Hey all,

looking for some help with a problem I’ve been dealing with on my r53. When I bought the car the brakes were awful, scraping sounds and very soft. I assumed it just needed new pads and rotors.

I replaced the pads, rotors, one caliper, and used my power bleeder to bleed the brakes. Fluid all came clear, bubbles stopped but the brakes were still spongy. Pedal was going to the floor every time.

I figured the master cylinder could be the issue because it felt like it wasn’t holding pressure, so I swapped it out. Foolishly I didn’t bench bleed the new one.

I then figured out the brake vent procedure in ISTA+. At this point I also realized that by not bench bleeding the master I had pushed air into the system.

I followed the procedure to a tee, pressure bled all four corners, and then bled all four corners with the ABS module cycling. Pedal still really soft. So I did it again, and then again. Still very soft.

At this point I figured out that I don’t actually have to bleed at each wheel, since the lines to the rear are known to be bubble free I started just running the ABS procedure while bleeding at the front passenger wheel only. Every single time I hit continue and cycled the ABS module air came out. I ran the procedure in ISTA+ at least 7 times while pressure bleeding, the amount of air coming out reduced every time, and the last two times no air came out.


The pedal is much much better, the car is way less sketchy to drive now, but it’s still soft. It’s driving me crazy, because I know it’s supposed to be like pushing against a wall.

The pads are bedded, the car stops well, the pedal doesn’t sink to the floor any more but it seems insane that I’ve purged the ABS system 7 times and there could still be air in there. Does it seem possible that could be the case? There are no leaks at all as far as I can see. Is there anything I’m missing?

Any help is appreciated, it’s getting to be insane how many times I’ve purged this thing.


thanks in advance!

 

Since you seem to have covered everything else, how about the brake booster?

 

good suggestion! The booster looked fine visually, but wasn’t the issue. I didn’t have to check it because I ended up getting it all to work.


I ran the brake bleed procedure 12 more times today for a grand total of 19 times. The air stopped coming out, and the pedal feels a million times better now.

it’s not rock hard, but it seems like this is how it is supposed to feel. It’s stiff and I can’t ever get it to bottom out


so yeah if anyone else has this issue just run the “vent brakes” routine 20 times lmfao

 

When bleeding, your car is running?

 

no just key in second position, should it be running?

 

I always have it running. I replaced a rotted rear line ( left rear, and the master in 2021. The blast of air that came out of the front bleeder was unreal.

I setup the pressure bleeder ( mine is from Motive, but I don’t think it matters, start car, then start bleeding.

 

oh I’ll definitely give this a try, maybe the pressure from the booster helps push things along.

the issues is that the fluid is just moving too slow so maybe this is the answer I need.

 

Update on this, the master cylinder had a leak at the input shaft seal. Which was allowing air into the system continually. The reason it appeared to finally purge at some point was because it was leaking into the booster and eventually the fluid level in the booster got above the leak point.

Mystery solved i need a new booster and master cylinder.

 

So, the new MC was bad? what brand is it?

 

I guess I owe a bit more background on this.

The master cylinder was attended to, but not totally replaced. I said it was replaced so avoid having to explain the roundabout nonsense I went through to try and replace it. I bought what I thought was the right part from Autozone, because I was trying to meet the deadline to have the car inspected. However when I got it at home I realized it was a non-DSC cylinder without the port for the pressure sensor. AZ doesn't stock the DSC cylinder at all, so I opted to take all of the parts out of the new cylinder and transplant them into the body of the original cylinder. It all looked good, all the seals felt like they fit and the whole thing appeared as if it was going to work. The bore of the old cylinder looked perfect, I figured some new seals would be all it needed for another 100k miles.

However it seems I was wrong and it was leaking out of the seal around the main plunger, I guess there are slight differences in the tolerance of the parts on the OEM MC and the Autozone MC. This isn't their fault at all, I'm assuming that in its original configuration it would have worked fine.

 

Damn that was confusing. Glad you worked it out