I wonder if you just need to adjust the screw/ bolt thing that goes into the master?

 

I'll give that a shot tomorrow. Thanks

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Well I pulled it apart this evening after all. Apparently the WJ booster pin isn't adjustable.

 

Screw the pin out, go to the hardware store, find a bolt that has the same threads, round the head, and make your own.
You should be able to pull the plunger out if that makes it any easier.

 

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.

 

Pedal “feel” comes from the rear brakes.

Are they adjusted properly?

 

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.

 

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.

 

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.

 

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.

 

...
@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.

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".

 

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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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.

 

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..

 

I won't go into all of the details because there is a lot of confusing information on brake system design which then translates into a lot of false information being spread over the internet. So with A LOT of research and A TON of math, I've come to the conclusion that the bore on my master cylinder just doesn't have the volume required to push all of the brakes. From my math, I have more than enough pressure but not enough volume to actually push all the calipers which explains why the rears work decent with just a hydraulic hand brake but barely move until the brake pedal is nearly on the floor. It also explains why the pedal feels so soft until the bottom because it's not pushing against anything until almost the bottom of the pedal stroke.

The simple and hopefully less confusing math is that my YJ has a 5:1 pedal ratio (5" of pedal travel equals 1" of MC travel) and my pedal has 6.3" of movement before hitting the floor. That means that my 1" bore MC is being compressed 1.25" which yields 0.98 cubic inches of fluid volume that can be pushed. All four of my brake caliper pistons add up to 19.2 square inches which gives 0.050" of caliper piston travel before the volume equals 0.98 cubic inches. Once the pistons have come out 0.050", the MC no longer has any fluid left to give the calipers. This is also assuming no caliper flex, no line flex, no pad compression, and equal fluid transfer....all of which aren't true. Add in all the losses and the fact that the front calipers will inevitably pull more fluid, there is very little fluid left to send to the rear calipers.

So I'm on the hunt for a larger bore MC to help bring my brake system back in balance. Will update as the story unfolds.

 

I won't go into all of the details because there is a lot of confusing information on brake system design which then translates into a lot of false information being spread over the internet. So with A LOT of research and A TON of math, I've come to the conclusion that the bore on my master cylinder just doesn't have the volume required to push all of the brakes. From my math, I have more than enough pressure but not enough volume to actually push all the calipers which explains why the rears work decent with just a hydraulic hand brake but barely move until the brake pedal is nearly on the floor. It also explains why the pedal feels so soft until the bottom because it's not pushing against anything until almost the bottom of the pedal stroke.

The simple and hopefully less confusing math is that my YJ has a 5:1 pedal ratio (5" of pedal travel equals 1" of MC travel) and my pedal has 6.3" of movement before hitting the floor. That means that my 1" bore MC is being compressed 1.25" which yields 0.98 cubic inches of fluid volume that can be pushed. All four of my brake caliper pistons add up to 19.2 square inches which gives 0.050" of caliper piston travel before the volume equals 0.98 cubic inches. Once the pistons have come out 0.050", the MC no longer has any fluid left to give the calipers. This is also assuming no caliper flex, no line flex, no pad compression, and equal fluid transfer....all of which aren't true. Add in all the losses and the fact that the front calipers will inevitably pull more fluid, there is very little fluid left to send to the rear calipers.

So I'm on the hunt for a larger bore MC to help bring my brake system back in balance. Will update as the story unfolds.

Wow! Thank you for doing the math and posting the work!! I spend a *bunch* of time on forums telling people there is more to making a brake swap work than just bolting parts together. The math to describe the volume of fluid transferred when the brakes are applied and the pressure generated at the pads/shoes should not be ignored. Many times people end up with improperly biased brakes that seem ok until the vehicle is in a panic stop or they fight too much pedal travel while thinking it's a soft pedal.

FWIW the Hydroboost brakes on GM vehicles with only a few exceptions all use 1.25" diameter bore m/cyls. In some cases (such as my '95 Blazer with '99 Blazer dual piston calipers) this results in very little change in the amount of foot pressure required to stop a vehicle. Non HB m/cyls can be modified to fit the HB booster but a better solution is to use HB from a Mustang or T-bird. Those m/cyls have a smaller bore and are more appropriate for smaller calipers.

Also be sure to avoid "quick takeup" master cylinders and calipers. Neat idea but the brake feel and response time is disgustingly poor. I have switched my '99 Suburban (originally equipped with QT brakes) to 2014 Silverado front rotors and calipers while moving to '79 Camaro master. The pedal travel is slightly higher but the brake performance is amazing.

 

Since you have a dual diaphragm booster from the wj you'll need a master with the o-ring seal. I know the 99 Dodge ram 2500 master I'm running would work, but it might be too large for your brakes. I believe I also read the ram 2500 van from the same year works and is a little shorter, but don't take my word on that.

The late 90s Durango master Tim mentioned should work as well.

Ram 2500 - 1-1/4" bore
Durango - 1-1/16" bore

In my own experience, I ran a 1-1/8" bore master when I had Dana 44s with 1/2 ton brakes and it worked really well. I used a 1978 Mercury Grand Marquis 4wheel disk master, but that doesn't use an o-ring seal so it won't work for your application.

 

Never realized the o-ring did anything besides just prevent dirt and grime from getting inside the booster. More importantly, that Ram 2500 looks like exactly what I want. It even has the same ports as the WJ master cylinder and the same cap so I can use my pressure bleeder system!

I don't really have a good way to measure how much force it takes to push my current brake pedal but going from a 1.0" bore to a 1.25" bore should increase fluid flow by 50% and leg force by 43%. Both are desirable to me right now.

 

I've never run the dual diaphragm setup. Just what I've read from folks who run or swapped to the 95 yj or the boosters which use the o-ring. Maybe the wj is different. Who knows

Edit

Not sure, but it looks like the seal being needed on the booster may have been unique to the 95 yj booster. Either way, if you get the Dodge part you are covered.

Here's an old thread about this specific swap. https://www.pirate4x4.com/forum/jee.../forum/jeep-hardcore-tech/677082-ram-2500-master-cylinder-mc-upgrade-yj-tj.html

 

Man...if only I had found that thread sooner. Granted I learned a lot about brakes but based on that thread, everyone got what they needed.

 

I won't go into all of the details because there is a lot of confusing information on brake system design which then translates into a lot of false information being spread over the internet. So with A LOT of research and A TON of math, I've come to the conclusion that the bore on my master cylinder just doesn't have the volume required to push all of the brakes. From my math, I have more than enough pressure but not enough volume to actually push all the calipers which explains why the rears work decent with just a hydraulic hand brake but barely move until the brake pedal is nearly on the floor. It also explains why the pedal feels so soft until the bottom because it's not pushing against anything until almost the bottom of the pedal stroke.

The simple and hopefully less confusing math is that my YJ has a 5:1 pedal ratio (5" of pedal travel equals 1" of MC travel) and my pedal has 6.3" of movement before hitting the floor. That means that my 1" bore MC is being compressed 1.25" which yields 0.98 cubic inches of fluid volume that can be pushed. All four of my brake caliper pistons add up to 19.2 square inches which gives 0.050" of caliper piston travel before the volume equals 0.98 cubic inches. Once the pistons have come out 0.050", the MC no longer has any fluid left to give the calipers. This is also assuming no caliper flex, no line flex, no pad compression, and equal fluid transfer....all of which aren't true. Add in all the losses and the fact that the front calipers will inevitably pull more fluid, there is very little fluid left to send to the rear calipers.

So I'm on the hunt for a larger bore MC to help bring my brake system back in balance. Will update as the story unfolds.

Amen to that. The rest of your post is impressive work and research and truly diving into what you are doing and fully understanding it and being able to smell and identify BS.

 

To re emphasize it.

If you increase the piston size of a caliper by going to a bigger piston, dual pistons, 4 pistons, etc. and remain with the same bore master cylinder your pedal travel will increase. It is in simple ratios.

A one inch piston traveling one inch will displace a 1 inch piston on the other end 1 inch and have equal pressure on both ends. A one inch piston traveling one inch will displace a 2 inch piston on the other end a half inch but double its pressure. It all may seem equal but it is not. It requires a twice as much pedal travel in the second scenario to increase braking pressure.

You did a great job of all the math, pedal ratios, etc. to work it out. Fantastic and scientific work .

 

Well I got my new master cylinder but my booster pin isn't adjustable so I'm going to have to do some modifications to make the booster pin adjustable.

Since it seems like the math is helping people, I'll go a step further since it's all written down anyway. Calculating pressure to the master cylinder can be found on many websites like wallace racing but it's a vague number and doesn't translate well to the 900-1200 psi that you realistically need for your brakes to work on modern vehicles. Using my 5:1 pedal ratio and a moderate 70 lbs of foot pressure, the calculator spits out 445 psi to the MC. That's crazy low but apparently the online calculators assume manual brakes. If you want to add in a vacuum booster, you get a lot more. The WJ booster is 8.84" in diameter. To get the boosted power, you take the area of the 8.84" dia and you get 61.3 sq in. Now multiply that by your engine vacuum in psi (not in-hg that vacuum is typically measured in). An average vacuum is 23 in-hg which translates to 11.5psi which you multiply times 61.3 sq in and you get 705 psi extra from the booster. This number is for a single diaphragm booster. If you have a dual diaphragm booster, you get about 1.4 times that of a single so 705 times 1.4 is roughly 1000 psi of free power for a total of 1445 psi to the master cylinder. We can definitely work with that pressure!

I didn't do this for multiple applications but in my scenario a dual diaphragm booster more than triples my efforts or to say it a different way, I'm applying 70 lbs of force to the pedal and booster is effectively adding another 155 lbs.

 

I love this- science, not guessing or heresay.

 

So I installed an adjustable booster pin... It was definitely required for the new Dodge MC. It's really not that hard to disassemble the booster. I was dreading this but it took 5-10 minutes with a pry bar the first time and less than that the second time when i forgot to put the spring back in.

1st pic is the booster disassembled (yes a WJ master cylinder does need an o-ring to prevent a vacuum leak and you can see why)
2nd pic is the old booster pin. It has a floating pin but it is not adjustable.
3rd and 4th pics are the new adjustable booster pin made from a welded coupling nut and 3/8" stainless bolt.
5th pic is what you get when you hammer the booster back together and leave parts out. :brickwall


Verdict on the test drive... Initial thoughts were that I'd gone too big on the MC because the pedal was super stiff and didn't move very far. Brakes were good but it required a lot of force. Then I remembered the booster wasn't hooked up. Now the brakes are really good but the pedal is still light so maybe I could've/should've gone bigger.

 

Now the brakes are really good but the pedal is still light so maybe I could've/should've gone bigger.

If it's still soft with that Dodge master, I'd double check that the pin in the booster is fully seated in the master (no slack), then do some more bleeding.

No science on my end, just a guy who's been there and done that several times with different setups over the years.

 

Really really nice work. Great thread. I am sure that it will be helpful to others for years to come.

 

Great thread guys. The only thing we need to be careful of is thinking a 2" piston is twice as big as a 1" piston. It is really 4 times bigger. When talking about pressure/force/volume/movement we need to use area of that piston, not diameter.

1" piston=.7854 square inches
2" piston=3.1416 square inches

Doug

 

Great thread guys. The only thing we need to be careful of is thinking a 2" piston is twice as big as a 1" piston. It is really 4 times bigger. When talking about pressure/force/volume/movement we need to use area of that piston, not diameter.

1" piston=.7854 square inches
2" piston=3.1416 square inches

Doug

Great point. It is the area of the piston that matters. I know better. :facepalm: