Technical Master cylinder bench bleeding??

I understand the logic behind bench bleeding a new master cylinder yet I often wonder if it’s really necessary? If you jack the car so the master is horizontal, there is no better vice than the final resting place. I also recall the time my screwdriver slipped and I scratched the bore of a new master, only to have to replace it a week later. The new brake lines are nothing but air anyways. And then there is the dripping of fluid on the fresh paint as you connect the lines to it, not to mention the air bubble. I’m pretty sure that I’m missing something here so please enlighten me.


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The master needs the air bled from itself first.
If you try and bleed it out at the wheels you better have plenty of brake fluid and time.

 

Nothing like crawling front to back several times to bleed them.

 

Do the single reservoir Wilwood MCs have to be bench bled.

 

Sure helps.

 

When I converted from an under the floor to a dual master on the fire wall, I decided to bleed the master cylinder mounted, with the brake pedal. Used a bleeder kit of course. Worked great!

 

Well I’m bench bleeding it now and forgot how persistent air likes to hide in these. I use a Phoenix injection system to bleed my brakes in reverse, pushing air up the way it wants to go so of course I question the old methods.


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I know that the dvd that came with my Phoenix injector probably addressed MC bleeding but it is MIA and YouTube let me down. Oh well, I’ll do it hybrid, bench MC, reverse injection the lines.


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Well if you have a Phoenix injector, you can bleed it on the car from the wheel.
If you start with a dry system, working at the wheels with the injector when the master is full you should be good.

 

I asked an old timer at work when I was fixing cars if bleeding a master was necessary, he said lets try it without bleeding. It took forever it seemed to bleed the brakes. Case closed. It takes only seconds to bleed a master on the bench. My method always works, clamp the master in a bench vise. Fill it up, cover off. Hold 2 fingers over the ports, work the pedal rod (or screwdriver) until the bubbles stop coming to the top. Put the cover on & install it.

 

If you have a firewall mount master that is above all the wheel cylinders you can use the gravity method. On my 56 I just filled the master, opened the bleeder nozzles and let it drain down for a couple of days. I was in no hurry and it worked great. Just have to remember to keep an eye on it and keep refilling the master cylinder.

 

The master cylinder, whether mounted to the car or in the vice needs to be pointing slightly downwards (for bleeding purposes - not necessarily in use), so not just simply horizontal, to ensure there's no air trapped in the far end of the cylinder - this air will come out as it is displaced by fluid (no pumping required) but if air is so trapped (eg by cylinder pointing up) no amount of pumping will remove it all.

You could dispense with the 'bench' element but you'd need to be sure the master was free of air, eg by running probably quiet a bit of fluid through, but i doubt you'd ever be certain about it? Did those bubbles come from the bleeders, or the master?

Chris

 

When bleeding a new master in the car you only have the travel of the pedal, when you bench bleed, you have full travel to bottom the piston on each stroke. No Trapped Air !

 

It’s just takes to damn long when mounted in the car!

I sometimes install masters dry in Forklift’s
Reason is they are usually mounted under the floor in some God awful location and have a remote reservoir up on the dash.
Also the longest line might be 3 feet, no valves , residual or otherwise non power etc, so they usually bleed fairly easy.


In a car there are many , many variables to bleeding a brake system so for me bench bleeding then installing makes the job faster and eliminates some of those variables
A pressure bleeder is the cats ass, said it before and I’ll say it again, if you don’t want to bench bleed for whatever reason then pressure bleed the system

 

If you can't full stroke the master when it is mounted; something is wrong with the pedal set-up.

 

The last master cylinder I did, was a dual chamber on an O/T vehicle. For the first time, I mounted the cylinder in the truck and bled it using a home made vacuum bleeder. Then I attached the lines and finished the job. It worked great, and was the quickest system I have ever bled.
The bleeder is too ugly to post a picture, but just because it was the easiest thing at the moment, for the pump I used the squirter from a spray bottle. It worked so well for me, that I am going to get an old compressor from a fridge and do all my brakes this way from now on.
Bob