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How do you properly bench bleed Master Cylinder?


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hi all,

I'm trying to replace the brake master cylinder on my 1999 Cadillac Seville STS. When I first took it off the bulkhead, I was able to move the master cylinder around full of brake fluid and nothing would leak. Since then, I have had to remove the master cylinder a few times. The last time I took it off, the thing was leaking through the female brake line female ports. I went to the local auto parts store and picked up a reman master cylinder.

The reman suggest I do a bench bleed. When I first fill the master cylinder with brake fluid and put the mc in my bench vice, the master cylinder doesn't leak. I follow the instructions exactly from the book and from the reman instructions, I give a few long presses on the piston to let most air out, i then do repeated small strokes. When I get all the air out... I take out the drain hoses fromt he resevoir and the brake fluid is leaking out of the hoses. If I point the hoses down, the resevoir will completely empty. This is the 2nd master cylinder i bought from the auto parts store that has done this after I was done bench bleeding.

Am I bench bleeding wrong? Can someone please HELP me? THANKS!

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Maybe this will help?

Do you have a Bleeder Adapter?

Here's some of the instruction from my 1994 GM Service Manual:

Remove the booster vacuum reserve by applying the brakes several times with the ignition 'OFF'.

1. Fill the master cylinder reservoir with brake fluid and keep reservoir at least one-half full of fluid during the bleeding operation.

2. If the master cylinder is known or suspected to have air in the bore, then it should be bled prior to bleeding at the calipers. Bleed as follows:

A. Disconnect the forward brake pipe connection at the master cylinder.

B. Allow brake fluid to fill the master cylinder until it begins to flow from the forward pipe connector port.

C. Connect the forward brake pipe to the master cylinder and tighten.

D. Depress the brake pedal slowly one time and hold. Loosen the forward brake pipe connection at the master cylinder to purge air from the cylinder. Tighten the connection and then release the brake pedal slowly. Wait 15 seconds. Repeat the sequence, including the 15 second wait, until all air is removed from the bore.

E. After all air has been removed at the forward connection, bleed the master cylinder a the rear (cowl end) connection in the same manner as the front in Step 'D' above.

F. If it is know that the calipers and wheel cylinders do not contain any air, then it may not be necessary to bleed them. Check for firm pedal feel and proper pedal travel.

Tighten brake pipe to to 24 lb. ft.

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hi all,

I'm trying to replace the brake master cylinder on my 1999 Cadillac Seville STS. When I first took it off the bulkhead, I was able to move the master cylinder around full of brake fluid and nothing would leak. Since then, I have had to remove the master cylinder a few times. The last time I took it off, the thing was leaking through the female brake line female ports. I went to the local auto parts store and picked up a reman master cylinder.

The reman suggest I do a bench bleed. When I first fill the master cylinder with brake fluid and put the mc in my bench vice, the master cylinder doesn't leak. I follow the instructions exactly from the book and from the reman instructions, I give a few long presses on the piston to let most air out, i then do repeated small strokes. When I get all the air out... I take out the drain hoses fromt he resevoir and the brake fluid is leaking out of the hoses. If I point the hoses down, the resevoir will completely empty. This is the 2nd master cylinder i bought from the auto parts store that has done this after I was done bench bleeding.

Am I bench bleeding wrong? Can someone please HELP me? THANKS!

I don't think you are doing anything wrong. The reman cylinder doesn't leak at first because it is full of air and it takes time for the brake fluid to displace the air. It is normal for the unit to siphon the fluid out if the lines are disconnected. I'd leave the bleeder lines connected while you mount the master cylinder to the brake booster unit and then remove them one at a time to connect the brake pipes. You may need to then bleed all four wheels.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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I don't waste my time bench bleeding master cyls. anymore. Just install the master cyl., connect your lines and fill it up to the max line. Now loosen each line at the master about 1/4 turn until it starts dripping. Tighten lines. Now bleed the master just as if you were to bleed the brakes at the wheels, except loosen and tighten lines at the master instead of the bleeders. You usually do not have to bleed the rest of the system after this, but with all the messin' around you've done it wouldn't be a bad idea.

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