #5 of 10 Old 02-27-2020, 08:57 AM
John Strenk
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Quote:
Originally Posted by VTCJ76 View Post
Thanks for the quick responses. I will look at all those items this weekend.

I did not change the proportioning valve, that was on my list of potential issues but previously I had been told it was ok to use the original.

You guys are a great resource thank you.
If you ever decide to go to 4 wheel disk brakes then you can use the original proportioning valve. So don't throw it out.

However right now you will need to use a disk/drum combination valve.

I used one for a 84 CJ7 on my 76 CJ5.

I'm not sure but you may need to change some of your fittings also. I took the easy way out and replaced everything when I converted over.

I would recommend getting one of these :



https://www.amazon.com/Team-Performa.../dp/B07BYVK7SN

And one of these:



https://www.amazon.com/OTC-OTC7853-V...omotive&sr=1-2


to make bleeding the system easier


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post #6 of 10 Old 02-27-2020, 08:09 PM Thread Starter
VTCJ76
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Thought I would provide a few pictures of the 76. Arizona vehicle, only original owner and then his mechanic who was a friend of mine. Frame and body is pristine, interior was really sun damaged as is paint. First snow it probably ever saw but I wont drive it in winter due to salt.
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post #7 of 10 Old 03-03-2020, 10:08 PM
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I'll go against the grain and state that the prop/combination valve does not need to be changed. I've done three conversions on '74-77 CJs and have always used the stock prop/combination valve and the stock master cylinder. The only change I've made is to remove the residual pressure valve at the outlet on the master cylinder for the front lines. My '75 braked better than my '78 after converting to disc in front.
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post #8 of 10 Old 03-04-2020, 09:42 AM Thread Starter
VTCJ76
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Interesting because the company I bought the conversation kit told me the same thing, you can use the original valve. I ordered one anyway because I cant get the pressure and am at a loss.
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post #9 of 10 Old 03-04-2020, 10:44 AM
John Strenk
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The only real difference that I can figure out is since the disk pad is usually loosely rubbing against the rotor. It doesn't take much travel for it to engage while rear drums have retractors that pull the shoe away from the drums and the rear slave cylinder has farther to move before it engages. To compensate for this the front brakes has a metering system that delays the front from actuating until the rear has time to catch up..

On a dry road there probably is not any difference but on a snowy, ice covered road, it might make all the difference between the front and rear working together and remaining in control or having one end or the other loose traction and go out of control. Same would be if you happen to slam the brakes on hard in a panic situation.

Since your jeep might not see any ice or snow or be involved in a panic braking situation, it probably won't make any difference.

Here is a pretty good description I found on line.
Quote:
Brake Pressure Differential Switch and Valve:
In the center of the Combination Valve (labeled as B in the image below) is a single wire connector that works in conjunction with a slide in the center of the valve. This portion of a Combination Valve is the first thing that sees brake pressure as generated by the master cylinder and is made up of two different components. There is the actual switch that works in conjunction with the Valve.

The purpose of this combination is to notify a driver should brake pressure be lost on either the front or rear brake system either before or after the valve. Should pressure be lost, the valve (which actually slides back and forth within the body of the combination valve) will close off either the front or rear of the valve which will provide an opportunity for a user to get home.

When this happens, the center pin on the switch will move and then ground the switch and turn on a light on your dash. This is a warning that something is wrong.



Metering Valve:
At the front of the Combination Valve (seen below and labeled as C) if using a Combination Valve designated as disc/drum is the Metering Valve. The Metering Valve, sometimes referred to as a Hold-Off Valve, is used to regulate the pressure going to the front wheels when the brakes are initially pressed in a brake system using disc brakes in the front and drum brakes in the rear.

Due to the design of disc brakes versus drum brakes, disc brakes will apply quicker than drum brakes. If this happens, the car will nose dive when applying the brakes. To prevent this, the use of a Metering Valve is required. It will simply hold of the line pressure coming from the master cylinder ever so slightly allowing the rear brakes to apply first. The total time it holds off the front brakes is so slight that as a driver, it is never even realized.

The Metering Valve portion becomes less of a discussion when you have a vehicle with disc brakes on the front and rear. In this situation, you do not need a Metering Valve and therefore would by a Combination Valve designated as disc/disc. These valves basically have an open cavity internally in the front that basically allows equal line pressure at equal times to both the front and rear.




Valve Inside Graphic



Proportioning Valve:
The Proportioning Valve part of the Combination Valve (seen above and labeled as A) is used to control the rear brakes. What the Proportioning Valve does and when it actually works is one of the biggest misunderstandings in a Combination Valve. In a moderate braking situation, line pressure being generated by the master cylinder is less extreme and therefore will reach the calipers or wheel cylinders in full. However, when the brakes are really put to work and a panic stop happens, this is when the Proportioning Valve goes to work. In a panic stop, the Proportioning Valve will actually reduce the line pressure to the rear brakes in an effort to try and minimize rear brake lock-up and also the possibility of the rear of the car passing the front of the car which is never a situation anyone wants to experience.
https://techtalk.mpbrakes.com/combin...ination-valves

You probably won't notice a difference usually.


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post #10 of 10 Old 03-05-2020, 09:56 PM
CSP
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Quote:
Originally Posted by John Strenk View Post
The only real difference that I can figure out is since the disk pad is usually loosely rubbing against the rotor. It doesn't take much travel for it to engage while rear drums have retractors that pull the shoe away from the drums and the rear slave cylinder has farther to move before it engages. To compensate for this the front brakes has a metering system that delays the front from actuating until the rear has time to catch up..
The purpose of the residual pressure valve in the outlet of the master cylinder is to prevent the springs from retracting the shoes away from the drum. There's about 2psi to keep the shoes close to the drum, if not loosely rubbing similar to the calipers on the rotor.

Never have had a single issue braking in the snow.
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