Technical Bench Bleed Master Cylinder Technique

Can anyone please explain the advantages/ disadvantages of the two methods to bench bleed a master cylinder.

https://www.performanceonline.com/blog/2009/02/master-cylinder-bench-bleeding/



Method A is to use block off plugs on both outlet ports of the master cylinder and then fill er up and stroke. The NAPA guy only had to stroke four times and done...

Method B (and this is the one that I am most familiar with) is to insert fittings with a barb for a plastic tube and route the recirculating tubes into the fluid in the reservoir and stroke and you can see the air bubbles flow out of the lines. The NAPA guy had to stroke it a number of times to get the bubbles out.

How come the Method A was so much faster and how do the bubbles get out of the system if the block off port is installed?

 

Interesting, I’ve always done it with the tubes for recirculating the fluid.

 

I have done it both ways. With the hose it can take longer, as the bubbles don't pass all the way to the end of the hose.

 

I haven't watched the video so I can't comment on that specifically. The method I have used in the past was to bend up some steel lines, connect the lines to both ports, fill both sides of the master, then bend the lines so the open end is submerged in the brake fluid. I then SLOWLY push the plunger in and out several times until I don't see any bubbles. If I recall, the new master came with some goofy plastic fittings and lines but I wanted a better seal so I made up the steel ones using real brake line and real fittings. That is what worked for me.

 

I've done it both ways - both worked.

 

Not to change the subject but this might be useful to some of you. In was having difficulty bleeding the system on my four wheel disc vehicle ( front calipers had two bleeders each)
I used a power bleeder that pressurized the system, a vacuum bleeder, and the old fashioned pump the pedal method. None of these methods produced a good pedal.
I then ordered a set ( I needed six) of these:
http://speedbleeder.com/
Problem solved. They are reasonably priced and the company ships immediately. I’m sold on them.

 

I've only used Method B, with success. Also a fan of Speed Bleeders, especially when there's no pedal pusher available

 

What was that tool he was using to push in the master cylinder piston?

 

I used to clamp the MC in a vise, fill the reservoir, push the piston all the way in, put my fingers over the ports and let the piston back out. Do that until it squirts fluid at both ports. Use shop towels to contain the fluid.

 

Either way works, take your pick.

 

If I have already mounted the mc in the vehicle, then I would do B method, or "my" method. I simply connect both brake lines to the MC, but leave them cracked open a bit. I also install a centering plug in the proportioning valve (in place of warning switch). Pump the pedal till fluid starts oozing out of the brake lines. Then tighten up fittings. There might be a little softness in pedal pressure, but the softness will diminish after a few brake strokes.

 

Thanks for the feedback...

I did not have the fittings for the tubes however I still had the blue port block off fittings and clamped the MC into the vice and did method A. It took way longer than the NAPA guy did in the video. Unlike the video there were bubbles coming out for a long time with each stroke and then finally I could look down and see clear fluid squirting up from the piston inlet port at the bottom of the reservoir.

OK...now to share some more ignorance with the community...

I know that there is supposed to be clear fluid squirting up from the piston inlet port when you stroke the piston and that indicates that the MC is completely bled...Great...

Even when you are doing the old "pump three times and push it and hold it to the floor" bit there is clear fluid squirting back up into the reservoir (that is why I always put the cap on the MC when doing the pump three times)...but why??? It really does not make any sense...

So when everything is buttoned up and sealed and the system is all bled and you are driving down the street and press the brake pedal...does clear fluid squirt up from the piston inlet port? I don't know because I never looked...but if it does then that is counter intuitive...right?!?!? The whole point of the MC is to force fluid down through the brake lines to the wheel cylinders/ pistons so if clear fluid is squirting back up into the MC reservoir than that fluid is not going to the wheels where it belongs...

One thing I know is that my ignorance is very great...and people really seem to be amazed by it...

 

What you are seeing is fluid coming back up through the vent port until the primary seals on the master cylinder pistons cover them and it starts to build pressure. Without the vent ports there would always be pressure in the system and the brakes would drag.

Typically takes about 0.030” of movement to cover the vent ports.


Sent from my iPhone using H.A.M.B.

 

3rd method that I use, is just a gravity bleed while in the vice. No hoses or fingers. Just a catch pan underneath. It takes longer, but less mess.

 

Either way works, The first way tends to get brake fluid all over the bench if the vise is on a bench or squirted on something you don't want brake fluid on. I did who knows how many that way before they started putting the plastic hoses in with the cylinders. The ends of the hoses have to be submerged in the brake fluid in the reservoir to keep it from sucking air back in but even then you may not get all of the air out. It's way less messy and you use way less fluid though.
Here at home I have a vise that is away from my work area and outside that I bleed master cylinders on and don't worry about a mess.

 

I'm completely clueless about this topic, but does this work while the MC is still in the car? If so, can you just open the bleeder on the left front wheel for this?

 

Make sure to use the little caps on your speed bleeder when you are done setting up your brakes. Apparently later ones will seize. But alas I use them too. I also do the tubes into the master for the bench bleed

 

When using method A, it goes a lot faster if you have the front end of the cylinder angled down so that any air in the M/C chambers migrates up towards the vent ports, rather than having the master cylinder level. If you have the front end of the cylinder angled up, you will never get all of the air out.