So far by cracking the lines above the proportioning valve, I am seeing that the master cylinder is pushing fluid to it.

When cracking the lines on the back end of the proportioning valve, it is sending fluid to both separate fronts, but NOT the back at all.

I've got it out now, and cant even blow air from the rear port IN to the rear port OUT.

I am going to now open it up and diagnose what is not allowing anything through.

 

This is looking down the throat of the proportion valve.

Am I looking at a crack or is it normally 'cut out' this way?



Also, if everything is normal here, how do I reset this thing?

 

I can't tell from the picture if there is a crack or not. But the manual says the way to reset it is to leave the engine off and really stand on the brakes. I had one of those that refused to reset and kept the brake warning light on all the time, but it did flow to all the brakes. I replaced it from a parts truck I bought and it works great.

 

its my understanding that back in the dark ages of a single-pot master cylinder, the proportioning valve actually worked according to its name, but ever since then it only serves as a tee plus an idiot light sensor.
that said, if you aren't worried about losing the idiot light, it should be ok to replace it with a simple tee for the front brakes, and a simple coupler for the rear brake line

 

That isn't what the 1981 Ford Light Truck Shop Manual says. Page 12-01-2 says:

 

I am close to just T'ing and coupling it as I had a Volkswagen without and proportioning valve, and it stopped just find.

However, that was drums all the way around. But I knew guys with discs in front and drums out back and still it worked fine.

The only reason I am thinking of going this route, is because I had no pressure from the breaks to reset the damn thing.

Guess this means my master cylinder must be bad if I've got something in the valve stopping my rears from getting any fluid but still had no pressure to the pedal...? The master pushed fluid when I cracked it before the valve and tested it. But maybe doesn't have sufficient pressure capability.

Anyone out there running without the proportioning valve with advice? Or anyone know how to take this valve fully apart?

 

its my understanding that if either circuit has no resistance, you'll have a very soft pedal, so you might spend a little time getting the fronts bled out, and if you still have no pressure, then the MC has to be bad.
or with the low price of a new MC and all signs pointing toward it, you might start by replacing it before screwing around with the old one

remember, new not reman. most reman units are questionable at best

 

Really, you should've left the proportioning valve alone. Very rarely do proportioning valves (real name is brake pressure differential valve) go bad.

In one of my previous posts, I explained that you more than likely had a bad master cylinder.

When the master cylinder is bad, the seal that keeps the fluid from leaking by the piston has started to leak. This causes air to get pushed into the brake lines. Depending on which path the air takes, you'll have air in either the back or the front lines.

Now here's where it gets really neat. When the air gets into a line, the brake pressure differential valve has a piston on the inside with electrical contacts that will slide back and forth in the piston bore. When pressure isn't equalized appropriately, the side with the most pressure will push the plunger, causing the contacts to jump and brake fluid to slow or cease coming out of the brake line.

When this happens, your pedal is usually soft, you have restricted brake fluid flow and less than appropriate brakes, as well as a brake warning light coming on on your dash.

So... what you need to do... is get yourself a wrench or sockets and undo the two nuts holding the master cylinder on the booster. Then, leaving the brakes lines still attached, slide the master cylinder off the studs.

Look at the back where the plunger goes from the brake booster. It'll be more than likely wet with fluid.

$15 bucks and your problem is solved.

 

The proportioning valve on the rear side was plugged.

I will take pictures of my findings. As of right now I am about to clean it out and install it with a new master cylinder.

An O-ring got wripped during tinkering and now it works at least to allow flow.

Good enough for me right now. I'll post pictures later.

 

there is no "good enough for now" with brakes, either they're perfect, or they aren't good enough

 

The proportioning valve on the rear side was probably plugged because it was doing its job. If there's a greasy, muddy substance on the inside, then I'd be wondering how it got in there. However, the fact that your brakes are soft with the rear side plugged still alludes to a bad master cylinder.

Also, if your master cylinder is bad, better get ready to change a brake booster too. Always do them in pairs (though you can get by with just changing the master cylinder if the booster isn't leaking).

When I changed out my booster and master, the master cylinder had almost halfway filled up the inside of the brake booster, and when I pressed the brake pedal, not only did I have a leak, but it was also trying to pull brake fluid into the motor.

Don't forget to bench bleed the master cylinder then bleed all 4 corners.

 

Just going back through the thread to make sure I didn't miss anything and saw this. My thinking would be that it had a defective check valve or a bad spring or some such on the inside.

 

that was back when i was first putting my first truck together, and i seem to remember i was missing the check valve and just had an ell fitting in there, which is the only reason the pedal came back up when i stopped the engine. my guess is that some internal leak caused vacuum to be applied to the diaphram at all times in a way that it pulled the pedal down. sure was a weird one, but its long gone and not the subject of this thread

 

You haven't looked at the Tech Tips sticky at the top of this forum, have you?

 

I did in fact take a look at that thread and found it mostly helpful. Had found it through searching and thanks for the reminder about the sticky.

In more detail:

I was looking for information regarding the parts and their functions as I was having a problem getting fluid to flow from the rear IN port of the proportioning valve, to the OUT. When in place, the "o-ring", the one that's around the removable 2" brake line tubing section that is held in place by a spring and the downward facing bolt in the rear part of the proportioning valve, acts as a plug and clogs the rear flow.

With the o ring out now, and the tubing section remaining in, I at least have flow to the rear breaks.
But, I assume the actual proportioning action of the valve will not still be in operation.

By the way : "o-ring" is in quotes because the rubber piece I am describing here is not simply a donut of rubber in shape, but rather has a domed shape with an extruding top and pushed in bottom that when the spring pushes it all the way to the top of the port (passed what seems to be a lip in the metal where something should sit but doesn't seem to make sense as nothing would stop simply there when a spring of that length is being screwed in behind it. ) but any way, when the "o-ring" gets pushed up to this location, it blocks the flow to the hole right above it that WOULD ultimately let fluid passed and instead blocks any chance of break fluid exiting the proportioning valve and making it the to the rear brakes.