Brake problem

[Duster_71]

Hello All,

Having problems with my brakes getting spongy a week or 2 after bleeding. I have a 74 Duster that was drum brakes all the way around. Converted front to disc with kit from Pirate-Jack (MBM Brakes), switched rear to large bolt axles and drum kit from Dr. Diff, mounted a new Raybestos master with 15/16 bore, and replaced all lines. Master bled and I bleed the brakes further-est wheel first with assistant pumping. Everything feels good for a couple weeks and then the pedal starts to feel spongy and takes 2 or more pumps to firm up. As always, I inspect all connections and can't find any signs of leaks. Re-bleed, get some air, and everything is fine for week or 2. Last time bled the brakes felt good for about 3 weeks and reverted back to the same symptoms. Am I missing something here or doing something wrong?

 

[Mopar Tim]

If you are getting air after bleeding, then there is a leak in the system somewhere. Cant get air without losing fluid. Find the leak.

 

[Duster_71]

If you are getting air after bleeding, then there is a leak in the system somewhere. Cant get air without losing fluid. Find the leak.

Thanks Mopar Tim, Understood, inspected and re-inspected, can find no leaks and level never appears to go down either, doesn't make sense.

 

[383Scampman]

If you are losing fluid there is a leak somewhere OR you are reintroducing air into the system during the bleed process . I had a similar problem and I was bench bleeding the master incorrectly . Took three times . Also, I was bleeding the lines wrong and putting air back iinto the system . I suggest a Mighty-Vac . don't know why everyone doesn't have one .

 

[Murray]

Drum brakes out of adjustment can imitate air in the lines. Also, see if you have self-adjusters installed correctly and not loosening the brakes instead of tightening.

 

[72bluNblu]

Actually you CAN get air into the system without losing fluid. Air can be sucked into the lines before there's a big enough gap for fluid to get out. It also depends a bit on where the leak is. It's not common because when the system is under pressure you can usually force brake fluid out, but with a small enough leak it's possible. And if the brakes last for a couple weeks before enough air gets pulled in to notice you're looking for a pretty small leak.

It could also be a problem with master cylinder seal, in which case the whole issue could be internal to the master cylinder, and you wouldn't find a leak outside in the system. New parts don't always mean good parts, there's always a percentage of new stuff that has issues.

 

[zombywoof]

^^^^^
this above happened to me. kept getting air. found i didnt have brake line fitting tight enough at the master cylinder and it would suck air there. had a helper pump the brakes while i looked for leaks and spotted tiny bubble coming from offending fitting.

 

[TrailBeast]

If you are getting air after bleeding, then there is a leak in the system somewhere. Cant get air without losing fluid. Find the leak.

I had to disagree with you Tim, because as 72bluNblu mentioned air can get sucked in around the cup seals in the master cylinder if the seals are not tight enough in the bore.
Basically what happens is the pedal can return from braking faster than the piston seals can handle and it pulls air in around the seals but fluid doesn't leak out.

This is likely why the fluid doesn't get lower but it gets air in it.
Seen it before a few times, and more often the cheaper the parts we can get are.

Thanks Mopar Tim, Understood, inspected and re-inspected, can find no leaks and level never appears to go down either, doesn't make sense.

I quoted you so you would see this.

 

[OL DODGE]

Which brakes are you getting the air out of? If it"s the rears then most likely the brakes are out of adjustment or you are pulling in air around the cups. Seems that ever since the manufacturers stopped putting expanders on the ends of the wheel cylinder springs(inside) that problems like this crop up. The other replies make some very good points about tight line connections and such and I personally never fully trust "new" parts now that so much is made in China

 

[TrailBeast]

Which brakes are you getting the air out of? If it"s the rears then most likely the brakes are out of adjustment or you are pulling in air around the cups. Seems that ever since the manufacturers stopped putting expanders on the ends of the wheel cylinder springs(inside) that problems like this crop up. The other replies make some very good points about tight line connections and such and I personally never fully trust "new" parts now that so much is made in China

You know, I didn't even think about those expansion (whatever they are called) but you are right.
I haven't seen those in awhile now.
I was going to mention that I have seen wheel cylinders suck air as well from being out of adjustment, but I figured it was likely the new master so I didn't.

Good call.