JunJun

I know the master cylinder has two pistons, forward one for the rear and back one for the front.
Does this mean I can bleed front and rear separetely?
In fact, I mistakenly emptied the reservoir tank for the front (back part of the reservoir), and I have to bleed the master cylinder...
Any information is welcome

JunJun

I meant "valves" by pistons.

Apeiron

Once air gets into the master cylinder it's difficult to get out. Do a search for "bench bleeding".

JunJun

Thank you, Apeiron.

Yes, I know about bench bleeding.
Even when bench bleeding, do I always have to do front and rear at the same time?
Aren't they independent even though they share the reservoir tank?

JunJun

I've just found a really helpful thread.
https://www.thirdgen.org/forums/v6/2...they-went.html

According to this, even though MC has two pistons, one for front and one for rear, they eventually mix at a proportion valve, which connects front right, front left, and rear.
So it seems I have to bleed all the four, including MC, even when only front gets the air.

87350IROC

iTrader: (2)

The circuits absolutely do not mix. That would be a huge flaw in the safety of the system.

But I'm not sure how you are going to bleed one circuit with the other plugged. The master will just lock.

sailtexas186548

iTrader: (2)

You have to bleed both front and rear at the same time. Also the cylinder needs to be horizontal

JunJun

Thanks for the reply.
Hmm, so they are independent, aren't they?

I don't think the master will lock.
Imagine bleeding just one of the front calipers.
The rear side is plugged but you can still push the brake pedal to the floor.
Haynes manual also suggests bleeding the master one by one with one hole plugged.

JunJun

The front and rear brake circuits seem independent.
https://www.thirdgen.org/forums/brak...-question.html

If this is true, I don't necessarily have to bleed the front and the rear at the same time.

Anyway, my master cylinder gets air in its chamber for the front, so I will bleed just the front (including master) and see if the pedal feeling will improve.

87350IROC

iTrader: (2)

But you have to remove the master from the car to bleed it. If you are going to do that its just as easy to bleed both the front and back. With one end blocked, and such little fluid in the master I think it would be a heck of a lot hard to bleed than just bleeding both at once. Remember with the rest of the system installed, there is a lot more compliance than just blocking the master.

JunJun

Haynes says "when bleeding the master, press the pedal a few times and then press slowly but heavily, and at that time loosen the hard lines connected to the master one by one (use some cloth to catch the fluid). When the pedal is pushed to the bottom, re-connect the hard lines. Then slowly release the pedal and wait for 15 sec. By doing this several times, the air in the master can go out."

So I will try this just for the front brake (the hard line nearer to the driver's seat) and bleed the front calipers, and check the pedal feeling.

87350IROC

iTrader: (2)

The Haynes is mistaken. The master is installed at an up angle. You have an air trap spot at the high point on the master. The master needs to be level when bled to avoid this scenario.

I also don't like the idea of making a gigantic mess in the engine bay doing it that way.

JunJun

Actually I have a doubt about the theory that the master traps air at its highest point.
You know, ANY car's master is angled like that, including Japanese newest cars, and there must be reasons to do that.
In my theory, it's angled like that to gather air at the top where the hard line connects.

In fact, I've researched a lot about a master cylinder of Japanese cars, but nobody says air gets trapped there.
Rather, they say once air is in the master, it just takes quite a lot of time to get it out because the structure of the master is complex (compared with just the tubes) and besides that there's quite a length from the master to the caliper, where air goes out.

So, I think the biggest reason to bench bleed is to get air out efficiently. If not, what is the angle of Camaro's master for?

87350IROC

iTrader: (2)

Pardon me being up front. But why post a thread asking for help if you are going to ignore it? If you are happy with your theories, then do it that way, and please get back to use with how it went.

In my opinion the master is angled for two reasons. First to get a good push-rod angle and second to clear the strut tower. In my opinion it has nothing to do with air in the system. I suspect it is angled in many cars for the same reasons. Packaging in that area is generally tight.

Remember fluid in the brake system does not flow in operation. So once air sits in a high spot it will never leave until the master is bled.

By the way, I am not totally convinced air actually gets trapped the master either. But I personally would not risk it.

JunJun

Oh I didn't mean to be offensive.
What I wanted to know was if the front and rear circuits were separated, not about the purpose of bench bleeding.
But the more research I did, the more often I heard about bench bleeding, and that also makes me think about the real purpose of bench bleeding.
I know it's controversial, but I just wanted to raise a question which could be helpful for everyone.

87350IROC

iTrader: (2)

No problem.

Let us know how it turns out.

Apeiron

The whole reason it's called bench bleeding is because it's done off the car, mounted level on a bench. All the air is out in about half a minute that way.

If you're bleeding the M/C on the car then you're not bench bleeding. It will take longer to get the air out then.