Proportioning valve

Can the proportioning valve be adjusted? Does it have any springs or moving parts which could get stuck blocking hydraulic flow to the rear breaks? My rear brakes are not working and I am wondering if this could be a cause. If i have to get a new proportioning valve is it something i can get in the aftermarket or is it strictly dealer item? thanks
Its for a 99XJ. I-6

No the stock cannot be adjusted, it is possible for it to malfunction, I had the same prob but a lack of pressure to my front brakes, do a search on proportioning valves and you will find many aftermaket adjustable valves.

Or just fix the rear brakes. They are probably in need of adjustment or a proper bleeding. It's pretty rare for the prop valve to malfunction. It's possible for the prop valve to get stuck when bleeding, but you'll see the brake lite on.

I have tried everything to fix the rear brakes. Nothing works. New shoes, rear brake cylinders, Master cylinder, and countless brake bleeds. The only thing left is Proportioning valve or the booster. I think valve is most likely.

I just upgraded to rear discs, so did some recent research on proportioning valves. You can try pulling the o-ring out of your exisitng valve, and that should send some more fluid that direction.

They make aftermarket ones, like this one from summit racing, http://store.summitracing.com/partd...&part=SUM-G3905&N=700+-48578+115&autoview=sku, but I never got far enough into it to figure out if something along those lines would do the trick.