Master Cylinder Question

I have a new project: 1972 Bronco.

Among many issues to address, the brake pedal goes all the way to the floor, no resistance.

One of the two brake fluid "wells" on the master cylinder was empty.

I filled it and began to bleed brakes.

The back two bled as normal.  The front did not.  Pumping the pedal results in no fluid coming out the front brakes.

I assume it is a bad master cylinder.  My question: does the master have two independent pumps for the front and back?  If so, my issue makes sense, but wanted to confirm before I replace the master.

Thanks,

Hi Jason..,  welcome..!

My question: does the master have two independent pumps for the front and back?

Yes

Likely but not necessarily the M/C.  The front lines (reservoir. to wheel cylinders) could still have air in them or, the line is plugged.

(being it was left empty for a while)

You could remove the line from the M/C and one of the front wheel cylinders. Blow air & see if it's clear. Same for other wheel.

In fact.., 1st, remove one line off the back of a wheel cylinder & see if you can get any fluid to run down the line or pump brake pedal & see in fluid appears. Try both sides. If fluid shoots out, there is issues with the wheel cylinder(s)

Another part of the system is the proportioning valve attached to the frame. This could have internal issues.

CHECK ME ON THIS but, I think, with the dual M/C valve system.., the rear should still hold pressure, even though the front is failing..., & vise-versa. (This is the whole idea of seperate valves "safety")

As you're doing, it would be nice to get them working. Then eventually you will likely have to referbish the complete brake system to be safe.

Keep us posted as to what you find.... :-B

Hey Jason,

As BB said the problem could be the proportioning valve.  However if there is no fluid at all in the reservoir for the fronts, the best way to rid the air is to open BOTH front caliper bleeders and fill the master with fluid.   Do not replace the cover....let gravity do it's job. Be patient.  It's a slow process, but it will absolutely rid the front system of any air.  Be sure to check the master occasionally so it doesn't empty.  When there is a lot of air in the lines it sometimes will not "bleed out, but get moved back and forth and/ or foam.   If the fluid was completely drained for some time, there is a good chance that internally the calipers, prop. valve, lines and master are rusted internally. 

With the age of your Bronc I would take Bully advice and replace all the components.  No go is a problem...No stop is a disaster! 

Good Luck