Back Brakes won't bleed

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magicr

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A couple weeks ago my brake lights got stuck on, crawled under dash and noticed the brake pedal was not coming back far enough to hit the switch. It seemed that the brakes didn't feel quite right so I decided to bleed them. I started with the passenger rear. My Dad pumped brakes up I turned the bleeder valve, and no fluid came out. Went to drivers rear, and same thing. I decided to go up front (My car has the factory non power discs up front 1971 Dart) brake fluid came out fine.

My car has new master cylinder, and new shoes front and rear. The work was completed three years ago, and I have never had any problems. Does anyone have any ideas of what could be wrong? Is the stuck switch issue related?
 
Three years ago isn't new. Bad master cylinder, rust or kink in the line running to the rear. I had the same problem and had to replace the line. Other than that, stuck proportioning valve....
 
This is probably a stupid question, but where is the proportioning valve? Found it, and realized that it was never replaced when I replaced the master cylinder. Is there a way to clean it or repair it, or does it need replaced?
 
Usually the proportioning valve is incorporated in the distribution block located on the firewall under the master cylinder. I'd try administering a few light taps with a hard rubber hammer before replacing the unit.

I had a similar problem with the Demon. Crimped rear line.
 
magicr said:
This is probably a stupid question, but where is the proportioning valve? Found it, and realized that it was never replaced when I replaced the master cylinder. Is there a way to clean it or repair it, or does it need replaced?
Click to expand...

The proportioning valve isn't usually replaced it is bad. I am 55 and have never had to replace one yet.
 
For many cars, that is not a "proportioning valve". It is a pressure differential sensor that activates the brake warning light on the dash if there is an imbalance between front and rear brake line pressure. This usually indicates a problem with the master cylinder or a leak somewhere. This device nearly never fails and I would be highly surprised if yours is bad.

If your rear brakes are not bleeding, then you have a clogged line somewhere. Start by disconnecting the flex hose to the rear diff and have someone step on brake pedal. If no fluid comes out, then the clog is closer to the master cylinder. Keep disconnecting lines until you see fluid coming out when the brakes are applied. Then clear or replace that line(s).
 
If it gets off-center; pump brakes, slowly press pedal until you feel a click, stop pressing, close the bleeder. Not clear, 2 people there. Been 45 years, done a lot of them, can't remember, seems bleed the front, for the click-oh, light will go out too.
 
If you do as said above by 68gtxman and find that the stoppage is the valve... They can get stuck and it is possible that you jammed it when bleeding. The gunk in the system goes to the bottom of the valve which feeds the rear. That is the most likely place it will get stuck. I know there are posts on here with a procedure to recenter it. I dissasembled and cleaned mine, but that should be a last resort. You can take the bottom cap off and clean it pretty well on the car. There are lots of little parts and springs though...
 
Good advice from other members. My opinion is either a collapsed rear brake hose at the differential or try removing the rear bleeder screws from the wheel cylinders and run a wire through the hole. Or the bleeder screws could also be plugged. My 72 Dart has a self centering pressure differential switch. your 71 probably does too, so I doubt that is the problem. (the brake warning light on the dash would be on). The brake light switch should not be related. I have been a brake mechanic for 40 years.
 
I had the same problem with a 69 dart gts and a 68 gt it was the line going to the rear end I replaced that and got fluid to the rear brakes.
 
Another question, I've got the rear flex hose loosened at the rear end, but should the fitting spin? I seem to only be able to spin the hose which is putting a bind at the other end. figured it out, and got it off, line is all clear. zkx14, can I pull the bottom cap off of valve without a lot of parts coming out, or not.
 
green1 said:
If it gets off-center; pump brakes, slowly press pedal until you feel a click, stop pressing, close the bleeder. Not clear, 2 people there. Been 45 years, done a lot of them, can't remember, seems bleed the front, for the click-oh, light will go out too.
Click to expand...

I would try and recenter the PV as said. If you crack the line to the front right at the PV and have someone press the pedal and listen for a click, that might straighten it out. Sometimes people bleed brakes and have a tendency to push the pedal tight to the floor which can off set the valve. I use a small block of wood under the pedal so as not to do this.

Of course now that I have a Mighty Vac, I use it.
 
This happened to my Dads car.What we figured out was the rear brake hose was plugged up.We replaced the hose and then we were able to bleed the brakes.
 
Start by removing the bleed screws at the rear wheel cylinders. That will give a bigger opening for rust junk to flow out. However, the clog is probably upstream since no flow from both. I only use silicone fluid (DOT 5) in all my cars since I had the same rust clog in my 82 Aries, but read up since many fear it for sometimes strange reasons. If you use anything else, bleed your brakes at least every 2 years to get new fluid. Glycol fluids have stuff to suspend the moisture, but when those get full, corrosion starts. DOT 4 or 5.1 are the best of the glycols. Don't fall for the "synthetic" marketing, all brake fluid is and always has been.
 
I was just checking years, and most likely you don't have this - a combination valve. I think in 71 you may have 2 blocks. One for the switch and on for propotioning disc/drum. Anyway here is a pic of the bottom end of this type opened. It will have spring pressuure on it when the cap is backed off. Note the black rubber piece around the brass piston - that is actually what is wrong with this one. They swell up and fall apart. There was a supplier for a kit which I believe included these(Muscle Car Research) , but there website won't come up now. Without that, the only thing you can do is clean them (flush with clean fluid). There is nothing in them to 'fix'.
This is the proportioning side of the valve. The 'slider' that trips the light and may be your problem is actually on the other side of the valve. So, you can't clean that directly without complete dissasembly. If you get flow through the system that side will flush. So what I said about the slider sticking would probably only happen if the system was dirty and out of service for a while (like mine was).
 

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I would try and recenter the PV as said. If you crack the line to the front right at the PV and have someone press the pedal and listen for a click, that might straighten it out.
Click to expand...

Just so I understand, I should loosen the front brake line fitting at the PV or will it work if I slightly open bleeder at front right wheel?
 
I was just checking years, and most likely you don't have this - a combination valve. I think in 71 you may have 2 blocks. One for the switch and on for propotioning disc/drum. Anyway here is a pic of the bottom end of this type opened. It will have spring pressuure on it when the cap is backed off. Note the black rubber piece around the brass piston - that is actually what is wrong with this one. They swell up and fall apart. There was a supplier for a kit which I believe included these(Muscle Car Research) , but there website won't come up now. Without that, the only thing you can do is clean them (flush with clean fluid). There is nothing in them to 'fix'.
This is the proportioning side of the valve. The 'slider' that trips the light and may be your problem is actually on the other side of the valve. So, you can't clean that directly without complete dissasembly. If you get flow through the system that side will flush. So what I said about the slider sticking would probably only happen if the system was dirty and out of service for a while (like mine was).
Attached Images
Click to expand...

That's exactly what I have, so if I take the bottom cap off, Fluid should pour out and I can flush it that way?
 
Yes, but... Do the other stuff first. Have you determined where the stoppage is at? Flushing that out will help it to not happen again, but it is not going to solve your current problem. If it is stuck you still need to recenter the valve.

What did you meen by 'line is all clear'? did you have flow there?

One other VERY important thing - be really careful turning the line fittings - especially on the block. They tend to strip and when you finally get them to move you may twist the end right off the line.
 
What did you meen by 'line is all clear'? did you have flow there?
Click to expand...

The rubber section going up to rear axle was clear, but still plugged up to there.

If it gets off-center; pump brakes, slowly press pedal until you feel a click, stop pressing, close the bleeder. Not clear, 2 people there. Been 45 years, done a lot of them, can't remember, seems bleed the front, for the click-oh, light will go out too.
Click to expand...

So just that I'm crystal clear I can open the bleeder at the right wheel and use the above procedure. Thanks
 
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