Did you bench bleed the master cylinder?
There are two holes in the brake pedal. Did you use the upper or lower hole..and do you have free play in the pedal? The rod must return fully.
I assume the calipers have the bleeders on top.
That happens sometimes…no offense intended.

 

I cracked open the master cylinder a couple times while my helper pressed brakes and some air and oil shot out. There is pretty good pedals now. The problem now seems to be the vacuum line to the Powerglide Trans modulator. The vacuum line inserts only a few 16ths of an inch then it bumps the adjustment screw. Also I have the tight fitting hose around this connection. Is this vacuum insertion normal? It looks like the modulator connection is not a good connection and causes the power breaks to be weak. The brake pedal works better without the booster. The booster in new but took several years to be installed. What do you think?

 

Many possible problems, hoses collapse internally, push rod to PB diaphragm adjustment, wrong master cylinder, etc. You definitely still have air in the system, gravity bleed or pressure bleed from the master cylinder.
These guys know more than I do:

ARTICLES

www.raybestos.com

Also check your Chevrolet Chassis Service Manual
Ron

 

Do you have a distribution block? Is it for disc/drum? Did you use the tool to lock the distribution block piston while bleeding?

 

Alex, I will take MC needs proper "bench" bleeding for $100

 

 

Cracking a fitting is not sufficient to bleed a master cylinder in my opinion. It takes quite a bit of slow and patient circulation to get all the air out.
Im a one guy show so I do it on a bench in a vise.
Plenty of YouTube stuff I’m sure.
Ron gave a good link to the bleeding process too.

It sounds like the factory hard line with a rubber sleeve. That should be fine…the modulator requirement is negligible.
It sounds like you need to look at your baseline vacuum.
Put a vacuum gauge on the manifold with the booster supply and the modulator plugged and record the number then record the number with the each utilization connected. There should be very little difference if any.
A drop in engine vacuum would indicate a leaking booster, or a leaking connection.
This all depends on the system being sealed well.
IE no intake, base gasket leaks.
You need a minimum of 14-16 inches at idle for a single diaphragm. ( some suppliers state 15-18)

 

OP, if you did not bench bleed the MC before install, you need to remove it and do so. Then install MC and proceed to bleed the brakes normally.

 

I noticed the short brake lines coming off the MC (master Cylinder) are not inserting perfectly straight (into proportioning valve) and I see some brake fluid around the fitting. Could this cause the mushy pedal when the car is running? Brakes are fairly good when car is not running. The vacuum line going to the Trans modulator wants too pull out because there no bracket to hold it in place. Is there a nice piece that secures that vacuum line in place so it does not pull out of the Modulator? The 2 inch rubber hose seal is not holding it. Thanks.

 

I do have a proportioning valve.

 

OP, you have the same ?'s going on in another thread but what brand disc kit did you use?

Did it come with new short lines, MC, PV and Booster?

This is my Right Stuff MC/PV and the short lines that came with it but with the Tuff Stuff dual 9" booster.. Short lines fit perfect.

Did you cross thread your short lines into MC? (close up pic will help)

trying to remember my PG but IIRC the hard line itself had no hold down at the vac mod area, just short rubber vac line bridging the hard line to modulator. Try using a longer piece of hose with a service loop in it to allow more flex. I believe there is a plate that holds the vac mod itself to trans. There is on my TH350

You can also use a small zip ties as a hose clamp to vac mod to hold it tighter

Any brake line or vac line leak will cause soft pedal so you need to resolve your weeping MC fittings and the vac hose from coming off the vac modulator.

If you have a mighty vac, pull vacuum on the booster to confirm it does not leak. It should hold vacuum.

 

I don't know the brand of my Power Booster. The body shop got it years ago. I noticed the pedal rod is not in the lowest position on the pedal arm. Maybe I should try moving it down one hole? Most people think its air in the lines so I will keep working on that. Engine guy thinks I have to push the air out with a helper which I have done once already. Maybe better job bleeding the Master?