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no brake pressure
#1
so i have 73 mach i am restoring. i replaced all the wheel cylinders and brake components on all 4 tires. the driver front was not leaking when i bled it so i changed the rubber line and it worked fine. i bled them all with a helper on the brake pedal and it was tight and seemed to work well.

i road tested the car and the brake pedal seemed spongy. sometimes it would go to the floor or close without stoppng and sometimes the pedal was stiff and would stop well. then the pedal hit the floor and didnt do anything anymore. the brake line came on and stayed on i believe.

i took the car in the garage and when a friend hit the brake pedal it seemed to leak out between the master cylinder and the gasket thats under the cap. i bought a new gasket and tried to bleed the brakes. the back 2 bleed just fine and was fluid the whole time. i went to the front two and both only drip when they are opened and someone is on the pedal.

also the MC never went dry. after my drive it still had fluid in it.

any thoughts what could cause it? could it be a bad MC? i believe its still original.
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#2
Sounds like the combination valve has shut off fluid to the front wheels.

The valve will now have to be re-centered.
  Reply
#3
rocket366;243803 Wrote:Sounds like the combination valve has shut off fluid to the front wheels.

The valve will now have to be re-centered.

+1

You may have an internal leak in the master cylinder so pressure was applied to only the rear brakes, which caused the valve to move.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#4
Change out the master re-center the combo valve and
flush the system completely to remove all air from the system.

I have a new (bought last year)... now used master from my power front disc brake 73 with less than 500mi on it for sale $20 delivered.

PM me if you want it. I take Paypal.

Paul

73 Grandé
351C 2v
Now 4v Carb/Cam/headers/T5


Gasoline is for washing parts.
Alcohol is for drinking.
Nitomethane is for racing!



Work in Progress photos here:
Last Update: 4/23/16

http://s1270.photobucket.com/user/theroc...t=3&page=1






















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#5
Sounds like the master to me as well.


  Reply
#6
Ok. Ill change out the master cylinder and start from there. I better buy some more pb blaster because those lines are rusttyyyyyy.

How do i recenter the proportioning valve?

Your MC is for discs? I have drums all around so i cant use yours, right?
  Reply
#7
Get a new master for your application.

To re-center the combo valve (other than taking it apart) try
loosening a rear brake bleeder screw , put a hose on the bleeder
screw and into a jar, then give the brake pedal a good STOMP.

That should re-center it but it may take more than one Stomp
as the valve may get stuck in some muck inside the combo unit.

The idea is to create an opposite "imbalance in front to rear pressure"
that caused the combo piston to move when it detected a problem.

Note: Your brake idiot light should be lit when the combo shuts down
the front or rear system and when re-centered the light should go out.

Good Luck
Paul
  Reply
#8
Well i replaced the master with a reman. I bench bled it and installed it. I bled all the brakes and all 4 bled just fine. Do you think i still need to recenter the valve?
  Reply
#9
Sounds like you are good to go.
Time for a road test

Paul

73 Grandé
351C 2v
Now 4v Carb/Cam/headers/T5


Gasoline is for washing parts.
Alcohol is for drinking.
Nitomethane is for racing!



Work in Progress photos here:
Last Update: 4/23/16

http://s1270.photobucket.com/user/theroc...t=3&page=1






















  Reply
#10
Awesome! Thanks everyone! Seems like you guys have worked on one or two of these cars!Tongue
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