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Owns A Torsen
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A simple proportioning valve only slows flow it can't control final pressure. The pressure for the front comes from one part of the master cylinder the pressure for the back comes from another.
I've lost track of who is having the problem LOL but the rears should work just fine with the amount of fluid provided by whichever half of the master cylinder is supplying it.
That being said, poor rear brake performance can only be a few things. In order from most likely to least likely:
1. The line is open for long enough during a rear end swap for all the fluid to drain. Most people simply do not bleed enough. Air has to compress completely for the hydraulic fluid to start compressing at all, and that it will but the piston in the master cylinder has to move much farther to do that. The piston for the front and the back in the master cylinder are on the same rod. If it has to move a half inch to compress the big bubble in the line chances are the front is already making resistance and could stop completely before the back does anything. Have a shop air bleed or vacuum bleed them, you have to start there.
2. IF you bleed properly and still no love.... some disk brake expo rear ends have been sitting for a while. The caliper(s) could be rusted to the bolts that let them move back and forth, the calipers themselves might have pistons stuck from rust and or dry rot.
3. The ABS module is freaked out and stuck in the emergency position. The proportioning valve is in the ABS or RABS module, there are 2 different types depending on if your Ranger was born with rear anti lock or 4 wheel anti lock. Either style is made so that if you lose pressure to the back (that pesky line before the rubber line on the rear end will break on you at some point) the pedal goes almost to the floor then the proportioning valve locks and you still have a little bit of brake to the front to get you home or on a trailer somewhat safely. Anyone who has broken a rear line knows exactly what I mean. This will also turn the brake light on, the one on the dash until the valve comes more center. I have seen them get stuck though it wasn't on a Ranger. Easier to replace them. Symptom would be pedal goes much farther than you like and the light will come on AND there will be very little fluid going to the affected side. Just enough to fool you in to thinking they are bleeding if you haven't done it a million times.
4. Most rare and I have never seen it on a Ranger, those rubber lines have linings. The linings can come dislodged especially if they sit dry for a while then all of the sudden see fluid. The inner lining comes loose and acts like a one way valve, stopping the fluid from flowing in the quantity needed for stopping.
5. Less likely than #4 but since we have a conspiracy theory thread going I have to bring this up. The friend helping you with the brakes? Actually your enemy. Maybe wants you dead because they have eyes for your girlfriend, boyfriend, wife, husband, dog whatever. Maybe they didn't like your Trump bumper sticker or your Obama sticker and it is politically motivated. OR they might be from a planet where the inhabitants live by consuming brake fluid and they are angry at the amount you are asking them to pour on the ground when they "pump it up and hold it... OK you can let it go now".
In recap, the proportioning valve on your Ranger really isn't a proportioning valve, it just gets labeled as that. It is kind of an emergency gate so that if one half loses pressure you can still stop only not as effectively.
I have used real proportioning valves on oiling systems in machines, some adjustable some with different diameter orifices. The ways on a lathe need a lot more oil than the ball screw but since it is coming from one source at a constant flow rate and has to go to different areas at different rates "proportioning" is what it needs. Geeky.
My 2:50AM red bull driven synopsis.
Jim
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00 XLT 4WD RCSB 3.GO! Jalapeño
01 XLT 2WD RC Steppie 3.0 auto Silver
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