WTB am I doing wrong? Bleeding the brakes on an old GM

I'm becoming increasingly convinced there is something wrong with the master. Two nights ago I bled the entire contents (almost, didn't go to air) of the reservoir through each wheel into my pressure bleeder and then pushed the same volume all the way back up into the master through each wheel.

What's really odd is that the pedal feels better when a bleeder is open. Like I can feel that the pedal is immediately pushing fluid, and it's nice and smooth. Then I close the bleeders and it still builds some pressure, but I can feel a "step" or something where it feels like something else grabs or something.

I happen to have another master sitting here that is also new but was for a drum/drum car. Rockauto says same bore size though. I might pop it on just to verify the master is the issue before I go plunk down for another master.

Out of curiosity, what's the difference internally between a disk/drum and a drum/drum master? This is old gm where the proportioning valve body with residual pressure springs is a separate unit.

Robbie said:

Out of curiosity, what's the difference internally between a disk/drum and a drum/drum master? This is old gm where the proportioning valve body with residual pressure springs is a separate unit.

main difference, and probably the only difference, would be how much of the overall stroke is allocated to each circuit.   for example, a drum/drum car has four wheel ends with approximately equal volumes, so the internal geometry of the MC would position the pistons such that it was "50/50 split" front to rear.   in a disc/drum car, this split could be 60/40 or maybe even as high as 70/30 front because of the much larger volume requirement of the front calipers relative to the rear wheel cylinders.

Robbie said:

I'm becoming increasingly convinced there is something wrong with the master. Two nights ago I bled the entire contents (almost, didn't go to air) of the reservoir through each wheel into my pressure bleeder and then pushed the same volume all the way back up into the master through each wheel.

What's really odd is that the pedal feels better when a bleeder is open. Like I can feel that the pedal is immediately pushing fluid, and it's nice and smooth. Then I close the bleeders and it still builds some pressure, but I can feel a "step" or something where it feels like something else grabs or something.

I happen to have another master sitting here that is also new but was for a drum/drum car. Rockauto says same bore size though. I might pop it on just to verify the master is the issue before I go plunk down for another master.

Out of curiosity, what's the difference internally between a disk/drum and a drum/drum master? This is old gm where the proportioning valve body with residual pressure springs is a separate unit.

Have you bench bled the master?

I'm with NOT A TA. This could easily be due to not properly bench bleeding. 

Early GM dual circuit master's are a bear to bleed, especially if they're not dead level while you're bleeding them. There's always a few bubbles that stay above the outlet hole and have to come out the top into the reservoir. 

Only way to be sure to get them all out is to look into the reservoir and visually check for them as you run the piston through the full stroke. 

Edit: although having recently learned about "reverse bleeding" I'm wondering if that would do it. 

Haven't bench bled yet but will try. I was reading about bench bleeding that brought me to bench bleeding with a syringe, which is using a big syringe and pushing fluid up through the outlet ports.

Which is why I tried bleeding in the car one last time by pushing tons of fluid up the lines from each wheel (with a pressure bleeder arrangement). No change in pedal. And no bubbles came out of the master. Yes I did tap the master as I was doing it and yes the master is very level in the car, maybe it needs to be tilted a bit while bench bleeding, dunno.

I'll be surprised if bench bleeding improves the situation, but I've been surprised before.

Robbie said:

I'll be surprised if bench bleeding improves the situation, but I've been surprised before.

so will i.   my money is still on bad MC out of the box.

FWIW, i *never* push fluid upstream from the wheel ends.   too much corrosion and crud that can get pushed back to the MC and damage a seal.

also FWIW, depending on the orientation of the MC as installed, even if only nose-up by a degree or two, it is quite possible that you can not push that bubble out by pushing fluid upstream *or* downstream.   but i believe the volume of air trapped in the MC wouldn't be large enough to cause the symptoms you're experiencing.

Verdict is in.

Bench bleeding the original new master didn't turn out any new bubbles. (And didn't fix the issue).

Grabbed a new raybestos master and bench bled it - I bought the bench bleeder kit only to have the raybestos master come with the plugs needed for bench bleeding, oh well. Popped it on and bled the brakes in about fifteen minutes (just like normal).

And everything works as expected! Woohoo! 

Pedal has lots of travel, but the effort isn't high. There are no power brakes, so the leverage trade-off for pedal travel to master travel makes sense. I was easily able to chirp the wheels from about 15 mph.

Thanks everyone!!