Bleeding the brake master cylinder

[sapote]

Hey sapote !
Thanks for your feedback.
No engine codes, it runs perfectly.
Car sat still for 8 years in a climate controlled garage. Brake fluid being sucked into the booster makes a lot of sense. Expensive parts to replace though
Is there a way to get a definitive diagnostic on this ? I guess brake fluid would get sucked into the engine which would "suck" ?

Maybe the air leak is not high enough to trigger the lean codes, but I would read the LTFT and STFT (I use obdfusion phone app and Veepeak dongle) and see how high they are.
Sat for 8 years and not moving the brake pistons then for sure some of the seals are damaged. Oil seal is not the problem, but brake fluid allows seals to bond to the cylinder. The only exception is the brake system on some of Citroen cars which use mineral oil for brake and not the standard brake fluid, i.e. my 1973 Citroen SM.

Same color but this's not my SM:

[Flow]

As a French person, I love that car. Some came with a Maserati engine that was quite potent.

Concerning the seals : agreed but what is done is done. Now I'll fix my mistake, that's ok.
I'll update this thread with my findings and also report back if I find brake fluid in the booster.

[Flow]

Could someone give me the pressure values read in INPA for a braking system that is in perfect working order ?
That would be super helpful. Thank you !

edit : from what I read 150 bar is very good. If someone can confirm this to me that would be wonderful.

[bigjae46]

Definitely need to bench bleed a new master cylinder or you'll be bleeding the system forever.

[eacmen]

Could someone give me the pressure values read in INPA for a braking system that is in perfect working order ?
That would be super helpful. Thank you !

edit : from what I read 150 bar is very good. If someone can confirm this to me that would be wonderful.

around 2000 psi is what i see +\- 200

[Flow]

Thanks for the answers !
I'll be bench bleeding the new master cylinder for sure as I believe the pedal feel I have had for a long time now comes from un-bleadable air in the master cylinder.
I get the correct amount of brake pressure but pedal feel is not there. I can bet on the fact that when my master cylinder has been switched out it was not bled correctly by the shop.

Once the master cylinder is bled, I install it in the car with the bench bleeder still in place, correct ?
What is the procedure to then connect the brake lines ? Simply « move fast » ?


Envoyé de mon iPad en utilisant Tapatalk

[eacmen]

Thanks for the answers !
I'll be bench bleeding the new master cylinder for sure as I believe the pedal feel I have had for a long time now comes from un-bleadable air in the master cylinder.
I get the correct amount of brake pressure but pedal feel is not there. I can bet on the fact that when my master cylinder has been switched out it was not bled correctly by the shop.

Once the master cylinder is bled, I install it in the car with the bench bleeder still in place, correct ?
What is the procedure to then connect the brake lines ? Simply « move fast » ?


Envoyé de mon iPad en utilisant Tapatalk

Everyone has a different approach.

Its sort of a tight working area… so what I did was make plugs for the MC after bench bleeding. Then took the plugs out and quickly connected the brake lines. As long as you keep fluid in the reservoir you shouldnt introduce air into the MC. So its going to be messy.

A little tip for those plastic fitting bench bleeders is a very thin layer of teflon tape to help em seal while bleeding. The dorman ones I used would leak without a thin layer of teflon. But never use teflon on actual brake fittings.

[elbert]

Could someone give me the pressure values read in INPA for a braking system that is in perfect working order ?
That would be super helpful. Thank you !

edit : from what I read 150 bar is very good. If someone can confirm this to me that would be wonderful.

150 bar is good.
In my experience, if you had air in the master cylinder, it would be max out around 80 or so. I found out when I changed pressure sensors. It took about 3 liters of brake fluid, but with a pressure bleeder and INPA, I was able to get the air out.

[elbert]

The dorman ones I used would leak without a thin layer of teflon. But never use teflon on actual brake fittings.

Oh, that's a great idea. I ended up making a huge mess trying to get them to work, so I gave up and just ran a bunch of fluid through, but is sounds like Flow might have some really stubborn bubbles.

[bigjae46]

I'd think you'd have to do it out of the car since you have to keep pushing the piston in to force fluid through. I used cheap auto zone fluid. IIRC, I installed the master with the fittings and lines routed back into the reservoir. Then put it in the car and quickly threaded the lines in. Just make sure that there is some fluid in the reservoir. It was a messy job when I did it.

[Flow]

Received some goodies today.
Brand new master cylinder and vacuum booster.
And some more stuff to keep the car clean.



I'll update this thread when I install it and test it.

[Zack]

Auto technician here, I would put dollars to donuts that the booster has failed internally. Maybe I missed something but are you losing brake fluid and don't see where its going? if not then the master has not failed to leak into the bake booster. Though replacing the master will certainly be nothing but beneficial as the car is 20+ years old at this point!

[Kdubski]

Dont bother changing the master or booster. I had the same issue in 2020 after an engine rebuild. Drove myself nuts bleeding liter after liter. Replaced master and booster and the sound came back right as I was pulling up to my first lap for the day at a track day. I almost lost it.

[sapote]

Dont bother changing the master or booster. I had the same issue in 2020 after an engine rebuild. Drove myself nuts bleeding liter after liter. Replaced master and booster and the sound came back right as I was pulling up to my first lap for the day at a track day. I almost lost it.

So no solution?

[Flow]

Finished replacing the brake master cylinder last night, and my god what a painful job to do seriously ! I am certain I did not do it in the right order, but getting the bleeder off and brake line connected with the master cylinder in the car is a major PITA, with brake fluid leaking all over the place for the hour+ I needed to get the threads of the lines to grab and then tighten everything. I seriously considered burning the car down and calling the insurance.
To replace the brake booster I needed to get the DSC out of the car, which also was quite annoying.
I tried to clean all the fluid that leaked as well as I could, hosing the bin down with brake cleaner, and then water, but still the paint on the bottom below the DSC bubbled and flaked off. The metal underneath seems to be treated and not bare, but this has me worried for corrosion. Should I do anything about this ? Or am I good to go in your opinion ?
I'll snap a picture later as I did not put everything back together yet.

Brake pedal feel is 100x better. I did not drive the car yet, but already I can tell it is much better, and the bubbling sound completely disappeared.
Looking at the parts after the fact, I believe it was simply the master cylinder that needed a good bleed, as there was no fluid in the brake booster.
I am still happy I replaced it all.