resetting brake valve?

[Jakegday]

The rear brake line on my fairmont rusted out the other day. I replaced it, then bled both rear brakes. the pedal still didnt feel right so i decided to bleed the front brakes too. the master cylinder is split into 2 sections. 1 section holds fluid for the front, the other section holds fluid for the rear. While bleeding all 4 tires, fluid came out of the rear side reservoir only. There is absolutely no air in the system, and the brake pedal still doesnt feel as good as it did last week.

Im assuming the valve thing underneath the master cylinder is the culprit, i just dont know how to "fix" it.
Can anyone help?

 

[NHGUY]

Replace the master cylinder.Its lived a good 31 years.

 

[Jakegday]

it actually only lived 30; i replaced it last summer.
The symptom im having doesnt seem like a master cylinder problem though. The 2 lines come out of the master clyinder, and go right into that (pressure differential?) switch. From there the fluid goes out 3 lines; 1 to the front left, 1 to the front right, and 1 to the rear... It seems like that switch/valve isnt letting any fluid out of the "front" section of the master clyinder.

just wanted to add, in case this helps: while bleeding all 4 brakes, i did confirm that all 4 are locking up when the pedal is pressed

 

[pcoxe]

I think there are some differential switches that will lock out one of the brake sections if it doesn't build as much pressure as the other side. The side with the higher pressure pushes a plunger over that cuts off the weak side, presuming that it has a leak to conserve fluid. There may be an electrical switch to turn on a dash light when it occurs. To reset it, you had to mash the brake pedal rather hard so the equalized pressure would force the plunger back to center. My wife once owned a Fairmont many years ago that had a similar problem.

 

[mechtech2]

There are brake light warning sensor valves in some older cars.
They simply go back and forth to trigger the dash light.
If either side ruptures, the little piston moves over and makes the electrical contact.
This may be your problem.
Or, a rebuilt master cyl being bad is not unusual at all.

 

[Kruse]

Originally Posted By: Jakegday

Im assuming the valve thing underneath the master cylinder is the culprit, i just dont know how to "fix" it.
Can anyone help?


10-12 years ago, I had the exact same symptoms on my '71 Cougar when I replaced a rear wheel cylinder. I smacked the proportioning valve a few times with a hammer and re-bled all four wheel cylinders. The light then went out and the brakes were great.

 

[mechtech2]

To reset a brake line failure switch after bleeding, simply bleed the other side/leg/half until the light goes out. ?the piston will move over and stop making contact.

 

[Jakegday]

Originally Posted By: mechtech2
To reset a brake line failure switch after bleeding, simply bleed the other side/leg/half until the light goes out. ?the piston will move over and stop making contact.


This is actually the first thing i tried, since its one of the things my textbook (automotive technology) suggests to do. but it didnt change anything. The brake light never came on through any of this actually, not once. But while bleeding all 4 lines, fluid only comes out of the rear (smaller) section of the master cylinder.. The master cylinder is only a year old, but i know that doesnt necessarily mean its good.

the problem im describing, with fluid coming out of the rear section only, does that sound like a bad switch/valve, or does that sound like it could be a bad master clyinder instead? Keep in mind it JUST started doing this right after the rear line failed. (i bled all 4 sides last year when i installed the new master cylinder, and it was all working properly)

 

[vssjim]

to check the master cylinder try to bleed it at the master cylinder and go down the line until you loose pressure and try to isolate it that way rather than starting at the rear start at the front and work to the rear. Try it on both side of the combination valve to check that and go on.

 

[SS1970chrysler]

Some of the valves have a little pin sticking out the front that you can pull with needle-nose pliers. That worked on my '85 Olds. otherwise, try the hammer tapping method or adjust the rear brakes. I had many Fox-body cars that seemed to always have one rear wheel out of adjustment.

 

[mechtech2]

The fact that only one reservoir is emptying makes the master cyl highly suspect.

 

[yonyon]

This does sound an awful lot like a junk part. If you keep pressure on the pedal and crack open each line out of the master, do you get fluid out of both?