So I have a 79 Corvette l82. Has 86k miles on it. I just installed new MC, all new calipers with o-ring conversion kit on them, all new rotors and pads. Lines look like they have been replaced at some point. I fully bled the lines. Now my pedal was nice and firm before starting the car. Once I started I the first time after getting everything fully bled the pedal went way down. Almost to floor but its holding brakes. From what I understand the 79 you can't adjust the brake booster rod, and I feel like the rod may be too short with the new master cylinder I have installed booster can't be bad since it's not a hard pedal. 

I'm confused to where I should go with this. Do I have to buy a new booster that's adjustable? Or is there another solution that I can look at that I haven't thought of yet. I'm so tired of working on these brakes I need something to work ugh.

I appreciate any help and advice! First time ever posting a question on the internet but I'm just at a loss at this point. 

To recap firm pedal when off, very soft almost to floor pedal when running.

Do you still have the original master cylinder available? Can't remember when, but the piston was changed somewhere around '77-'79. The later style pistons have a deeper "well" for the booster pushrod to fit into, compared to the earlier masters. Using a late master on a vehicle that takes the early style "shallow" piston will cause what you are describing. Not saying that is your problem, but it has caused similar issues before....ask me how I know....😫

You should also try bleeding the calipers with the engine running, or at least run long enough to have enough boost to bleed one caliper at a time. Trying to bleed a power brake system without power can be futile, and in some cases will cause the proportioning valve to flip.

What method have you used to bleed them, and in what order?

Imho, a pressure bleeder is the ONLY way to completely bleed this system. 

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Kicking myself in the but for throwing old mc away lol it does sound like a booster rod issue but if it is IDK how to fix it cause what I have gathered up from my research about it, the 79 booster rod isn't adjustable. So what do I do at that point if I can't adjust it. Replace it with an adjustable one? Also I turn car off hard brake, turn car on goes to floor. Its the weirdest thing. I thought about bleeding while running but didn't know if that would be required.

I have tried gravity bleeding, I have done pedal bleeding multiple times where no air bubbles are coming out at all.  Started thinking maybe I should have car tilted to make the air bubbles move lol I ordered a motive power bleeder yesterday. I didn't want to spend $100 on it but I'm at my wits end.

Did you take any pictures during disassembly?  This video has a shot at the  rod in the booster before removal that shows how long it sticks out of the booster.

https://youtu.be/Dpz7Tr9P6MY

No I didn't take a video or any pics and I really wish I did! If I remember correctly the rod sticks out less than this. I believe almost flush with the hole. And from what I gathered they are not adjustable on this year of Corvette. Have a feeling I'm gonna have to get a new one if that's the case. When I took the old MC off it was all wet along the booster. Can tell it has been leaking for awhile before I picked this car up. I bought it with brake issues thinking not a big issue.... Ya I thought wrong lol tomorrow after work I will take the MC off again and check the rod to make sure it's sticking out like in this video. If it is I guess I'm buying the tool to measure it. Also have a power bleeder showing up in 2 days hopefully that will do the trick