definitive comanche brake bleeding

[Eagle]

Eagle, just to clear things up. The whole point by doing my procedure is you don't trip the plunger. Just follow the brakelines on an MJ and imagine how the brake pressure goes and you will see how it will work. In normal operation brake pressure goes out the nose of the block, to the load sensing valve, via the T coupler from the bypass line and out to the wheels. That means the same pressure will also act backwards towards the distribution block through the bypass line, but it has nowhere to go since it end up between the two o-rings you mention. Air trapped here will stay here forever and thats kinda the whole reason behind this discussion. By loosening the nut under the block you will backflush the bypass line, rear to front, using the reduced brake pressure from after the load valve.

 

Your procedure can't bleed air out of the bypass passage in the distribution block itself. The factory established the bleeding procedure in the FSM for a reason.

[badguru]

It sure duz bleed it out. I just simulted a front brake failure by opening a front bleeder valve then I took the truck out and slammed the brakes on at around 30mph and got 2 perfect black marks from the rears skidding me to a stop. I closed the bleeder filled the mc back up gravity bled that caliper then took it back out and locked all 4 wheels with no problem all this with my bleeding technic. This brake system is the same as what's on a toyota hilux read up on that system and you understand how it works. And that line isn't just an emergency circuit it's opening every time you step on the brakes. And it's also being pushed back by fluid pressure from the front circuit and that makes it a (drumroll please) a proportioning valve how do you think it gets closed after u do that silly bleeding proceedure and you restore pressure to the fronts

[Eagle]

 And that line isn't just an emergency circuit it's opening every time you step on the brakes. And it's also being pushed back by fluid pressure from the front circuit and that makes it a (drumroll please) a proportioning valve how do you think it gets closed after u do that silly bleeding proceedure and you restore pressure to the fronts

 

That line does NOT open every time you step on the brakes, and it has nothing to do with proportioning. As I posted previously, the XJ has the same shuttle valve, but the XJ also has another plunger and spring to handle the proportioning. How could the shuttle move every time you step on the brakes? The pressure is the same on both ends. If it moved, it would trigger a warning light every time you step on the brakes. The shuttle does two things:

 

1. It triggers the brake warning light if either end loses pressure;

2. It opens the bypass circuit (XJ or MJ) in the event the front brakes lose pressure

 

There's a reason why the factory refers to it as a "bypass."

 

Obviously, nobody is going to convince you. We've posted the bleeding procedure from the factory service manual, and I've posted cut-away photos of the actual units from both the XJ and the MJ so you can see how they work and how they are different internally. Feel free to continue to believe what you believe. Hopefully, other members have learned something about their brake systems from all this.

[badguru]

I will see if I can find a big enough piece of plastic to machine a clear proporioning valve and use the stock shuttle.

[Comavalanche]

[DirtyComanche]

Hi everyone,

 

First, I would like to thank Eagle for his very patient discussion above. I think I now understand the MJ rear braking system much better than I had.

 

Second, who has real world experience with this? Anyone eliminate the height sensing valve and just run the brake line directly? Anyone actually running an XJ proportioning valve?

 

If so, any problems with rear wheels locking with an unloaded bed?

 

Thanks

 

Gene

 

Mine has the height sensing valve deleted, one of the POs did it after a line broke or something.  There's a whole thread of people on here that have done it.  I've debated going to the XJ proportioning valve if/when I go to a later booster and master cylinder, but more likely I will just go to an aftermarket adjustable one.  And yes, unloaded you can lock the rears up.  IMHO the rears should lock just before the fronts do, which is certainly what I am getting, but I haven't decided if it's too much or not enough at this point.  It's winter here so it's hard to do any testing with real valid results.