Need brake bleeding advice.....soft pedal no matter what I do?

[tcs69rt]

I have a 69 RT front discs & rear drums. Been fighting caliper and master cylinder (mc) leaks all summer. Got them fixed. Now I bench bled & replaced the mc 3 times, removing air. I have alot of air in the system with replacing all the lines and rebuilt calipers as well. I have a mighty vac pump & tried to pull all the air out but am still getting bubbles. So it was suggested that I open all 4 bleeders and let it sit overnight. That did nothing for it, no fluid came out and the mc is still full of fluid. Could that be a proportioning valve problem? It is original. My next move is to buy a pump and do reverse bleeding with hopes of pushing all the air to the master cylinder. I tried to do this with a large syringe (feed store) but it couldn't push fluid well enough into the brake bleeder to the mc.

The traditional 2 man concept of bleeding was tried but once the pedal went to the floor it would stay there. I could start the car & wait 10 minutes and the pedal would raise half way on it's own. From that we could never get any fluid from foot pumping the pedal, the system had no pressure. The mighty vac pump was the only way to pull air & fluid out of the bleeder but after driving a few miles the pedal falls again and pressure is lost. I have been all over the lines and cannot find a visible leak.....Suggestions? Thanks! TC

[John_Kunkel]

You have a faulty master cylinder, it's bypassing internally.

[tcs69rt]

Well, I started the car last night & left the bleerders open a second night. This morning the mc was empty = oops! So I removed it, bench bled it until all bubbles were gone, then did it again 15 min later to ensure no air. I then put it back on the car and used my mighty vac on the rt rear drum, then the left rear drum & moved fwd to the rt then left discs. I did pump the pedal between bleeding each wheel. The pedal came up and the brake lights went off. I started it and let it run a few minutes pumping the brakes and the pedal went down aliitle activating the brake lights but the it came up enough for the lights to go off after a few seconds.

Should I drive it around the block, see if the pressure holds and then re-bleed all wheels again? Thanks for the help John!  TC

[Back N Black]

I would bleed again, also you do not have to remove the master cylinder to bleed. Cut up a couple of old brake lines like in the pic below.

[tan top]

 would rebleed the system again ,  any sign of soft pedel , don't drive it

I would bleed again, also you do not have to remove the master cylinder to bleed. Cut up a couple of old brake lines like in the pic below.

 yeah thats true  ,

what i do when bench bleeding  on car , put a sprit level length ways on the master cylinder & jack up  front or back of car to get it level  
same if bench bleading off the car in a vice , jack up the work bench as required to make the master cylinder  level ,  

[tcs69rt]

My concern is that my pedal is halfway off the floor. Will the pushrod still go all the way in with it mounted to the booster?

With a "half pedal" am I better removing it and putting it in a vise to bench bleed it that way with a bleed kit (hoses) I have?

Thanks TC

[John_Kunkel]

The OP states "but after driving a few miles the pedal falls again and pressure is lost." If there is air trapped in the system it won't appear and disappear it will just be there; so the pedal will always be soft.

The only circumstance that I can think of that would make the pedal be firm on one application and then drop on the next application is a MC that bypassing internally. BTDT