Ever since I bought my car, the brake pedal has been pretty spongey, but it had sat for a while so I figured it was just degraded fluid. I recently took off the master/booster for repainting and drained as much of the old fluid as possible. After bench bleeding the master and reinstalling everything, I bled the brakes with a reverse bleeder. The pedal did get a little firmer but was still quite spongey and there was no movement from any of the calipers. Some of my local FD friends suggested that a seal in the master may have perished, so I pulled everything apart again. There were no obviously bad seals or scoring in the bore, so I just gave everything a clean, reassembled, and re-bled everything, but this still did not fix anything.

The thing that confuses me is on pedal depress, there is an air sucking sound from somewhere in the the front left of the car. I cant tell if it's from under the car, the master, or the caliper. There is no air in the fluid, and I assume if there was a way for air to get into the system brake fluid would leak out as well. I have checked and I'm not losing any fluid. So before I order a master rebuild kit or take off all the calipers for painting/rebuilding, is there anything else I should try?

Really just trying to get some brake functionality as the car will need to be moved in the near future and it's currently at the top of a steep driveway. Any advice is appreciated as always.

 

there will be no visual indication of failed master internals. from what we've seen, any "original" brake master that goes dry for any duration of time fails. the fix 100% of the time is to do a rebuild. if a brake master is stored wet, it seems to have an almost indefinite shelf life when it is the "original" one. same goes for overly dirty fluid. the process of doing a flush on master that has dirty fluid will also kill it. its crazy because it can be dirty and "work" for years but as soon as some fresh fluid cycles through it, it dies. 99% chance your problem will be solved with a rebuild kit.

 

That's what I figured but thought I would check just incase there were any other quirks that someone might know about. That's interesting that introducing new fluid can cause a failure. More than likely fully died when I bench bled it the first time with new fluid.

 

yea, its happened to us enough times for it to be something we don't do anymore. if the fluid is too dirty then we don't flush it because we know what will happen so we always pair it with either a rebuild or replacement. this isn't unique to the FD either, its happened on a few different cars. the theory is something similar to the same condition in an auto trans. works, fluid flush, doesn't work lol. maybe something about all the dirt and stuff in the fluid is what's keeping it alive or something and then you remove the dirt and it just..... dies. no idea

 

The prevalent theory is that old/dirty fluid leaves deposits that seal leak paths and abrade seals. When you flush with new clean fluid, that stuff gets washed off leaving leaks and seals that no longer seal. That's especially true with tranny fluid which has a very high detergent content. Apparently that's also true when flushing the brake system with clean brake fluid.

 

Like when they pull artifacts out of the ocean and have to keep them in sea water to stop degradation, it's one of those things that sounds stupid at first but makes sense if you think about it lol. Honestly kinda glad that its the master that's bad, saves me from chasing issues in the lines, or rebuilding the calipers this early on in the build.

 

Don't forget that there is a big vacuum line connected to the booster that is supposed to connect to the UIM. If the line is not attached to anything, I reckon it is possible to hear something like what you described.

 

Update: After rebuilding the master and giving everything a thorough bleed, the pedal is firm! The car is now not only a roller but a stopper as well

I had forgotten to mention that I plugged the booster vacuum line very early on in my diagnosing process, but thankfully the sucking noise has stopped now that the fresh master parts are installed. My guess is that the chambers weren't completely sealed from each other which was somehow causing the sound though who knows for sure at this point.

Also just glad I didn't have to go through the whole caliper refresh process just yet.