How does the brake proportioning valve work and how can I fix mine

Hey guys
I just picked up my first gen tacoma and I come to find it has literally no rear braking.
So I looked into it and I believe my brake proportioning valve is set wrong, leaving no braking at the rear.
I'm pretty mechanically inclined but taking a look under the car I have no idea how this thing actually works. I see that there is some sort of plunger on a rod with a single line going to it. Mine is also set completely to the top with the adjustment nut seized.
Any suggestions?

 

Do you have a lift? Have you bled the proportioning valve? Bled the rest of the system? Does the bar from the axle to the valve appear to be bent or damaged?

 

I don't believe the proportioning valve is adjustable. At least I've never seen one on a generic vehicle

 

Do you have a lift on it?

Function: when you load weight in the bed, the rear of the truck sinks. This effectively pushes the rod up, which activates the levering rod across to the valve to increase brake pressure to the rear.

If it is not working, you may get all pressure going to the front brakes, which would lead to premature wear and possible warped rotors.

This is the stock setting of the adjustment nut on an 01 4wd Xcab V6. It looks to be at least an inch. I don't have the truck anymore, or I'd go measure.

 

So how would I best go about dis-assembling it?
What does the single line going to the plunger do?
I'll post a pic later but mine looks nothing like that, it's set completely to the top with none of the rod sticking out. Factory suspension as far as I know. At least I'm guessing that because the suspension looks as old as the rest of the truck.
The adjustment nut is seized and if I really crank on it it twists the whole assembly and I'm worried about breaking it. I was thinking about taking it apart and then braking the nut loose but I wanna know exactly how it works first.
Bled the valve itself, didn't change anything. I also tried to bleed the rear brakes and I get a bit of fluid out the right but it's dry as a bone on the left, so I suspect it's so tight it's not even letting any fluid get to the rear

 

So seeing how far up mine is its either super out of wack, or maybe the truck is lifted? The suspension doesnt look like anything special

 

That is adjusted all the way up which should give you MORE braking at the rear. If you are not lifted, and you do not have good braking at the rear, either your proportioning valve is bad, or your rear brakes are not adjusted properly. Do you ever use your e-brake? You need to use your e-brake regularly to keep the rear shoes adjusted.

 

I recently bought the car but the e brake does work. In fact it's the only way the rear can brake right now, they get no hydraulic pressure

 

If you don't have any brake fluid coming out of one of the bleeders, the proportioning rod is not the issue. I would break the soft brake line free where it connects to the splitter on the rear axle and see if you have fluid there. The internals of the proportioning valve seize up sometimes and they simply stop passing enough fluid to engage the rear brakes.

If that is the case, consider replacing the valve with an adjustable manual proportioning valve.

 

Where would I find one of these valves that would bolt up with no modification?

 

The only thing that will bolt up with no modification is an OEM replacement of the proportioning valve, or some cheap knock-off, if one is even made. Manually adjustable valves are much much simpler - no load sensing valve, no brake line return to the front brakes, just a globe valve with a screw adjustment. Mine came from Front Range Offroad, who put the kit together using a Jegs valve.

Kit:
http://frontrangeoffroadfab.com/proportioning-valve-kit/

Install write-up:
https://www.tacomaworld.com/threads/bretts-average-build-thread.305098/page-28#post-12388470