Can't catch a "brake"

[Dusty Dude]

When my wife and I got our '69 Coronet 500 it was equipped with after market power four wheel disc brakes and they worked as best I recall. I have only driven this car from the transport trailer in front of my home to my rear entry garage. During the rebuild of just about everything drive train related I decided to go back to manual brakes which is what my A-12 car had when we bought it brand new in '69. The engine I have built for the car (496" stroker) is not going to pull enough vacuum to operate power brakes and besides I don't like the clutter of it all in the engine compartment. Other than deleting the power portion of the system the only other change I made was to go to a 15/16" bore master cylinder. It is visually identical to the one I replaced just a smaller bore. It is a four bolt, cast iron unit and I have bench bled it twice. The problem I'm having is no matter what method of bleeding I use other than pressure bleeding the pedal goes to the the floor on the initial stroke and then on the second stroke I have what I would call "normal" pedal travel and feel for manual brakes. A learned friend of mine says I need to install 2lb residual valves but my question to him is why as they were not on the car in the first place and the brakes functioned properly. I have vacuum bleed the system a few times (manual hand pump and electric pump) now and also manually bled it along with trying to let gravity do the job but no luck. I'm out of ideas and hopefully someone here can shed some light on what I'm missing. At seventy five years of age getting up and down under this car is becoming something I should sell tickets to see!

Thank you All in advance for any advice and Merry Christmas to you and yours.
Mike

 

[WileERobby]

What proportioning valve are you using ?

 

[Dusty Dude]

What proportioning valve are you using ?

Jegs

 

[PurpleBeeper]

Dang....#1, re-bleed the master cylinder. Maybe try those little hoses that loop back into the reservoir? If that doesn't work, maybe isolate the front/back....put a plug in the side of the master cylinder for one line and see if the brakes are still too the floor or not. That will tell you front or back brakes is the problem. Somewhere there's a big air bubble....and possibly a leak, so look closely. Water will rinse off any residual brake fluid on any parts now, then let it all dry, any new wet spots are brake fluid.

 

[pnora]

To much shoe travel can cause issues. Are all 4 wheels adjusted up?

 

[Dusty Dude]

To much shoe travel can cause issues. Are all 4 wheels adjusted up?

I have four wheel disc brakes.

 

[Gus chiggins]

When my wife and I got our '69 Coronet 500 it was equipped with after market power four wheel disc brakes and they worked as best I recall. I have only driven this car from the transport trailer in front of my home to my rear entry garage. During the rebuild of just about everything drive train related I decided to go back to manual brakes which is what my A-12 car had when we bought it brand new in '69. The engine I have built for the car (496" stroker) is not going to pull enough vacuum to operate power brakes and besides I don't like the clutter of it all in the engine compartment. Other than deleting the power portion of the system the only other change I made was to go to a 15/16" bore master cylinder. It is visually identical to the one I replaced just a smaller bore. It is a four bolt, cast iron unit and I have bench bled it twice. The problem I'm having is no matter what method of bleeding I use other than pressure bleeding the pedal goes to the the floor on the initial stroke and then on the second stroke I have what I would call "normal" pedal travel and feel for manual brakes. A learned friend of mine says I need to install 2lb residual valves but my question to him is why as they were not on the car in the first place and the brakes functioned properly. I have vacuum bleed the system a few times (manual hand pump and electric pump) now and also manually bled it along with trying to let gravity do the job but no luck. I'm out of ideas and hopefully someone here can shed some light on what I'm missing. At seventy five years of age getting up and down under this car is becoming something I should sell tickets to see!

Thank you All in advance for any advice and Merry Christmas to you and yours.
Mike

Did you change the MC pushrod? I mean if your new manual MC didn't come with one. No need for residual valves, I don't remember ever seeing those on Detroit iron of the 60's, 70's vintage. Don't know for sure.

 

[Dusty Dude]

Yes, I installed a new Dr.Diff rod and it is properly adjusted.

 

[Dave6T4]

Master cylinders for 4-wheel drum brakes must have residual valves in both front and rear circuits to keep wheel cylinder cups from collapsing. Residual valve maintains about 3 lbs. pressure on circuit. Master cylinder for front disc/rear drum system would have no residual valve in the front disc circuit, because line pressure must return to "0" to allow piston seals to retract piston and pad. Similarly, a master cylinder for 4-wheel disc brake system would have no residual valves, front or back. If a no residual valve MC is used on 4-wheel drum brake system, there is no pressure in the wheel cylinder to keep the pistons lightly expanded. When this happens, most of your brake pedal travel is used to move these pistons out to where they should have been if residual valve kept that 3 lbs. pressure inside the wheel cylinder.

 

[Gus chiggins]

Yes, I installed a new Dr.Diff rod and it is properly adjusted.

Roger that.

 

[Dusty Dude]

I believe I have found the problem. Whoever put this disc brake conversion kit on the car apparently did not adjust the parking brake assemblies as per the installation instructions and as a result the levers that the cables attaches to have way to much movement. This allows the pistons in the rear calipers to retract much further in their bores when at rest which results in more volume in the calipers than the smaller bore master cylinder can provide to move them out far enough to provide braking within its stroke length. In other words if those E-brake levers aren't set to spec (1/8" min to 1/4" max from the stop) you will have very poor braking or none at all. I am confident that once I get the E-brake levers adjusted properly I will be good to go. This has been a real learning process for me and I thank those of you who responded.

Merry Christmas
Mike