How to bleed the Brake Master Cylinder?

[Rasteirinho]

Hi,

I've performed an overhaul to the BMC of my AE86, however now i can't bleed it...

I've assembled all the brake pipe system from the BMC to the wheels, i started the bleeding process but no fluid was getting out.

After several attempts i removed the pipes from the BMC and no fluid was also there. Only inside the reservoir i had fluid.

I removed and dissasembled again the BMC to see if everything was assembled correctly. My only doubt is if the springs are in the correct position. I have inserted the longer one (and stronger) first and then the shorter one.

I put the BMC back to its place, filled up the reservoir again and pressed again the brake pedal several times to see if some fluid was coming from the top of the BMC (where the brake pipes are tight) but no fluid showed up.

The acting pin is working and is pressing the cylinder, however no fluid is coming out. Any ideas what could be the problem?

Thanks...

[Ivan141]

First, make sure you get the springs in correctly.. I remember forgetting the order they where supposed to be in myself when I rebuilt mine, but I guess I got it right, so I think you will too.

Next, you can only bleed the master cilinder properly by bench-bleeding it.
The procedure is as follows:
-Mount the cilinder in a vice
-Attach some old pieces of brake line, and route them back to the master cilinder. Make sure they end up somewhere under the fluid level.
-fill up the reservoir
-very gently start pushing the cilinder in with a screwdriver or something similar..just be carefull you dont scratch the bore of the cilinder.
-Keep doing this untill you see NO bubbles, it takes a lot of pumping to get it this far, so dont give up to fast. Remember to stay gentle with it, or you will risk damaging your new seals, or sucking up the bubbles back into the cilinder before they clear out of the reservoir.

[Rasteirinho]

Hi Ivan,

Thanks for the advice.

Can i use only one pipe attached to one hole and block the other hole? Or i have to use two pipes to attach to the two holes?

Regards

[parrot]

There is a bolt on the side of the MC that locates the piston correctly inside the MC (piston stopper bolt). It is not a bleed screw as often thought.

Therefore, even if you rebuilt it properly but then tried to 'bleed' it, you can stuff up the positioning of the pistons.

To reset it, you need to loosen the bolt, push the pistons all the way in then tighten the bolt.

I am sure the strong spring goes in first, with the piston barrel with the long nose.

[Ivan141]

Sorry, all the documentation specifies to attach both lines at the same time. Normally you get plastic nipples that you can attach tubing too with rebuilt master cilinders, but since you're using your old master cilinder you'll have to do without. You could probably get two bits of lines made for you for fairly little money, or just sacrifice one old line (which already has 2 correct fittings), and replace it.

[parrot]

I can't see why this process couldn't be done on the car (gently) with some dummy lines back to the reservoir, or am I missing something?

[Ivan141]

it's just easier when you actually see what you are doing.. kinda hard from inside the car..

[Rasteirinho]

Quote:it's just easier when you actually see what you are doing.. kinda hard from inside the car..

That's true... i had already tested... lol not easy to see the BMC from the driver's seat, even without windshield.

[Rasteirinho]

Job done...

But now i have another problem...

The brake system is completely bleed but the front brake pistons aren't touching the pads and the brake pedal has much stroke...

Any ideas what could be the problem? Maybe the pistons are stuck...

[parrot]

Maybe? The front piston in the MC sends fluid to the front brakes, did you check the piston positioning and set the piston stop bolt properly?

Are the rear brakes OK?