Soft Spongy Brakes - even after bleeding brakes

[LemansJeff]

Last year my brakes needed "pumped up" prior to departure and landing, as they were feeling spongy.  So, during my last annual maintenance/inspection I had the brakes bled and fluid replaced.  Well, this only worked for a couple of months before the brakes once again needed "pumped" on a regular basis.  NOTE: My parking brake neve failed and always held solid.  Now my brakes are worse than ever, but I can't find any leaks nor does the reservoir loose any fluid.  I was told the seals in the master cylinder may need replaced, but I would appreciate other's thoughts.  Thanks.  1988 M20J

[Tim-37419]

Seals would be next if it were me. 

[1980Mooney]

5 minutes ago, LemansJeff said:

Last year my brakes needed "pumped up" prior to departure and landing, as they were feeling spongy.  So, during my last annual maintenance/inspection I had the brakes bled and fluid replaced.  Well, this only worked for a couple of months before the brakes once again needed "pumped" on a regular basis.  NOTE: My parking brake neve failed and always held solid.  Now my brakes are worse than ever, but I can't find any leaks nor does the reservoir loose any fluid.  I was told the seals in the master cylinder may need replaced, but I would appreciate other's thoughts.  Thanks.  1988 M20J

As Tim says it sounds like the piston seals are leaking.  This is not an "external leak".  It leaks past the seal as you push.  Therefore it just goes back to the Reservoir.

See the Service Manual Chapter 32 page 24

. 

 

2 minutes ago, Tim-37419 said:

Seals would be next if it were me. 

I think you are right

[EricJ]

There's air getting into the system from somewhere.   Usually if there's air getting in, fluid is getting out, so you can look around for any red fluid drips on the lines.    If both brakes are doing it it's either just at the input to the cylinders (from the supply side), the parking brake system, or both sides have a problem.

[1980Mooney]

14 minutes ago, EricJ said:

There's air getting into the system from somewhere.   Usually if there's air getting in, fluid is getting out, so you can look around for any red fluid drips on the lines.    If both brakes are doing it it's either just at the input to the cylinders (from the supply side), the parking brake system, or both sides have a problem.

You may be right.  But the OP said "I can't find any leaks nor does the reservoir loose any fluid".  So it is one or the other or both.....

[EricJ]

39 minutes ago, 1980Mooney said:

You may be right.  But the OP said "I can't find any leaks nor does the reservoir loose any fluid".  So it is one or the other or both.....

It can only be a little drip to let a bubble in that makes the brakes soft.   It can do that without noticeably affecting the level in the reservoir.

[SheryLoewen]

The rotor disc maybe warped ( cupped) put a straight edge across it to check. That would cause the caliper to spring back. 

Paul Loewen

[Pinecone]

If it is pad spring back they will be soft, but if you pump them a couple of times, the should get firm

[Shadrach]

Do they stay firm for any period of time after you pump them up? If there is air in the system they will be soft almost immediately. If the piston seals in the master cylinders are leaking fluid to the back side of the piston, that can also feel spongy. 
 

The curiosity here is that it’s in both brakes and that the recurrence of the symptoms happens at the same time in both brakes. Seems highly unlikely that those master cylinders would fail at the same time and in the exact same way.  Also seems unlikely that an internal leak would manifest in both circuits at the exact same time in the exact same way.

something weird is happening and I think it’s going to be difficult or anyone to diagnose without being there.

 

Maybe start with low hanging fruit. Apply vacuum to the reservoir to try and pull any potential air out of the system. If the system won’t hold vacuum, you know you have a leak(s) somewhere.

[Kelpro999]

Your description sounds like air being sucked into a master cylinder input shaft o-ring during rapid brake applications if wheel bearings and caliper alignment have been ruled out. M. cylinder reseal should remedy. Also bottom up pressure bleeding at better than 8oz/min. with a few full strokes (against pressure) seems to completely remove air. At least this is what I found during bench testing using non-syn fluid. 
 This applies to one cylinder at a time. Reservoir to cylinder restriction could lead to both having same symptoms 

Edited November 22, 2023 by Kelpro999

[A64Pilot]

You really can suck in air without fluid leaking out, air is lower density of course. I’ve seen hydraulic pumps that would suck air in, but not leak fluid for instance.

Pulling a vacuum if you have the ability should bring an air leak to light.

Personally if you don’t have the mighty vac pump or whatever I’d just go ahead and pull the cylinders and “overhaul” by replacing all seals, there are kits that of course are overpriced but they have the difficult to find Stat-O-Seal, which I think just could possibly be the problem, kits were $30 maybe, while the O-rings are just penny’s, so your buying the Stat -O-Seal.

The work is in R&R, the seals are simple, I wouldn’t want to have to do it twice by trying to save the kit money.

Obviously do one at a time so if you forget how everything goes you have a complete one to look at.

Now is also the time to can the 5606 for 83282 also, since your pretty much emptying the system and re-bleeding anyway.

[Fritz1]

I had spongy brakes after annual about a year ago, eventually I got rid of the air, took off master cylinders so that fluid connectors face up, used two pressure pots, one connected to both calipers and one connect to reservoir, pumped fluid back and forth between both pots several times at 30 psi until no bubbles, when one of the lines blew off from one of the pressure pots and I had a 10 ft tall red fountain in the hangar I knew that I was getting close. At least in the Bravo the master cylinders are installed tilted sideways so that the fitting are not on top, I think it is impossible to get air out once it is in that spot because the flow velocity is just too low, maybe bleeding from the bottom with 100 psi might do, wondering how much of that will make it to the reservoir hoses and if that might then blow off

[Ethan]

Sometimes, one bleed isn't good enough, it may take more than one.

Second, if the brake lines offer any flex or give at all, they can make an airtight system seem spongy.  I have stainless braided brake cables because my mechanic was fighting with soft lines before figuring out they were causing the spongy brakes and replacing them.

[LANCECASPER]

20 hours ago, SheryLoewen said:

The rotor disc maybe warped ( cupped) put a straight edge across it to check. That would cause the caliper to spring back. 

Paul Loewen

Paul,

Thanks for contributing your thoughts on here. The decades you owned LASAR provide a wealth of knowledge and wisdom to our forum.

[dzeleski]

23 hours ago, LemansJeff said:

Last year my brakes needed "pumped up" prior to departure and landing, as they were feeling spongy.  So, during my last annual maintenance/inspection I had the brakes bled and fluid replaced.  Well, this only worked for a couple of months before the brakes once again needed "pumped" on a regular basis.  NOTE: My parking brake neve failed and always held solid.  Now my brakes are worse than ever, but I can't find any leaks nor does the reservoir loose any fluid.  I was told the seals in the master cylinder may need replaced, but I would appreciate other's thoughts.  Thanks.  1988 M20J

Check out this post I made awhile back. I had a similar issue and it ended up being the wheel faring causing the pad to move away from the rotor. The mechanic I used at the time lost some washers for the faring without realizing it.

 

[Mcstealth]

Probably not the issue, but some folks have found that their brake pads are installed backwards. 

Something to check.

 

[Hank]

I noticed a small puddle of brake fluid in the hangar besidemy leftbmain, and tightened up the fitting. Unsurprisingly the right brake was soon spongy. I helped new IA refill and bleed them, but it was soon spongy again. Left him the plane for annual, and the second brake bleeding without my assistance has held for a couple of years now.

Sometimes it just takes twice. 

[EricJ]

Another thing that can lead to spongy brakes even after a rebleed is if there's air in the fluid you're pumping in.    Some people like to fill the pressure pot, or pump, or whatever you're using, and let it sit open to ambient pressure for a day or two before putting it in the airplane.   If it gets aerated when you fill the pot/pump, that air will go into the system if you don't give it a chance to bubble out.

 

[PT20J]

@EricJ makes a good point. I believe letting the pressure pot sit after filling is in the M20J Service Manual bleeding procedure. 

My IA claims that rapidly pumping the pedals can aerate the fluid if there is any air in the master cylinders and it can take a while for the air to bubble up out of the fluid. 

It seems to me that spongy brakes would be due to air in the system and that the fact that bleeding fixed it, but it came back would imply a small leak in the system somewhere, probably on the low pressure side between the reservoir and master cylinders.

If the O-rings in the master cylinder piston were leaking I would expect the pedal to just slowly go to the stop when constant pressure was applied. But as noted above, it seems unlikely that both master cylinders would develop this issue at the same time. 

Skio

[Pinecone]

I no longer use a pressure bleeder on my cars.  When you pressurize the thing, you put air into the fluid.  For my cars, I gravity bleed.  Just open the bleeder and let the fluid flow naturally.  Get a VERY stiff pedal.

[LemansJeff]

Want to thank everyone for your inputs.  As BOTH left and right brakes are equally involved, and after doing much research, I'm leaning on the master cylinder needing new seals.  Also, my Mooney only has brakes for the left seat (right seat only has rudder foot controls); thus, only one master cylinder.  To me it does not seem to be a problem with the rotors or calipers. 

 

 

[Shadrach]

18 minutes ago, LemansJeff said:

Want to thank everyone for your inputs.  As BOTH left and right brakes are equally involved, and after doing much research, I'm leaning on the master cylinder needing new seals.  Also, my Mooney only has brakes for the left seat (right seat only has rudder foot controls); thus, only one master cylinder.  To me it does not seem to be a problem with the rotors or calipers. 

 

 

You have two brake cylinders, one for each pedal.