Bench bleeding of cylinders


#1

I need to know what to expect when bench bleeding clutch or brake cylinders. Obviously this could take a lot of space, but hopefully I can narrow it down:

some details: The car is an '87 Porsche 944, and I have all new clutch/brake cylinders (FTE brand) to bench bleed. If you know about this car, then it had an original Girling brake master cylinder with Girling booster. I read the NAPA instruction sheet (see below), Haynes, as well as many other things.

I can see how the cylinder should be held in a vise and positioned to let bubbles rise to the top and out the holes, but I am particularly wondering about what happens when the rod starts to be pushed. I have read that the rod should not be pushed in all the way. I also imagine (from testing my old one) that the fluid gets pushed out - with a lot of force - every hole on the cylinder. I see that NAPA makes some fittings and such to help bench bleeding, and it appears that there should be some sort of flow-through collection bin to catch this fluid. I guess when fluid flows without bubbles, the unit is transferred to the vehicle in a condition that prevents air from getting back in - or perhaps it is more forgiving?

link to NAPA instructions:
http://s7d9.scene7.com/is/content/GenuinePartsCompany/1229488pdf?$PDF$
NAPA tool:
http://www.napaonline.com/Catalog/CatalogItemDetail.aspx/Brake-Bleeder-Kit-Master-Cylinder-Only-For-Bench-Bleeding-Master-Cylinders-Before-Installing-On-Vehicle/_/R-NDP7001795_0375747757


#2

I bought a master cylinder bleed kit. It screws into the MC holes where the brake lines go and 2 rubber hoses feed the fluid back up into the reservoir. I just pushed on the cylinder rod and let off and pushed again until no more bubbles were going into the reservoir.


#3

I want to tell this person to have someone fix the brakes and move on to something he can actually do. But that would not be polite.


#4

Bench bleeding of the master cylinder only allows you to pump brake fluid in a circulating manner into the MC chambers, pumping the air out, such that when you install the MC its chambers are filled with fluid rather than air and you don’t pump all that air into the lines. If you didn’t bench bleed the MC before installing it you’d be pushing all the air that the chambers are filled with directly into the brake lines, and you’d then have to push it all the way through the lines and out the bleeders.

But that does not mean you don’t need to bleed the lines after installing the unit. You will need to. Some air will get into the lines. There’s no way totally avoid it unless you submerge the entire vehicle in a pool full of brake fluid to install the MC. And that isn’t possible.


#5

It’s a little sloppy but some people are quick. Don’t get brake fluid on the paint or the paint comes right off.


#6

@JuniorMint

I’m going to be blunt

You bench bleed the brake master and the clutch master with a kit. This is what I use

Then you hook up the diaphragm brake bleeder and the proper adapters, and bleed the lines on the vehicle

powerprobe has the adapters you need

My method is efficient and you don’t need a second person

Done

No more to be said about this

I’m glad we all agree

that was easy

Next


#7

I did this on my Ford truck a couples years ago, brake MC. The instructions how to do it came with the new MC. And it came with some special fittings as I recall, just for doing the bench bleeding. I don’t think it took longer than about 5-10 minutes, not a big job. I do think I recall running the expressed fluid right back into the top using some supplied rubber tubing. Did yours come with bench bleeding fittings, tubing, and instructions? A couple of cautions, one is to use a level to make sure the MC is absolutely level before you start. Second is to pay careful attention to where the instructions tell you to place the vice on the MC. Usually you clamp on a flange of some sort, definitely not the MC’s body. If you clamp the MC in the wrong place, e.g. clamping on the cylinder, you can easily damage it beyond repair. Also to avoid damage to the MC, I used a wooden dowel to press with.


#8

I finally got around to getting this kit from NAPA:

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx/Brake-Bleeder-Kit-Master-Cylinder-Only-For-Bench-Bleeding-Master-Cylinders-Before-Installing-On-Vehicle/_/R-NDP7001795_0375747757

each fitting turns out to have different measurements - i.e. only one of them fits any of the same sized holes on this particular cylinder. Unless there’s a place that sells the single fittings, and I assume all those ports need to be connected - that means three of these kits are needed - yes I looked on tooltopia and powerprobe - thanks @db4690 - but no single fittings.


#9

The clutch M/C is connected to the brake M/C and doesn’t have its own reservoir, which makes it a bit trickier.


#10

Master cylinders normally sit at an angle and if you don’t bench bleed you will not get all the air out.
You can use a kit to pump the fluid back into the resv or

blow the fluid all over the floor. Creates a huge mess.


#11

I forgot to ask : how should the fittings be removed once the cylinder is installed, so no air gets in? just be careful and re-bleed the system after installation?