I replaced the master cylinder, booster & all front & rear brake lines on my 1970 convertible. It had disc front brakes on it but no proportioning valve. I installed the new GM replacement combination proportioning valve when I did the lines. I get fluid to the rears OK but nothing from the fronts.

I did bench bleed the M/C, both on & off the car, fluid moves in both reservoirs. The "service manual" says (for the original metering valve) to make sure it's open when bleeding. I can't see how this new replacement valve can be held open. Anyone ever have this problem??

I would start with the master cylinder. Make sure the front is bleeding from there. Unhook the line where it goes into the prop. valve and see if it bleeds there. If it doesn't bleed there, its the master cyl.

If it does, hook that back up, and unhook it where it comes out of the prop valve and give it a try. If it goes in, but doesn't come out you've found your problem.

I had a similar problem that turned out to be the master cylinder.

service manuals tell you to hold the button pushed in on your combination valve while bleeding. There should be a rubber boot covering a steel pin sticking out of one end of the combo valve. This is the button that must be pushed in. GM has a special tool to hold the button in. There is a picture of it at the end of the chassis service manual chapter on brakes. I made my own. Basically, it is a piece of sheet metal bend 90 degrees on one end. That end pushes against the button. The other end has an elongated hole that fits one of the bolts. Push the button in and the bolt that holds the combo valve to frame is tightened on the tool to hold button in. A spring could do the same, or a loving wife to hold the button in. But when I bleed mine doesn't seem to make a difference with button held in or not. Mine bleed fine either way.

I think you pushed the master too far when you bench bled it. The front internal piston is stuck. Easy to do when the system is dry.

If you do not get fluid if you crack the line at the front port at the master when an assistant hold the pedal down, then swap the master.

After checking all of the suggestions, I still couldn't get the fronts to bleed.

I decided to remove the M/C and bench bleed it again. When I put a wrench on the booster, it split apart at the seam. I'm sending it in for replacement under warranty. When I pushed the master cylinder in just a bit (1/2" or so) I got a gush of fluid from both the front & rear ports. I checked the new booster pin and it looks like once you bolt the M/C on, it is already pushing the piston somewhere around 3/4" - 1" or more. I belive the M/C & booster are mismatched, causing the piston to move further than it should closing off the front port. When I get the new Booster, I'm going to recheck dimensions again. I did get eh booster & M/C from 2 different sources but both were supposed to be for a 1970 with Frt. Disc's.

You just learned the hard way that there is a long and short booster rod for the same year car. When you look at your master, if the piston has only a shallow dent it takes a short booster rod. If the piston has a deep indent, say 3\4 of an inch, it takes the long rod. When the master is bolted to the booster there should be a few thousands clearance between end of rod and master cylinder piston. I have read on this site that some people ensure this by putting a piece of clay or the like on the end of the booster pin and assembling. If there is a bit of clay left on the tip of the rod, a few thou thick, you have the right clearance.Get a new pin or cut yours to length.