Bleeding Master Cylinder

[Ed]

What happens when you DON'T bench bleed your master cylinder before you install it?

[240ZX]

The results are similar to having no gas in your fuel tank....it won't work. Air in a brake system has a funny way of compressing and not moving the pistons in the calipers or slave cylinder on drum brakes. It is standard practice to purge (bleed) the M/C of all air before bleeding the calipers or slave cylinders. Regarding the sequence for the wheel cylinders, start with the cylinder furthest from the M/C and work your way though to the closest.

[ZCCOR#109]

Since air bubbles want to go to the "highest" point they will want to stay in the master cylinder, compress, and preclude you having appropriate hydrolic pressure in the system.

The master cylinger should have a bleed screw on the side, so you can bleed it now. Or, loosen one of the lines and bleed out the leak. open and close the "leak" as the peddle is pressed and released.

Once the MC is bleed, start at the right rear, lf rear,rt ft, the finally lf front. This is done in order to bleed starting from the farthest point from the MC first. You are "chasing the air bubbles" out of the system.

[SteveJ]

I must have committed an abomination. I changed out my MC & brake booster recently. I did not bench bleed the MC. Instead, I bled the MC and proceeded to bleed at the wheels. After bleeding at the wheels, I bled the MC again, just to make sure. After all, brake fluid is cheap. I ended up with a normal feel to my brakes.

[Bambikiller240]

What happens when you DON'T bench bleed your master cylinder before you install it?

95%+ of the time bench bleeding is NOT necessary.

I must have committed an abomination. I changed out my MC & brake booster recently. I did not bench bleed the MC. Instead, I bled the MC and proceeded to bleed at the wheels. After bleeding at the wheels, I bled the MC again, just to make sure. After all, brake fluid is cheap. I ended up with a normal feel to my brakes.

Although I am aware of the possibility of a bench bleed being needed, I have almost always been able to simply bleed the the MC after installation, and then proceed to the RWC's and Calipers.

Bench bleeding is not a mandatory thing. It really isn't any different than (doing a full and proper) bleeding with the MC installed, just more of a PITA, IMO

[Bambikiller240]

It is standard practice to purge (bleed) the M/C of all air before bleeding the calipers or slave cylinders. Regarding the sequence for the wheel cylinders, start with the cylinder furthest from the M/C and work your way though to the closest.

In 240Z chassis many people seem to think that the RR is farthest from the MC, but if one takes a good hard look (and a tape measure for verification) they will note that the LR cylinder is actually further from the MC than the RR. They both share the same brake line down the trans tunnel and the "T" block is on the passenger (RR) side of the tunnel, the line to the LR must cross back over the tunnel and on to the LR WC. The line from the "T" to the RR WC does not have to travel the extra distance of the width of the tunnel.

In short, the length of the line from the "T" to RR is shorter than the length of the line from the "T" to the LR.

[Ed]

At first I didn't bench bleed the M/C. I also used a vacuum pump to bleed the brakes (my first time). I was sucking air bubbles for hours. I went through about 30 oz. of brake fluid. I moved around to all the wheels and nothing but bubbles. I was pulling out my hair. I thought it was because I didn't bench bleed the M/C. So I pull it and bench bleed, what a mess. I then started over again. Bleed, bleed, bleed, bleed still more air bubbles. WTF :mad: The brake felt good and solid but why so many air bubbles????

So I decided to go the old fashioned way and had the wife pump the brakes while I manually bled the brakes. After a few minutes I was done! Take that you silly vacuum pump. Later I decided to re-read the instruction manual for the vacuum pump. And I noticed one little sentence that I didn't pay attention to the first time; "It should be noted that a tiny stream of bubbles may be noticed in the hose after all of the air is bled from the lines. This is caused by air seeping around the threads of the loosened bleeder fitting and being drawn back through the fitting by the suction of the pump."

:stupid:

Atleast I'm done.

[GunnerRob]

... I noticed one little sentence that I didn't pay attention to the first time; "It should be noted that a tiny stream of bubbles may be noticed in the hose after all of the air is bled from the lines. This is caused by air seeping around the threads of the loosened bleeder fitting and being drawn back through the fitting by the suction of the pump."

:stupid:

Atleast I'm done.

I think most instructions have a "GOTCHA" sentence in them that only appear after you reach a certain frustration level.

Thanks for sharing that with us Ed. I'd of been right where you were at with that pump! :mad: