Any input on using a larger rear drum brake cylinder?

Like most my braking is far from spectacular on my 88XJ especially with 33" tires. I find the brakes o.k. until I try to stop hard in which case the pedal bottoms out before I get any real good braking. It would appear as though I'm not getting enough fluid flow from my stock M/C. This was confirmed when I swapped to ZJ front discs as the ZJ caliper piston is larger than the XJ and the problem got worse. With the ZJ calipers the pedal would almost go to the floor before I got any substantial braking. I took the ZJ stuff off and went back to the XJ calipers.

So the fix...

I've got a ford E350 master cylinder which has a internal bore of 1.145" compared to the stock MC bore of .875". This should provide more than enough fluid for my stock system but I'd like to get as much as possible so I'll be putting the ZJ front calipers back on. It's also about time to change my rear brake cylinders so I'd like to upgrade to some larger units. The stock bore is 7/8" and I'm looking for some 1" or even a bit larger if possible. I found one website in which they used 1" bore cylinders from a 1972 ford pickup.

**Note.. I am aware of the potential consequences of keeping the stock booster and using a M/C with a larger bore. If pedal effort is unbearable I will more than likely swap in the dual diaphram booster from the 94 to 95 model XJ **

Does anyone know any brake cylinders that are larger than 7/8" and will bolt up to my rear Dana 44 backing plate?

Does anyone have any experience with running larger brake cylinders?

Let's go back to basics. The brakes are a closed hydraulic system. One of the basic laws of physics (or fluid mechanics) is that pressure is equal throughout a hydraulic system (unless you have high velocity flow, but that's likely not a closed system).

For simplicity, ignore MC and wheel cylinder bore diameter and assume that your OEM MC, calipers, and wheel cylinders each have an area of 1 square inch. Let's also assume that the swept brake area at each wheel is 100 square inches.

Now -- you step on the peddle with a force of 100 pounds. Because of the relationship of the foot peddle to where the rod goes to the MC, this is multiplied by a factor of 5 (assumed) at the MC. So you have pressurized the system to 500 psi.

That means that at each wheel you also have 500 psi pushing out on a piston of 1 sq. in., which developes a braking force of ... TA DA! ... 500 pounds, which is applied to 100 sq. in. of pad or shoe area. So the pads or shoes are applying 5 psi to the drums or calipers.

Change #1 -- you want to increase the MC bore diameter. To keep the example simple, let's just double it. The MC now has an area of 2 sq. in. You step on the brakes with the same 100 pounds, the same 500 pounds goes into the MC, but the area of the piston is now double so the system is now pressurized to only 250 psi.

With no changes to the calipers or wheel cylinders, the braking force at each wheel will be exactly half what it was before. (In a real world example, it will be reduced by the proportion of the original MC area divided by the new MC area.)

Okay, time for change number 2: Increase the size of the wheel cylinders. Make it twice as big as it was, and now you have 250 psi applied over 2 sq. in. or area, and ... wonder of wonders ... we're right back where we started, with 500 pounds of force being applied to the rear shoes.

FWIW, before messing with any of this stuff I'd suggest replacing the stock OEM combination valve with an adjustable proportioning valve from Summit Racing or Wilwood (or even Mopar Performance). The stock combination valve seems to go bad and cause an overall loss of braking. You can replace the MC , calipers and wheel cylinders, but if the combination valve is messed up you'll never get good braking, and you'll just keep on thinking you haven't found the right combination of parts yet.

Ya know eagle i have to bump in here a sec and just credit you as you always give excellent descriptions and explanations in your posts.

eric

Wagner 1"

Wagner 1" units bolt right on a D44 and provide 5 feet better stopping distance from 60 MPH - it feels like more; but thats what the G-tech measures. All good - no drawbacks in 2 years running.

Have you explored the area of friction materials in your pads/linings? Also...you mention pushing the pedal way down..either there is air and/or moisture in the system or the brake lines are giving...A good set of braided lines will take care of the line flex. Check with NAPA about the availability of pads/linings in their "Severe Duty" line of metallics, beyond a doubt the best brakes I have ever seen.

And a Hats off to Eagle on the explaination.

All my lines are stainless and my pads/shoes are the higher quality Napa issue ones.

I can see the need for the bigger MC if you need more fluid volume because of a larger caliper or rear disks, but as Eagle alluded to it may not be the thing for a single upgrade. The first thing I would recommend is to go to the dual diaphragm booster.
It is by far the best brake mod I have made since dumping the Bendix POS antilocks.

If you use a '96 booster it should be nearly bolt in. Then it does add a bit of rear power to go to the larger WC's (the '72 Ford PU WC's will bolt right in to your D44).

Only after this would I worry about changing front calipers if braking still was not good enough (actually, I would do a rear disk set up before swapping front calipers).

Bones
#6

Oh,

D*mn good explanation Eagle. I had always thought of the brake system in the terms of gears (Little turning big=easy, Big turning little=hard). Never thought of it in quantitative terms.

Bones

Brake problems s*ck

Eagle, that was perhaps the most thorough discussion I have seen on hydraulics outside of a college physics class. Everything you have said is 100% accurate for a nonboosted brake system, such as would be found on a motorcycle.
However, I believe that the brake booster being involved changes the dynamics of the hydraulic system. It should "level the playing field" for the forces needing to be applied to the brake pedal when using different diameter MC's. Including the booster in the equation certainly complicates things.
Regardless, the larger bore MC should require more force with your foot, but at least it might fully apply the larger brakes before running out of stroke (bottoming out). If the driver is willing to strain his legs a little, he just might be rewarded with a smaller stopping distance. Maybe not, but just a thought.

Yes, I was familiar with the physics involved in the braking system and my initial problem didn't stem from an overly stiff brake pedal but rather an apparent lack of volume in the M/C bore.

Upon reading Eagles post it did strike a thought which could somehow be responsible for my current problems.... I'm not completely sure if the stock brake porportioning valve is keeping the required residual pressure in the rear drums.

If the valve is not retaining a certain amount of pressure in the drum brakes there might be excessive slack created in the rear drum brake parts. This slack would have to be "made up" in my next brake application which will use up a certain amount of the pedal stroke.

Hhmmm.... I'm not in any hurry to begin blindly swapping parts in an attempt to randomly solve the problem so any other information would be appreciated.

Perhaps those who have had experiences with a faulty proportioning valve system could tell me the symptoms.

CHW from NAC had effectively zero rear brakes on his '89, and finally he got to a point where he had a soft peddle and could not bleed it up to get a firm peddle. After we discussed all other possibilities, he stripped out the OEM proportioning/combo valve and replaced it with a Wilwood adjustable proportioning valve, in the rear circuit only. He said his brakes are now better than they ever were.

Adjustables prop valve ... Eagle

If I wanted to replace the stock prop. valve does it mean I would need 2 of the Wiwood prop valves or will one be sufficient.

Is this mod pretty much a bolt on.

Thanks

Start by making sure the system is clean (when was the last time you changed the brake fluid?) and see what happens next. Brake fluid should be changed every two years - since it is hygroscopic, it picks up humidity, which lowers the boiling point of the fluid. This results in air bubbles in the system.

Why is that bad? Air compresses, fluids don't. Air pockets will absorb some of the pressure in the system, which will reduce the force available at the working end of the system

Also, switching to a dual diaphragm booster is well spoken of, and should probably be on your list anyhow.

I hadn't known of the prop valve clogging up, but it makes sense. The Wilwood will likely be a drop-in - I've not read the Wilwood catalogue I just got, but their stuff is first-rate (been using it for years.)

5-90

Re: Adjustables prop valve ... Eagle

KLXJ said:

If I wanted to replace the stock prop. valve does it mean I would need 2 of the Wiwood prop valves or will one be sufficient.

Is this mod pretty much a bolt on.

Thanks

Click to expand...

The purpose of a proportioning valve is to modulate the braking so that one end doesn't lock up before the other. In practice, under braking much of the weight shifts to the front wheels, so seeing the front lock up before the rear is almost unheard of. CHW ran the proportioning valve in the rear circuit only, which happens to be the way most car manufacturers were doing it back in pony car days (although those weren't adjustable).

It is not a bolt-in swap. The Wilwood valve doesn't have the same configuration as the stock combination valve, doesn't serve both circuits, and doesn't incorporate a brake failure warning light switch. He just went over the top, ripped everything out, and replumbed the entire brake system with new hard lines all the way from the master cylinder to each wheel.

It's not necessary to get quite that extreme, but there will be some adapting and by-passing to do.

Or ... you could just visit a dealer and replace the combination valve.