Can I "bench bleed" the Master Cylinder in place? - PeachParts Mercedes-Benz Forum

Chad300tdt · #1 04-14-2008, 08:23 AM

I asked this question in the tech forum, but wanted to ask it here since a number of knowledgeable people only view this forum.

I just replaced all the rotors and calipers and I'm getting ready to replace the master cylinder. I plan to use a Motive Power Bleeder once the master cylinder is in.

Do I need to do a bench bleed prior to installing the MC if I am pressure bleeding the system anyway?

#2 04-14-2008, 08:45 AM

Yes, always bench bleed a new MC no matter what you plan to do.

Chad300tdt · #3 04-14-2008, 08:48 AM

Quote:

Originally Posted by ForcedInduction

Yes, always bench bleed a new MC no matter what you plan to do.

What does bench bleeding do, that won't happen bleeding it in place? I'm no brake expert, so I would like to know.

#4 04-14-2008, 08:54 AM

It primes it by removing any air that may be in the piston.

For those that might not know what bench bleeding is: http://www.youtube.com/watch?v=oGVnS4BetlA

Chad300tdt · #5 04-14-2008, 09:01 AM

Won't a pressure bleeder from the reservoir fill the piston and push any air out through the lines? Are there areas that need to get fluid that require the piston to travel or move out of the way?

#6 04-14-2008, 09:05 AM

Just do it any way you like. Replacing the MC is about a 30-45minute job, in the time you've spent with this thread you could have bench bled the MC and installed it on the car already.

Chad300tdt · #7 04-14-2008, 09:09 AM

Quote:

Originally Posted by ForcedInduction

Just do it any way you like. In the time you've spent with this thread you could have bench bled the MC and installed it on the car already.

I want to do it the right way, but I'm trying to understand what it's doing. I don't like having answers like, "because that's how it's done". I want to understand why it's done that way.

I don't mind spending time to learn.

ctaylor738 · #8 04-14-2008, 09:24 AM

FWIW, I have replaced at least a half-dozen MC's, and bled them with a Motive unit at 15 PSI. Never the slightest problem. I can't see what bench-bleeding would add to the process.

Might make sense if you were going to foot-bleed.

Chad300tdt · #9 04-14-2008, 09:49 AM

Quote:

Originally Posted by ctaylor738

FWIW, I have replaced at least a half-dozen MC's, and bled them with a Motive unit at 15 PSI. Never the slightest problem. I can't see what bench-bleeding would add to the process.

Might make sense if you were going to foot-bleed.

Thanks for the reply, it's good to know it's been done successfully.

JimmyL · #10 04-14-2008, 10:39 AM

So in the youtube video it appears the person's bleed lines had threaded tips. Do master cylinders come with those bleed fittings or do you have to make some on your own?

Chad300tdt · #11 04-14-2008, 10:50 AM

Quote:

Originally Posted by JimmyL

So in the youtube video it appears the person's bleed lines had threaded tips. Do master cylinders come with those bleed fittings or do you have to make some on your own?

The master cylinder I bought came with the fittings and the lengths of hose.

tangofox007 · #12 04-14-2008, 10:51 AM

Quote:

Originally Posted by JimmyL

Do master cylinders come with those bleed fittings...?

In my experience, some do and some do not.

Knightrider966 · #13 04-14-2008, 10:58 AM

If your master cylinder doesn't come with the fittings to bench bleed, you can get them at Napa. It just a plastic metric fitting with a couple of rubber tubes to put into the resevoir. I did this with my MC in the car and installed. It worked very well, just make sure you keep pumping the brakes intil you are recirculating brake fluid only. The whole point of this procedure is to get the air out of the system, otherwise you will have poor and erratic braking performance if at all!

It helps to have an assistant watch to see when bubbles of air stop flowing through the MC, but if you dontabout 30 repititions of full strokes on the brake pedal once fluid starts flowing should do the trick as far as bleeding the MC.

tangofox007 · #14 04-14-2008, 11:29 AM

Quote:

Originally Posted by Knightrider966

I did this with my MC in the car and installed.

The master cylinder needs to be level when bled. You won't get all the air out of it if it's not horizontal.

Those who insist on taking short cuts when working on brakes might do well to reconsider.

Ken300D · #15 04-14-2008, 11:53 AM

The master cylinder does have to be bled before use in order to purge it of air. However, it is not necessary to "bench" bleed the master cylinder if you can meet the same bleeding requirement with the assembly mounted in the car.

Note how the master cylinder normally sits in the car - it is tilted upward at the forward end (the end away from the vacuum booster). All you have to do to bleed the master cylinder properly in the car is eliminate that tilt. Face the car down hill enough to make the master cylinder sit level. Now you can bleed it properly with something like the Motive Power Bleeder. Using a Power Bleeder, you can go ahead and hook up the brake lines to the master cylinder before you start bleeding the system - just bleed air out the calipers as in a normal brake bleed. With the master cylinder level, it is exactly the same as a bench bleed.

This is what I have done in the past with perfect results. Its a lot less messy to do it this way, and I believe less likely to induce air bubbles as you try to transfer the master cylinder from the bench to the car.

Ken300D