No brake pressure on front circuit after master cylinder replacement

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Originally Posted by 80stech

Maybe the ABS is leaking fluid by ?

I don't understand how the ABS/DSC could leak fluid internally without the fluid level in the reservoir dropping. Where would the fluid go?

Originally Posted by 80stech

Do you have a plug you could use to block off one circuit and see if the pedal still drops ?

Not off hand. I guess I could make one up with a bolt with the correct thread pitch and teflon tape, but I don't have anything to seat on the flare. This is a good idea, as it would confirm or refute my suspicion. I'll see what I can cook up.

[80stech]

If the ABS is plumbed like it was back in the day then it shouldn't happen but who knows now. It seems pretty strange to have 2 replacement master cylinders leaking by. The best way to make a plug would be with a piece of brake line. That would for sure tell the story.

If there is a proportioning valve that could be bottomed out (maybe the clunk you heard) just something more that might be going on.

Maybe not "leaking by" but retracting ?? Have you tried driving it ?

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Originally Posted by 80stech

If the ABS is plumbed like it was back in the day then it shouldn't happen but who knows now. It seems pretty strange to have 2 replacement master cylinders leaking by. The best way to make a plug would be with a piece of brake line. That would for sure tell the story.

I found an M12x1.0 double flare plug online. I'll see if I can get one locally.

Originally Posted by 80stech

Maybe not "leaking by" but retracting ?? Have you tried driving it ?

Don't understand what you mean by retracting. If everything is working properly you should get pressure when you press the pedal, and the pads should retract when you release the pedal. Haven't driven it because it's not working as expected and it's time consuming to put everything back together, put the wheels back on, and take it off the jack stands only to have to get it back up on jack stands again.

[80stech]

We've had some people who have had problems with a sinking pedal and ended up needing to drive the car to get the ABS working but I think sometimes that didn't even totally seem to fix it.

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Replaced MC with OE (TRW). Brakes feel and function fine now.

Was difficult to bleed the system. I bench bled the MC before installing it, then:

1. pressure bled
2. pressure bled + cycled DSC
3. manual 2-person bled
4. manual 2-person bled + cycled DSC
5. manual 2-person bled with pressure bottle hooked up
6. repeated above steps

Front circuit was really hard to bleed. Brakes were feeling awful until step 5. For some reason that blasted out a huge amount of air that no step before it did. This leads me to suspect that even though I bench bled the MC the act of installing it reintroduced air into the MC. Scotty Kilmer on YT advocates "burping" the MC by applying brake pressure and briefly cracking open the line at the MC which I guess provides any air the shortest route to escape. I suspect any air at the highest point in the system may tend to remain there despite bleeding efforts.

So first new MC had clearly failed, 2nd new MC was probably OK but the system wasn't bled properly, and 3rd new OE MC worked when system was thoroughly bled. I disassembled the 2nd new MC and couldn't find anything wrong with the front circuit seal. Oh well.

What a nightmare.

[80stech]

Bench bleeding is meant to make the job a bit quicker so you don't have to wait for the master to fill through the compensating port. Cracking the line at the master is normal procedure and sometimes you can get away with not even bleeding at the wheels but more so on older cars. I think what happens here is that getting a bit of air into the ABS is causing the pedal to drop in a way that doesn't really feel like it's air but something is bleeding back/around (maybe piston retracting whether it be unwinding or maybe through an orifice) through the ABS and maybe more so if there is a bit of air and more noticed when a person is focused on working on the brakes!

There are some places like the front brake lines just before they got into the hoses where there are going to be high spots and some pretty good flow (high pressure on the pedal) is needed to get the air through which is why I always recommend using 2 people especially for problem cars.