In need of another bleed?

[beanhead]

Hi all, looking for some advice...I have a manual disc/drum setup on my '68 Satellite and I'm starting to have an issue with the pedal getting increasingly soft and going almost to the floor. I bleed the brakes and they work fine for about a week or so of driving and then it will start to happen again... wondering if I need to do another bench bleed of the MC, or if there may be another problem that could cause this? There are no leaks anywhere. All lines were replaced recently and when I bleed the brakes I get nice flow of fluid at each corner...thanks

 

[moparnation74]

All bets your master cylinders needs replacement! The seals are shot inside...

 

[oldbee]

++1

 

[POMONAMISSEL]

if it an old master or a new one . if the seals aren't bedded in it will pull outside air in as it comes back . thus making a soft paddle more and more . those are rare stock systems . in those cases we have them sleeved with a stainless bore liner , set it in on the bench vise and run it , should have no worries then .

 

[beanhead]

Thanks guys... The master cylinder is only about 5 months old, I will look into replacing it. Since I have some spare hardline I am going to go ahead and bench bleed it in place first and see how it works

 

[threewood]

I had an issue similar to my 4 wheel manual drum brakes, new master cylinder. I didn't have the original bellows seal bolted onto the ms under the dash, I just had the small shipping stop on it. What happened was the pedal kept hitting the stop bending it back and it would suck air at the base of the piston where the push rod attaches. I installed a full circle steel ring around the base and I have had no more issues.

 

[beanhead]

Updating this thread....tried re-bench-bleeding the master..the front brake reservoir kept pumping foamy bubbles...so much for the easy fix I'm getting a new one tomorrow. Just a question for my own database here, if the seals are bad, how does it pull air but not leak any fluid? Thanks again for all the help!

 

[POMONAMISSEL]

shape of the seal lip pushes out on pressure and relaxes and loses seal on the return . thus putting air into the returning fluid . thats what some i've had do . and if its was assembled with the wrong piston it would never work . had one rebuilt and it leaked , but the rebuilder used the same type piston , which was wrong from the get go . he changed it out , never an issue again . we stainless lined that bore as well .