1969 Buick Skylark Convertible brake pedal does not pump up

[69LarkVert]

Hello,

 

I am new here.  My father has brought his '69 Skylark convertible to my house to see if I can assist in getting the brakes to work.  Prior to his ownership the car was converted to front disc brakes.  I cannot guarantee the prior owner bench bled the master cylinder.  Any suggestions are where to start trouble shooting would be much appreciated.  I have attached a picture of the master cylinder and shot showing the car has been restored from the frame up.  All new hardware, lines, equipment, etc.

 

Thank you,

Bob

[m-mman]

To clarify the situation, the pedal is low(?) or does the pedal not return after being pressed(?). 
 

Is the pedal firm? Or squishy?

What things have you attempted already to fix things?

 

And how experienced are you with mechanical repairs?

 

This will help us direct your efforts 

[joe_padavano]

I see that you have the metering valve (the small valve next to the M/C). Do you also have a proportioning (combination) valve? These cars did not use a prop valve from the factory. The metering valve slightly delays application of the front disc brakes to allow time for the rear drums to take up any slack in the brake linkage. GM adjusted the front/rear brake bias by using smaller wheel cylinders at the rear of factory disc brake cars. Do you know if the rear wheel cylinders were changed to the correct ones for disc brake cars? If the car has both the metering valve and the combo valve, that could be part of the problem as the combo valve also has a metering valve built into it.

[roysboystoys]

Are the calipers mounted with the bleeders at the top?

Sometimes they get mounted on the wrong side. Won't bleed with screw at bottom.

[ted sweet]

whats a drun brake master cylinder

 

[m-mman]

14 hours ago, ted sweet said:

whats a drun brake master cylinder

As part of normal operation disc brakes use more fluid as they wear. The reservoir(s) for the disc brake has to be larger. 

In the early days of disc brakes they had single reservoir master cylinders. The difference between the disc and drum masters was significant. 

[Frank DuVal]

11 hours ago, m-mman said:

As part of normal operation disc brakes use more fluid as they wear.

Yes! In a drum brake system the springs retract the wheel cylinder pistons and shoes to their starting point every time you step on the brake pedal. So the fluid used to expand the shoes is returned to the reservoir.  In disc brakes, the pads stay out at the rotor (the runout of the rotor pushes them back just slightly to make them clear the rotor). Therefore the fluid does not return to the master cylinder reservoir as the pads wear. 

[EmTee]

On 7/3/2024 at 8:38 PM, ted sweet said:

whats a drun brake master cylinder

Drum brake MCs also do not have a residual pressure check valve on the front reservoir.  The rear side uses a check valve so that pressure is available to expand the shoes as soon as the pedal is pushed.  As stated above, the front disc pads need to be able to be pushed back by rotor runout when the brake is released.  A check valve on that circuit would prevent release.

[joe_padavano]

Two days and the OP still hasn't been back to provide more info, but we're doing our best to take this thread off into the weeds...