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post #1 of 35 Old 11-16-2019, 08:39 PM Thread Starter
Waternut
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Spongy brakes

I've been trying to diagnose a soft brake pedal for a while but I've run out of ideas. Bottom line is the brakes stop me pretty well once the pedal gets near the floor but I feel like the rears could be better and the pedal is extremely soft.

Here's what I've got:
1. WJ brake booster and master cylinder (used)
2. WJ dual piston front calipers recently replaced with Chevy calipers after swapping to a D44 front axle (all new)
3. Ford 8.8 rear disc and calipers. All new pads, calipers, and hoses.
4. Summit Racing adjustable brake proportioning valve. It's wide open and I've tried both directions.

Here's what I've done:
1. I've gravity bled the brakes, bled them with a helper, and bled them by pressuring the master cylinder. No air bubbles ever come out. I've obviously bench bled the master cylinder as well.
2. Replaced the master cylinder with another used unit. No change at all.
3. The Chevy brakes on the D44 are stronger than the dual piston WJ calipers but obviously this doesn't change the rear brakes.
4. I had the rear brakes going through a hydraulic hand brake. Thinking maybe this was a problem so I bypassed it but it didn't change anything.
5. I put a 2psi residual check valve on the rear brake lines. No change.

Here's a video of how light the brakes are. Air in the lines makes perfect sense but there can't be any air left in the system.




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post #2 of 35 Old 11-16-2019, 08:44 PM
TSEJEEPERS
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I wonder if you just need to adjust the screw/ bolt thing that goes into the master?
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93 YJ SOA 2" springs front, XJ springs rear w/main leaf added, High pinon 9 inch rear detroit locker front Dana 44 ARB 4.56 Gears, 36 inch Irok tires too much to list.
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post #3 of 35 Old 11-16-2019, 09:13 PM Thread Starter
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I'll give that a shot tomorrow. Thanks

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post #4 of 35 Old 11-16-2019, 11:01 PM Thread Starter
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Well I pulled it apart this evening after all. Apparently the WJ booster pin isn't adjustable.


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post #5 of 35 Old 11-17-2019, 05:43 AM
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If you have spongy brakes, there's either air somewhere or the soft lines are worn and are "ballooning" when you press the pedal.
I have also seen bad masters right out of the box.

Bad "new" parts are part of the deal we made when it was decided manufacturing in places where workers are paid 20 cents an hour is a good thing.
I used to pay 40 bucks for a mechanical fuel pump. Now, years later, they are 20 bucks but half of them are bad.
There's a lot of defective crap out there. Poor metallurgy. Quality control and assembly.

It makes working on things a pain in the butt.

End the Ethanol Rip-Off
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post #6 of 35 Old 11-17-2019, 06:45 AM
fishadventure
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Pedal “feel” comes from the rear brakes.

Are they adjusted properly?

[size=“3”]Shackles & D-rings are different things.
Cranking IS turning over
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post #7 of 35 Old 11-17-2019, 07:20 AM
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Quote:
Originally Posted by fishadventure View Post
Pedal “feel” comes from the rear brakes.

Are they adjusted properly?
One quick way to tell that the rears may be out of adjustment is to pump or apply the brakes a few times consecutively and see if the pedal raises.

If you cannot fix it with a hammer then it has to be an electrical problem.
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post #8 of 35 Old 11-17-2019, 10:50 AM
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I had a similar issue after putting in a 44 front with massive dual piston calipers from a F250. I chased rear brake problems for a long time trying to figure it out. I ended up pulling the soft line on the rear and capping the hard line with a couple of adapter fittings and suddenly the brakes were great (front only obviously). So I thought I had narrowed it down to the rears.... spent lots of money that I didn't need to.

I talked mrblain from here and Black Magic Brakes and after running down what I had we decided I needed a different master. I had been running a TJ master and booster. Now running a '97 Durango master that has a slightly smaller bore which creates greater pressure, problems solved can pretty easily lock up 37's on dry pavement if I wanna. Oddly enough it was the front calipers pulling too much fluid even though capping the rear seemed to make it better (they were really just a parasitic bleed at that point).

BTW since Chris mentioned bad "new" parts I went through 4 Durango masters that all leaked right out of the box upon install, they were on back order from Napa so tried Auto Zone and O'Reilly both (two each), Napa one still doesn't leak pushing 4 years now.

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post #9 of 35 Old 11-17-2019, 11:22 AM
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One thing you might check is your rear calipers and see if the bleeder is on the top or bottom as they are mounted. I have 8.8 brakes on the rear and the calipers were on the wrong sides. That puts the bleeder on the lowest part of the caliper. No way to get the air out when they are on the bottom. I had to swap the calipers around and that helped some with the rears.
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post #10 of 35 Old 11-17-2019, 11:36 AM Thread Starter
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Rears are 8.8 calipers so there isn't really any adjustment and the bleeder is on top.

I did manage to make the pedal firmer and higher by using a plug and spacers on the master cylinder but the rears are still pretty worthless.

@timatoe Do you have a TJ booster and Durango master? I'll look into this.


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post #11 of 35 Old 11-17-2019, 03:53 PM
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Quote:
Originally Posted by timatoe View Post
I had a similar issue after putting in a 44 front with massive dual piston calipers from a F250. I chased rear brake problems for a long time trying to figure it out. I ended up pulling the soft line on the rear and capping the hard line with a couple of adapter fittings and suddenly the brakes were great (front only obviously). So I thought I had narrowed it down to the rears.... spent lots of money that I didn't need to.

I talked mrblain from here and Black Magic Brakes and after running down what I had we decided I needed a different master. I had been running a TJ master and booster. Now running a '97 Durango master that has a slightly smaller bore which creates greater pressure, problems solved can pretty easily lock up 37's on dry pavement if I wanna. Oddly enough it was the front calipers pulling too much fluid even though capping the rear seemed to make it better (they were really just a parasitic bleed at that point).

BTW since Chris mentioned bad "new" parts I went through 4 Durango masters that all leaked right out of the box upon install, they were on back order from Napa so tried Auto Zone and O'Reilly both (two each), Napa one still doesn't leak pushing 4 years now.
From what I'm seeing, the Durango mc is actually a little larger bore than the tj, which makes sense.

Soft pedal after a swap with larger calipers common. The larger calipers require more fluid, so increasing the size of the mc to match is needed. I used a 1-1/8" bore mc from a grand Marquis when I ran d44s. Now with 1 tons I have a 1-1/4" bore master from a Dodge 2500.


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post #12 of 35 Old 11-17-2019, 04:08 PM Thread Starter
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Hmmm I do have a 99 Silverado master cylinder laying around from my LS swap that I could try. Maybe it'll fit... I'm also looking into the potential of fitting a hydro brake booster frim something like an astro van but that may be overkill and a lot harder of a swap if I source all of the parts rather buy an expensive plug n play kit. I've got a few days out of town for work where I can research that to death at least.

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post #13 of 35 Old 11-17-2019, 04:17 PM
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Quote:
Originally Posted by Waternut View Post
...
@timatoe Do you have a TJ booster and Durango master? I'll look into this.
Yes, I ran both the booster and master from a TJ because I couldn't find a YJ dual diaphragm booster and a guy was parting out a TJ, got both and the proportioning valve for $35. That setup worked great with the 30/8.8 combo (stock brakes on both). The Durango master bolted right to it.

Quote:
Originally Posted by bgredjeep View Post
From what I'm seeing, the Durango mc is actually a little larger bore than the tj, which makes sense.

Soft pedal after a swap with larger calipers common. The larger calipers require more fluid, so increasing the size of the mc to match is needed. I used a 1-1/8" bore mc from a grand Marquis when I ran d44s. Now with 1 tons I have a 1-1/4" bore master from a Dodge 2500.
Blain is the brake guru and he swears you get less pressure with a larger bore but a better "feel" at the pedal. You're right on the larger bore for all the first gen Durangos except the '97, I have a part number somewhere. Blain said it's the same one they use in their "Big Brake kit".

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post #14 of 35 Old 11-17-2019, 07:29 PM
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Weird stuff happens in brake swaps.

Let us get to hydraulic basics though.

Take a 2" piston pushing a 4" piston, or master to caliper relationship for example.

For an inch of travel in the master (the 2" piston) the caliper will move 1/2" but the force will be doubled.

Change the mathematics though. Take a 1" piston in the master doing the same against a 4" caliper piston. The caliper now moved a 1/4" but force is multiplied by 4.

In other words. It takes twice the pedal travel of a 1" master vs a 2" bore master to produce the same pressure at an equal sized caliper piston.

Brake design is a real b!tch.

Contemplate this all day long. Js.

Pedal travel vs. pressure is not so simple. .

That is where brake pedal ratios come in to play also.

If you cannot fix it with a hammer then it has to be an electrical problem.
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post #15 of 35 Old 11-17-2019, 07:59 PM
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It was a long time ago and I don't remember all the details but I bought an 85 Chevy Corvette that was supposed to have had major brake work including a master cyl. It always had a spongy pedal I gave up trying and took it to the garage. They told me that the master cyl was not bench bled. It came back with a nice pedal.
It's probably not your problem and have bled the MC but I thought I'd just mention it..

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