Brakes and prop valves, options?

[Skorpyo]

Ok so this is kind of a beating a dead horse question but here goes. I’ve installed an 8.8 with discs, a dual diaphragm 95 booster, have removed the LSV and am now debating what to do next. As I understand it the LSV handled proportioning to the rear brakes and eliminating it comes with the price that should I lose a brake line I’ll effectively lose all braking power if I’ve just plugged the port on the distribution block and am running a single line to the rear from the nose port. So:

 

1. Am I wrong about the above?
2. Will replacing my distribution block with a Wilwood 260-11179 be my best option?
3. Would I be better served to install a Wilwood 260-12627 for just the rear line?
4. What might be a better option for me?

 

I'm not planning on doing much more than running around town and maybe the occasional green trail or two and I don’t haul much weight on a frequent basis, except maybe when I’m taking a run to the dump or hardware store. Input appreciated. Thanks!

 

Tim

[Pete M]

If I was starting over, I'd swap in a non-MJ prop valve up front (XJ, WJ, etc) and then an adjustable one for the rear line.  Seems that gives you the best chance at dialing it in juuuust right. :)

[Eagle]

1 hour ago, Skorpyo said:

Ok so this is kind of a beating a dead horse question but here goes. I’ve installed an 8.8 with discs, a dual diaphragm 95 booster, have removed the LSV and am now debating what to do next. As I understand it the LSV handled proportioning to the rear brakes and eliminating it comes with the price that should I lose a brake line I’ll effectively lose all braking power if I’ve just plugged the port on the distribution block and am running a single line to the rear from the nose port. So:

 

1. Am I wrong about the above?
2. Will replacing my distribution block with a Wilwood 260-11179 be my best option?
3. Would I be better served to install a Wilwood 260-12627 for just the rear line?
4. What might be a better option for me?

 

I'm not planning on doing much more than running around town and maybe the occasional green trail or two and I don’t haul much weight on a frequent basis, except maybe when I’m taking a run to the dump or hardware store. Input appreciated. Thanks!

 

Tim

 

Yes -- you are incorrect. You will NOT lose all braking power by eliminating the rear height-sensing valve.

 

The MJ has dual-circuit brakes. There is one hydraulic circuit to the front, and a separate circuit to the rear. With the height-sensing valve in place as designed, under normal conditions the output from the "nose" of the distribution block goes through the height-sensing valve, which reduces (proportions) the braking force to the rear wheels. The forward, bottom outlet on the distribution block, which only opens if the front circuit fails and loses pressure,  by-passes the height-sensing valve and sends full pressure to the rear wheels.

 

If you properly eliminated the height-sensing valve, by connecting the "nose" outlet directly to the flex hose at the rear axle, you will ALWAYS be sending FULL braking pressure to the rear wheels -- both when the front brakes are working, AND if the front brakes fail.

 

The downside to eliminating the height-sensing valve, especially if you also put disks on the rear, is that under normal conditions you may have too much braking in the back. In rain, snow, or if the road has oil or sand on it, the rears may lock up before the fronts, resulting in spin-outs. Obviously, this is more likely when the bed is unloaded than when it's loaded to capacity, but that's the way most of us drive our MJs probably 90% or more of the time. It's also more of an issue when going down steep inclines, because more weight is transferred to the front end when going downhill. I'd suggest finding an empty parking lot and testing how your new setup behaves under heavy braking.

[carnuck]

It will depend on which rear brakes you have and the load on the truck too. I switched to different wheel cylinders on my otherwise stock early XJ rear axle I put in mine with 10" rear drums. They were on my spare parts shelf and I looked up the application to see that they were for an AMC Eagle wagon with the same size brakes, but smaller bore so the pressure on the drums was slightly less.

 

Law of hydraulics. Liquids at normal atmospheric pressure cannot be compressed in an enclosed environment.  (as long as no gases are trapped in the liquid, like air in the lines of a brake system) Brakes also include the Archimedes law of levers. 

    Small bore master cylinder and big bore slave cylinder is how floor jacks work. The bigger the slave cylinder vs the master, the more pressure is applied but the shorter the movement . The smaller the slave, the more movement, but the pressure it applies is diminished. 

 

Cliff notes: 

smaller bore wheel cylinders mean less pressure is applied to the brake shoes on drum brakes. Not for the average person to play with on the highway where lives are at risk without doing the math and real world testing. 

 

If if you go too far one way or the other, excessive force can occur or next to nothing. Most modern power brakes (non-power steering assisted) can safely be stopped in the event the brake booster fails. 

 

[Eagle]

1 hour ago, carnuck said:

 

Law of hydraulics. Liquids at normal atmospheric pressure cannot be compressed in an enclosed environment.  (as long as no gases are trapped in the liquid, like air in the lines of a brake system) Brakes also include the Archimedes law of levers. 

    Small bore master cylinder and big bore slave cylinder is how floor jacks work. The bigger the slave cylinder vs the master, the more pressure is applied but the shorter the movement . The smaller the slave, the more movement, but the pressure it applies is diminished. 

 

Cliff notes: 

smaller bore wheel cylinders mean less pressure is applied to the brake shoes on drum brakes. Not for the average person to play with on the highway where lives are at risk without doing the math and real world testing. 

 

 

You're on the right track, but your explanation is technically incorrect.

 

Many years ago, in my AMC days, a young friend bought a used 2-seat AMX. For some reason,m the AMX used a rear proportioning valve but the Javelin didn't. I guess the short wheelbase of the AMX made it more prone to "swapping ends" in panic stops. Anyhoo ... the previous owner of this particular AMX had FUBARed the entire brake system by filling it with something other than brake fluid (ATF, I think it was), which caused all the seals to swell up and lock up the brake system. By the time my friend bought it, we could still buy every part of the system ... except the proportioning valve. So, we removed it. To compensate, he used wheel cylinders from a 6-cylinder Gremlin (smaller bore). He drove it that way for many years, autocrossed it, and never had a problem. So it's a valid approach. BUT ... it means reduced braking to the rear axle ALWAYS. Not a problem when running an unloaded pickup, but something to keep in mind if fully loaded. It's a work-around to deleting a proportioning valve, not a functionally equivalent solution.

 

The smaller wheel cylinders do not result in reduced pressure to the rear. In any hydraulic system, pressure is always uniform throughout the hydraulic (fluid) part of the system. But FORCE is pressure applied to an area. So if a brake system is pressurized (by your foot on the peddle) to, say, 1000 psi, then a wheel cylinder with a piston that's one square inch in area will apply a total force of 1000 pounds to the rear brake shoe. That 1000 pounds is then spread out over whatever the area of the brake shoe lining is.

 

If you use a smaller wheel cylinder with a piston area of 3/4 of an inch, that same 1000 psi of hydraulic pressure applied over an area of .75 sq. in. will now apply a force of only 750 pounds to the brake shoe. That 750 pounds is then distributed over the same lining area, so the force applied to the rear brakes is only 75%.

[Skorpyo]

9 hours ago, Eagle said:

 

Yes -- you are incorrect. You will NOT lose all braking power by eliminating the rear height-sensing valve.

 

Thank you Eagle, that is exactly what I was hoping to hear. While I was quite sure what was in my mind was wrong I simply wanted to hear it from folks who know more about this than I. I think the second Wilwood valve will serve my purpose to manage pressure to the rear and I certainly will take her to an open lot to get a feel for the braking pressure and adjust accordingly. 

 

And thank you pete and carnuck for your input as well. 

 

Tim

[carnuck]

I did the late Model XJ master and booster on mine for increased stopping power. Later front braking system would have been better with larger bore calipers. 

   Hauling loads was never a problem and I even towed numerous vehicles. 

[Skorpyo]

12 hours ago, carnuck said:

I did the late Model XJ master and booster on mine for increased stopping power. Later front braking system would have been better with larger bore calipers. 

   Hauling loads was never a problem and I even towed numerous vehicles. 

 

By "Later front braking system" are you referring to a later model XJ braking system? I only ask because I upgraded the front D30 to a D30 from a '95 XJ D30 when I did my 4WD conversion. Would this have netted me larger bore calipers in the process? If so w00t! Thanks again!

 

Tim

[carnuck]

There are a couple caliper changes along the way ('90 up for example) and different brake pads as well. XJ rear brakes changed as well from 10"x2" to 9.5"x2.5" and then rear disc brakes option. 

 

 

 

   I'm glad MJs didn't get the failed ABS system that XJs did. I drove several Chevy Colorados for parts delivery at NAPA and hated driving  this one very mild paved hill (with big bumps from semis or dump trucks from Sea-Tac taking it) and a 110 degree corner at the bottom. Even in good weather you had to drive 5 mph under the 35 mph speed limit or the ABS would kick in if you rode the brakes and the little trucks would speed up almost like the infamous Suburban on Potato Salad hill video. 

   If it started doing that, then most of the drivers with less experience would push the brakes even harder (especially in the rain) and sail off the corner into the weeds. Luckily no one hit a vehicle coming the other way (at least while I was there) and it looked to be repaved when I drove by yesterday.