One thought is that occasionally when you allow the pushrod in the master to travel more than it normally does (as when bleeding the brakes) and its old there can be rust/scale, etc. on the pushrod that trashes the seals in the master. It may not leak externally, but just kind of bypasses the fluid internally leading to it constantly seeming like its got air in the system.   If this is what is sounds like its doing I'd swap in a new master and don't forget to bench bleed it. 

I will also say that just because something doesn't leak fluid doesn't mean it doesn't leak air.  Years ago I installed a set of NAPA fuel filters on my Diesel F250.  Truck started and ran fine.  I was out driving and it just died.  Low on fuel I thought maybe I ran out, so I filled it up, and it fired right back up.  I got home, no issue.  The next morning it started fine, but died while I was maybe 1 mile from home.  I got it started and limped it back, constantly stalling, having to wait, driving a short distance, etc.  I couldn't think what could be causing it, so I talked to my mechanic neighbor, and he gave me one of the best pieces of advice I've gotten about diagnosing a problem: "What did you do to it last?"  I thought about the filters, but couldn't believe that was the issue, but as he's a very smart guy I climbed underneath it and took a look at the frame rail mounted filter.  I noticed the cap was not fully seated even though it was tight.  I drained it, and pulled the cap off.  I tried to reinstall it, and it only went the same distance in (it had not been cross threaded or anything).  I made sure the filter was seated correctly, which it was.  Out of ideas I pulled the new o-ring I'd installed when I changed the filters off the cap and found it threaded in just fine without the o-ring.  I went to Ford and bought a Motorcraft filter set.  I looked at the o-ring for the cap and immediately realized what the issue was: the NAPA o-ring was WAY too hard and prevented the cap from seating fully.  This allowed it to suck air, even though it didn't leak a drop of fuel.  It kept stalling due to being air bound.  When I let it sit the air dissipated and it would restart until it sucked in enough air to kill it again.  Of course I only used Motorcraft or Racor (who makes the filters for Motorcraft) from then on and never had another issue.  Lesson learned, just because it didn't leak fuel didn't mean it couldn't suck air.  Something to think about. 
 

I'm thinking bad MC but that's only guess so it's not to helpful.

TKOPerformance wrote:

One thought is that occasionally when you allow the pushrod in the master to travel more than it normally does (as when bleeding the brakes) and its old there can be rust/scale, etc. on the pushrod that trashes the seals in the master. It may not leak externally, but just kind of bypasses the fluid internally leading to it constantly seeming like its got air in the system.   If this is what is sounds like its doing I'd swap in a new master and don't forget to bench bleed it. 
 

Thank you for all the inputs!

Sorry, I failed to say how I was bleeding the brakes.  I'm using a Mityvac with a pickle jar reservoir (I got tired of continually emptying that itsy reservoir that it came with.  This setup worked really well on my daily driver a couple of weeks ago).  It does leak a little air into the tubing around the bleeder screw but what I was seeing is much, much more than that.

The "what did you do last" comment made me realize that the very first thing I tried was pumping the brake pedal.  But when no fluid came out of the bleeder, I decided to use the vacuum pump instead.  I bet I pushed the pedal in too far and messed up the master cylinder.

I'll give replacing that a try and see what happens.

I can't thank you all enough.  I'll let you know how things turn out.

Last edited by John Ha (11/25/2024 6:59 AM)

GPatrick wrote:

Did you try the two-person method?   

Thank you for the suggestions.    I live alone so the two-person method isn't possible.

GPatrick wrote:

Check the condition of your rear brake hose and try to minimize any high loops that can easily trap air when vacuum bleeding.  

All the hoses and lines look great.  I'm 99.44% sure that stupid me screwed up the master cylinder on my first attempt to bleed them.

Last edited by John Ha (11/25/2024 10:28 AM)

When I suspect the caliper bleeder screw is allowing air into the vacuum line, I have placed teflon tape on bleeder screw.

When done bleeding, I allow it to gravity bleed while I remove the tape.

IMO there are a lot of exotic "solutions" to bleeding brakes.  None of them work as well as a length of clear vinyl tubing and a jar.  Put clean fluid in the jar, hose from bleeder into the jar (cover the end in fluid), crack the bleeder, and pump the brakes.  Keep the jar at a level below the bleeder.  Don't let the master get too low (usually 15-20 pumps is going to be all you can do without refilling the master).  I've used this method on vehicles old, new, large, small, import, and domestic and never had an issue bleeding brakes.  Everything else IMO introduces added complexity into something that does not need to be complicated.  The two man method also works, but when does anyone have a spare person to help? 

Yeah, my vacuum bleeder never worked well so it only does duty on chasing actual vacuum leaks and such. Good old two person system always works. Push Hold, (close bleeder) Let Up. (loosen bleeder) Push Hold.............

Nos681 wrote:

When I suspect the caliper bleeder screw is allowing air into the vacuum line, I have placed teflon tape on bleeder screw. When done bleeding, I allow it to gravity bleed while I remove the tape.

Dan's method works. As the owner of 5 (count 'em, yep five) BMWs with maintenance recommendation of every other year brake fluid flushes - I've done dozens of them. Many dozen. It's a good ritual, you can see the discoloration of the old fluid and its hygroscopic nature makes the reco sensible (to me anyway - many think it's unnecessary.)

The only time I ever encountered a problem was by depressing the brake pedal too far using the two person method. I've used the mityvac method using heavy grease on the bleeder screw to eliminate air intrusion.

I've also used a more time efficient one-man method where you remove the bleeder, grease (or teflon tape it) and replace it, crack it loose, fit a tight clear tubing over the bleeder nipple and arc it up and over the suspension, terminating in a pickle jar. Preferably a Kosher pickle jar. Using a shallow pump of the brake pedal, it goes much faster than Vac bleeding and the arc in the tubing holds way more fluid than needed to keep air intrusion at bay. 

On the fastback thrash to get it on the road, I tried every method of bleeding the newly installed brakes. Nothing worked until I finally realized I had installed the rear Eldo brakes with too much clearance. Fixed that and she tightened up nicely. DOH! Now I realize why the Caddy Eldo rear disk brakes are unpopular. Quite fiddly. 
Cheers.   
 

Good to hear John.

Glad ya got it fixed.

With old master cylinder, did you have an external inline residual valve?
Just wondering.

Residual valve.

https://youtu.be/DIVDjqniUX8?si=bsSIoWFQh2RRixMe