What is "Bench Bleeding" a Master Cylinder

[Tidybuoy]

I'm going to be installing a new master cylinder this weekend. I've read on a few posts something about bench bleeding the master cylinder.

Do I need to do this and how do I do that. I've bled brakes many times but never the master cylinder on a bench.

Any advice would be appreciated.

Vern

[Joe Bob]

Pre filling the MC with fluid prior to install.

[mnez]

I didn't with my 911, nor have I ever done it with any other car. Seems like a messy waste of time to me. But others swear by it...

[McLovin]

A messy waste of time.

[drtyler]

+1

Originally Posted by mclovin

a messy waste of time.

[theiceman]

in my eyes ANYTHING that increases the risk of splashing brake fluid on my paint is a bad thing. I would never do it ..

[McLovin]

Whoa, what are the odds that two people both posted the exact words "messy waste of time" at exactly the same time?!?

[yelcab1]

You can do it in the car as well. Just connect the hose to the outie end of the cylinder, loop it back to the top of the reservoir, pump a few times to get the air out, and you are good.

[porwolf]

Originally Posted by Joe Bob

Pre filling the MC with fluid prior to install.

I recently replaced the master cylinder on my '79SC. "Bench bleeding" seemed to make sense but it was not practical because when locating the master cylinder plus brake booster assembly back to it's place it had to be turned and twisted and turned in just one way to locate it right. In the mean time the "bench bled" brake fluid was threatening to run out and make a mess. Instead I installed the brake master cylinder plus brake booster empty of brake fluid first. After installation I attached two brake bleeder valves with transparent Tygon hoses to the brake line outputs. The Tygon hoses fed back into the brake fluid reservoir. Then I filled the brake fluid reservoir and pumped the brake pedal until brake fluid appeared in the Tygon hoses without bubbles. Then, disconnecting the bleeder valves and reconnecting the brake lines was easy. After that procedure the brakes felt fine and hard with no sign of air in the system.

Brake bleeder valves can be had for a few dollars at any Auto parts store. I think the thread is M10X1.

[emac]

I've done a lot of jobs on my car during the last thirteen years, I feel that replacing the M/C was one of the biggest pain in my ass. Literally, at 64 I just can't bend the way I used to. Lol
Ernie 81sc

[porwolf]

Originally Posted by emac

I've done a lot of jobs on my car during the last thirteen years, I feel that replacing the M/C was one of the biggest pain in my ass. Literally, at 64 I just can't bend the way I used to. Lol
Ernie 81sc

It is somewhat of a PITA. But can be done even at 74.

[GH85Carrera]

My 85 Carrera and 86 El Camino both have the OEM master cylinder that they came with. I flush my brake fluid every spring on the 911 and every other year on the Elky.

I have no idea how long they will last but I plan to find out.

The 911 has 150,000 miles the Elky has 322,000 miles.

[Tidybuoy]

My brakes work great; especially with my new PMB A-Calipers and new PMB M Rears.

I probably don't need a new master cylinder but I get a mushy feel sometimes and I and pump up pressure. I've bled my brakes at least 10 times since I installed the new calipers a couple of months ago. I just decided to get a new master cylinder since everything else is new and I'm hoping to eliminate the mush (it's not horribly bad but I feel it could be better).

[emac]

Tidy- the mush is probably the M/C bleeding or starting to bleed through itself. I would change it out.

[mnez]

Here's my preferred technique, used when changing to 930 brakes on my 911 (or when draining all the fluid for any reason).

Install everything dry, attach clear tubing to all the bleeder valves with a catch bottle for each wheel. Fill the MC reservoir and open all the bleeders. Get your beverage of choice and wait for gravity to do the bleeding for you. Top up the reservoir as needed. It's very slow so there is lots of time to putter around with other stuff.

Once there is a good flow with no bubbles in the clear tubing, close up the bleeders and do the final bleed by your preferred method. I use the Motive pressure bleeder because my wife long ago got tired of helping.
Works every time.

Good luck.

[mnez]

Originally Posted by McLovin

Whoa, what are the odds that two people both posted the exact words "messy waste of time" at exactly the same time?!?

Great minds...

[LM3929]

I have a tool that I use on every master cylinder I rebuild or replace, it makes the pre bleed a 5 minute non messy (if that is at all possible)

The tool is a plastic syringe with a rubber sealer on the end and all I do is fill the reservoir's and suck the fluid and air thru the brake line port or fill the syringe with fluid and push the air out thru each brake line port.

Lorne M.

[Eagledriver]

The problem, as I see it, is that if you are putting fluid in from the top bubbles can be trapped in the MC. What you would like to do is put fluid in from the brake lines. Here is how I did it.

Install the MC dry but leave fluid in the brake calipers and lines. When you hook up the lines you can squeeze the pads in like when you replace the pads. This pushes fluid back up through the MC and into the reservoir. Then top off the reservoir and bleed as normal.

-Andy

[71scgc]

Easy job.
Go to your preferred auto parts store (NAPA, Advance, O'Reilly, etc.). Buy a bench bleed kit.
Clamp MC in a vise.
Follow instructions on the kit.
It doesn't have to be messy, and in the vise not near your paint. Air is very hard to remove from a MC through normal use.
Once done, you'll wonder what the big issue was about.
Carter

[porschenut]

It's not worth the effort. I've replaced two master cylinders on 911s and both times I got a nice firm pedal just by bleeding at the calipers.