I upgraded my 66 f-100 to a dual res mc and power booster. This is the type of MC you'd find on a mid seventies covette with the reservoir closest to the headlights operating the front brakes.

I just upgraded the fronts to disc and went to bleed them after putting everything together.

The rears (bec. I installed a prop valve) bled with my vacuum bleeder (hooked to compressor) no problem. Pump sucked fluid right into the container like it should.

The new fronts are proving to be a pain: the pump is not pulling any fluid to the calipers. The only time I'll see fluid is after pumping the brakes. Cap is off MC, and forget about anything coming out on it's own in order to gravity bleed.

Here is the issue: No flow, no gravity bleeding possible, pump wont suck fluid out of MC either. I thought maybe blockage in the line, and started at the MC. If I take the fitting off of the MC that goes to the front brakes nothing drips out. If I hook a bench bleed kit to it, I'll get flow when pressing the pedal, but imagined there would be fluid flowing from it without hitting the pedal. Am I wrong to expect this?

 

I have grey hair now but I seem to remember on a GM dual master cylinder that the rear reservoir feeds the front brakes and the front reservoir feeds the rear brakes. I would expect there to be flow so it sounds like you are air locked and need to bench bleed some more.

 

Thanks. I talked to a local mechanic who lets me bend his ear and he suggested bench bleeding some more as well. That's two for bench bleeding more so it seems like a good bet.

I am certain that this MC's front reservoir feeds the front brakes though.

The mechanic also suggested jacking the car up a little in the front to tip the MC up and bleeding still attached to the firewall by pumping the pedal.

Thank you for your suggestion. I'll update after I give this a shot.

 

I am glad I can help and I will admit I am not a Corvette guy so their mc could be quite different from the Pontiac Grand Prix and Olds Cutlasses that I have owned.

 

You said you installed a proportioning valve. Did you use the PV bleeding tool, otherwise you tripped the valve.

 

I installed an adjustable prop valve (Wilwood 260-8419 Adjustable Proportioning Valve ****)

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

on the rears, and had no trouble bleeding the rears. I did not use a special tool though, and the instructions made no mention of special steps for bleeding once installed.

The fronts are where I am getting no flow from the MC unless I step on the pedal. I have not had a chance but will try the above suggestions out tomorrow, or this weekend the latest. Let me know if you think I am missing something.

 

The valve lock tool only applies to OEM style drum/drum pressure differential valves or OEM disc/drum brake valves. It keeps the pressure differential valve spool from shifting off-center and tripping the brake warning light --assuming you have (or have installed) an OEM brake valve and the warning light. Tandem MCs, pressure differential brake valves and brake warning lights didn't come into use on domestic vehicles until 1967.

Here's the valve lock tool installed in the '84-'86 F150 disc/drum brake valve in my '69 F100, while I was bleeding out the stock '69 rear drums and the '77 F100 front discs.



https://musclecarresearch.com/brake-valve-tool

 

UPDATE: I just attempted to "bench" bleed (with it still attached to the firewall) using a bench bleed kit looping back into the reservoirs and no fluid coming out of the front reservoir. Back brake res. (closest to firewall) pumps fluid right back into the res, but nothing in the tube for the fronts.

The only way to get fluid out is to push the pedal and suck with the mighty vac.

Is this MC garbage?

 

confirmed MC is garbage. Broken piece. Replaced, and no problem.

 

What was defective on the MC?

 

The nozzle that is visible when you remove the fitting for the reservoir that goes to the front brake was crushed and a piece actually broke off. If you look inside the port when you remove the fitting you can see it in there. I must have over tightened it maybe . . . maybe there was something in there when I tightened it down, not sure, but it was probably my fault. Maybe it stuck out too far to begin with when it was rebuilt, not sure.