Technical An interesting observation, no rear brakes

Bob, perhaps the rear only aspect is because the occurrences of a rear line failures is historically much higher than front lines, just a WAG but it does makes sense. Also making sense is that Within the valve, the front outgoing ports are blocked until the rear side of the valve sees the required pressure. The O rings and only spring pressure are doing that job of blocking the front ports, but if the rears never build pressure how would that ever happen?

I'm wondering about this statement you made "What doesn't make any sense is why you would want this feature," . In the event of a rupture retaining the remaining fluid within the system by preventing its flow to the ground sure seems like a great idea. That rupture can drain the entire master cylinder.

 

I've run into this problem I believe and didn't know how to fix it at the time.
It was a 73 Chevelle and I only drove it in the summer as the ineffective brakes in the rear made it super difficult to drive in slippery conditions.
I was told long after the car was gone about this recentering of the valve to get fluid flow to the rear brakes.
I've since had this procedure described to me by several people who's ability I respect.
No idea how common those particular valves are...but they definately exist.
You may not drain the ENTIRE master cylinder if the rear line rusts out...but you will be a step ahead if the rear chamber doesn't empty itself. Makes it a lot easier to bleed the new lines if the chamber didn't go dry!
This kinda stuff is a much bigger concern in the rust belt. It's really common to replace lines and equipment here due to corrosion.
I've replaced the same line a couple of times on some of my beaters. Replacement lines rust pretty fast!
As for the whys and don't make sense etc?
My experience with cars is that they are full of stuff like that and it almost appears the manufacturers do it to make things as complicated as they can. Guess it keeps the Engineers working!
I KNOW they do just that with new cars to make them almost impossible to fix outside a dealership.
Dealerships will order parts from the manufacturer. $$$$$$$ makes the world go round...so they tend to design things that will get the money flowing THEIR way! LoL

 

I recently experienced the same lack of pressure to the rear brakes after replacing a leaking rear wheel cylinder on a 1972 Chevy truck. With the truck on stands I could not even stop the rear wheels from turning with full pressure on the pedal. It did have enough flow to vacuum bleed but afterward would not stop the wheels from turning when in drive (automatic trans)

This was a new problem for me but after some study I opened the front bleeder valve and applied brake force, this shifted the shuttle valve. This solved the issue. I re-bled the complete system again just to be safe.

BTW this was a 3500 series truck if it matters.

 

In the early 60 AMC/Rambler offered dual circuit brakes a couple of years before most others.
"Double Safety" brake system.
The ads described them as two independent systems ( one front, one rear, not diagonal, as is necessary for non FWD or similar layouts)
http://www.lov2xlr8.no/brochures/misc/62rambb.html
http://clickamericana.com/wp-content/uploads/sensible-new-Rambler-vintage-cars-1964-1.jpg

Like everybody else shortly thereafter , As there 2 separate systems the differential pressure switch must continually keep each one separate, so I guess in a sense "plugs" the leaky one, even before it leaks.

 

Well my truck is still busted but we're all learning a lot of stuff Tonight I adjusted the rears again, no difference. With the truck on stands, the right rear wheel will spin with the brakes applied. The parking brake stops it no problem. So the next thing was to bleed the brakes. Bleeder snapped off! It looked clean, no rust but it's gone. I also checked the differential pressure switch with an ohmmeter to see if the pin was grounded, indicating that the valve had shifted but it was also fine. Tomorrow I will replace the wheel cylinder, bleed and report back.

 

In my years of playing with cars and fixing dump trucks at work [old ones had hydraulilc brakes] I've run into your problem a few times. Several times the problem was an internal problem with flexible rubber brake hoses. I've had them begin to degrade inside, causing a little flap of rubber in the hose to block flow to the wheel cylinders but only in one direrction. The fluid would return back to the master on pedal return. Drove me nuts 'till I found it but your truck is acting like a blocked hose.
Did you get your combo valve to center? Might have to center it a few times.

 

Your getting so many things going on here that your chasing your tail, fix the broken wheel cyl.
Bleed the system again. Crack the rear brake line at the combo/prop or whatever you want to call your valve, have someone press the pedal down and you watch the dribble or squirt. If you have lots of pressure there then your problem is between there and the rear brakes. If it dribbles than crack the line from the M/C to the valve. If it has pressure there than the problem is in the valve, if it dribbles the problem is in the M/C or the mechanics of M/C, pedal ratio, plunger depth, internal leak, etc.. You can't rule out a bad M/C AND a bad valve either. JMO

 

Started bleeding brakes tonight. I bled a lot of air out of the rears, so much so that I need to buy more brake fluid. Also I noticed the metering valve plunger on the combination valve was pulsing while the brake was being pumped. So far, there's still not enough pressure to hold the rear brakes. I did however have good squirts from the wheel cylinders as I bled them.

 

To keep the combination valve from shifting while you are trying to bleed the rear brakes, find a third person to crack a front bleeder when you crack the rear. Crack both bleeders and have the person on the pedal push it down slowly. No pressure will build up in the front so the valve won't shift.

If you don't have 3 people to do this, you can try cracking the rear bleeder and have the second person push the pedal down slowly and only go part way down so it doesn't build pressure on the front brakes.

When I want to bleed brakes and don't have anyone to push the pedal, I cut a piece of wood to put between the seat and the pedal that is 2 inches longer than the distance to the pedal when it is all the way up. I crack the bleeder and then put the piece of wood in to hold the pedal down, go back to close the bleeder then remove the piece of wood. It's a slow process but it works.

 

Here's another odd observation...As previously mentioned, the right rear wheel cylinder was changed and bled. When I got to the left rear, it produced air, lots of i t. So I kept bleeding it till I ran short of fluid and called it a night.
Tonight I continued the bleeding process. Right rear, 6 or 7 good squirts, no air. Left rear, a shot of air out with every squirt! Topped off the MC again, another 6 or 7 squirts, each one with air coming out. How can you have that much air in 2 feet of line between the junction block and the wheel cylinder?
I'm thinking it's time to change that wheel cylinder as well because the air does not seem to be coming from the MC.

 

The other wheel cylinder has now been changed and the brakes were bled again. NO air now! I also noticed that there was brake fluid behind the dust boots of the old wheel cylinder, not a good sign.

Today I cobbled up a pressure gauge assembly with a fitting that would screw into the rear brake port of the combination valve. It wasn't till 10:45 tonight that I could get someone out here to pump the brake pedal while I watched the gauge.

It was pretty interesting, sometimes the gauge would go to 50 or 100 lbs, sometimes it would go to over 400 lbs. My wife was pumping the pedal and I was trying to get her to apply the same amount of force every time. I basically told her to push it down for all she was worth every time. Sometimes the gauge would come back to zero, sometimes it would drop only to 20 lbs or so.

Having never had to measure pressure in a brake system before, I need to digest the results I guess. My expectation would be that it should be possible to attain the same reading, + /- a few pounds on every application of the pedal. Tomorrow I'm going to try to find a fitting that will thread into the rear brake port of the master cylinder itself and see what the result it there. I would expect it to have higher pressure and be very repeatable. I just need to find the right fitting, I think the master cylinder threads may be metric

 

This morning, I attached the gauge directly to the output of the master cylinder. The results were the same. Sometimes I had 400-500 lbs, sometimes, I couldn't get over 50. This is not bleeding the brakes, this is right at the master cylinder with the rest of the system disconnected. The results were the same with the engine running or off. The pedal is fully returning after each application.. Seems to me like the MC is flaky. Thanks to our friends in China, a new MC is cheap. Now I know I probably shouldn't have ordered the same thing I have in there but I just wanted to be sure it fit the booster and would line up with the tubing.

Stay tuned

 

I applaud your efforts 1great40 and thank you greatly for posting them.
This kind of diving in for understanding will allow you to help yourself and others.

Rear brakes that only work whenever they want is seems more dangerous than no rear brakes

 

Times 2 Gary

 

Yep, gotta hand it to you, you are sticking with it the rite way and the report back is of great help to others.
There's a lot of cars/trucks running around with no or little rear brakes but don't know it because the HotRods are a lot lighter than a normal family rig and the fronts are doing all the work..................

 

Thank you guys. I will admit that although I originally posted looking for a silver bullet fix to the problem, after a while it got pretty interesting. Though spring is coming, there's no need to have this truck fixed by the end of the week or anything like that so trying to educate myself on the nuances of brake system function hopefully helped other guys as much as you guys have helped me.
If I had not been so reluctant to open up the plumbing and actually install a gauge, this problem may have moved along a little faster, but I have never had a failure of this type before and it wasn't until the gauge was installed that I could actually see what the brake pressure was doing. It was really strange to stuff my foot through the firewall and barely register any pressure at all on one actuation and then have the next actuation generate 500 lbs of pressure. Sometimes it would generate pressure repeatably for like 10 actuations and then all of a sudden, 50 lbs! To monitor this behaviour right at the MC has pretty much convinced me that the thing is flaky .
Although I was suspect of my pedal travel and free play, I am assuming that since those parameters are the same all the time, I should be seeing the same pressures all the time.
The new MC was ordered today and I will bench bleed it and it should bolt right in. Then we'll see which direction this tale takes.

 

Did the new m/c fix the problem?

 

I replaced a working master cylinder on an off topic truck because it was seeping out the back end. I couldn't get the rear brakes bled. I finally put the original back on and it bled easily. I put 3 new ones on before I got one that worked. Cheap master cylinders are expensive.

 

Hello all! I finally got some help in bleeding the brakes with the new MC. The first thing I did was have my daughter pump up the brakes and I cracked a front bleeder. Then I checked to be sure that the brake failure switch was no longer grounded, that told me that the switch was centered.
Then I went ahead with a full bleed. Since the MC had been swapped and the combination valve was empty, I went through everything again.
While it took a fair amount of pedal pressure to keep the wheels stopped while the engine was at fast idle, once it was up to temperature and at normal speed, the rear brakes would stay stopped with normal pressure, something it wasn't able to do before. So, for now, I'm going to declare victory and go home proud!
I'm going to chalk it up to a bad master cylinder although I never installed the gauge on the new one to measure the pressure that was developed. There certainly was something flaky with the old one since on some applications, it would develop 400-500 PSI no problem and on other applications, I couldn't get 50 lbs out of it, although it did become more reliable the more I actuated it. I guess I shouldn't have expected too much from a fine "imported" master cylinder!
Thanks so much to all of you who helped.