Bench bleeding master cylinder

[wegngis]

I started bench bleeding my new master cylinder according to the instructions that came with it. Basically I just push on the piston. There's nothing fancy about it, no special kit that cycles the fluid back in. I'm getting one bubble for every 2 pushes now, but I can still push it in a good inch or so. It's not easy though. Is it really that arduous of a process? Do I really need to eliminate every single tiny bubble before I bleed the rest of the system?

Background: disc brake conversion, I've had most of the lines apart so it's got a lot of air in the system anyway.

 

[hyghlndr]

Just have the fluid go from the ports through the bleeder tubes back into each bowl. Yes, sometimes it seems like it takes forever, just go slow and I do it until every single air bubbles out. Sometimes it seems to help the tilt the master cylinder up or down slightly.

 

[Teal68]

It is a must to get all the bubbles out.

 

[DirtDonk]

I started bench bleeding my new master cylinder according to the instructions that came with it.

Basically I just push on the piston. There's nothing fancy about it, no special kit that cycles the fluid back in.

Sounds strange. Without the return tubes, it's not bench-bleeding as anyone knows it that I'm aware of.
What do the instructions say to do, if you don't have the return tubes? Are the ports plugged instead? Or are they open? I suppose a closed port has some similarities to true bench-bleeding, but it's not the same and is likely to take much longer than the traditional method where you have the curved line returning the fluid straight back into the reservoir and, being not exposed to any outside air, can't pull air back in when you release the piston.

If you've got fluid in the master, are getting any bubbles at all, and can still push the piston at all, it's not ready for prime time yet. That 1" of the piston translates to 6" at the pedal (given an approx. 6:1 pedal ratio) so is basically unacceptable before you even get to the air in the main lines.

Maybe the return line kit was missing? Maybe they've figured out a new-fangled way to do it? Maybe I don't know everything?
All of the above are plausible, but the first two don't make sense to me yet.

Paul

 

[xcntrk]

Sounds strange. Without the return tubes, it's not bench-bleeding as anyone knows it that I'm aware of.
What do the instructions say to do, if you don't have the return tubes? Are the ports plugged instead? Or are they open? I suppose a closed port has some similarities to true bench-bleeding, but it's not the same and is likely to take much longer than the traditional method where you have the curved line returning the fluid straight back into the reservoir and, being not exposed to any outside air, can't pull air back in when you release the piston.

I've always bench-bled a new MC with the ports capped. That's typically what the instructions will tell you. I agree having a slick return tube or loop back into the reservoir would be ideal.

 

[bax]

yes you bench bleed or I should say I bench bleed with the fittings in place. I use plugs. I plug both ports. cycle the plunger a few times. Crack one plug open at a time push the plunger in then tighten the plug up and release the plunger. I do this so I wont suck air back into the system. Takes about 5 times per plug. Always works for me. Makes a bit of a mess. Perhaps I will make up a few lines that go back into the fluid in the reservoir. Thats a good idea. My way is a little messy.

 

[Teal68]

I have sets of old lines that I cut and bent to where the tips are near the bottom of the master cylinder. Makes for less disturbance (read...splashing and spilling). I also find that it is easier to "bench" bleed it already mounted so you can use the leverage of the pedal instead of having to push the plunger by hand.

Basically, do whatever you have to to get all the air out!

 

[KyleQ]

I would bolt it in, fill it and use the pedal to pump until you get bored - give it the occasional rap on the side to dislodge any air. When it is capped and there is no air in the system, the pedal should not move at all. That is what is great about the plugs - its super easy to tell when it is done.

I recently swapped masters and lines and bench bled mine - changing the angle from time to time seems to help -

Patience is the name of the game-

 

[needabronco]

I've personally never seen any benefit to bench bleeding. If you just bolt it on and go you'll actually save time over the time it takes to bench bleed, mount, and rebleed.

 

[wegngis]

I've learned a lot since I first posted this yesterday afternoon. This particular master cylinder doesn't have return lines for bleeding, it just has closed bleeder plugs. Here's what the instructions say to do.

1. Secure the new master cylinder in a vise by the mounting ear on the casting.
2. Remove the protective plugs from the outlet ports and fill reservoir with new brake fluid
3. Install the bleeder plugs into the ports. Tighten gently to seat.
4. Using a blunt instrument or wooden dowel to avoid piston damage, slowly depress the piston into the bore using no more than 1 in strokes. Repeat the process until the piston cannot be depressed anymore than 1/8 inch and no more air bubbles appear in reservoir. (IMPORTANT: When bleeding step bore design master cylinders, you must pause 15 seconds between strokes to avoid fluid aeration.)

At first tons of bubbles came out, now only one side is continuing to produce any. I can still depress the piston, however. I can really see the benefit to having a kit that loops the fluid back. I keep having trouble with brake fluid shooting out the top as I compress it by hand. After reading some of the ideas here, I may go one of two directions. First, I might get that bleeder kit. Second, I might just mount the master cylinder and pump it with my foot (with the cap on). I just can't generate the power I need to push that piston by hand in a controlled fashion.

Thanks for all the replies. I'm continuing to mull this.

 

[Casey835]

Here is a nice set of instructions for you. These were for my mc so I'm not sure if it will apply to yours.http://www.classicperform.com/TechBook/BrakeBleed.htm

 

[wegngis]

Well, I tried bolting it in and pushing on the pedal. I've probably pumped it 300 times. I guess it makes sense if a single bubble is coming out on each pump. Right? It's gonna take a while.

 

[Teal68]

That seems extreme. I got a bad mc once, and each return stroke took in air. Could not b bled. You will see leaking fluid where u push if that's the case.

 

[wegngis]

I bought a bench bleeding kit at napa with the return lines. We shall see if that does the trick.

 

[Apogee]

I personally prefer to plugged-port method over the recirculation method for the follwing reasons.

1. Plugged port provides a positive means for knowing when the master cylinder is fully bleed since the piston will hydraulically lock once the seals clear the fluid ports into the reservoir, typically about 1/8" of travel. Just as the fluid returns to the reservoir, so will the air bubbles upon release. With the recirc method, no more bubbles may mean you're done, but may not and is not definitive since there's no way of knowing whether there's still air trapped anywhere based on the feel or travel of the piston.

2. Plugged port method has always worked quicker for me, which I've attributed to the fact that the bleeding process generates pressure which collapses the bubbles, reduces their surface area and makes them more likely to release from whatever seal, groove, void, etc from which they may be trapped. The recirc method just pushes fluid in a loop under basically zero pressure, so while a decent way to get the bulk of the air out, not the best to finish difficult to bleed MC's IMHO.

3. Finally, if there is an issue with the master cylinder with respect to bypassing fluid internally between circuits, you can and will be able to determine that with the plugged port method prior to installation. So even if you prefer bleeding via the recirc method, it would be good practice to plug both ports after bleeding, cracking each plug to bleed the ports, and checking the MC to verify that it's fully bleed and doesn't slowly sink under pressure.

 

[Hozr]

I've personally never seen any benefit to bench bleeding. If you just bolt it on and go you'll actually save time over the time it takes to bench bleed, mount, and rebleed.

I had never bench bled a M/C until the bronco (30+ years). I will NEVER bench bleed a M/C again! I don't get all the fuss but I definitely got all the mess trying to install a M/C leaking fluid everywhere....

Do it in the truck, works fine.

 

[Jeff10]

Hey Guys,

Getting ready to install a new master cylinder for the hydroboost.

My thinking, based on what some of you have written, is to bolt it up to the hydroboost unit and depress the plunger using the pedal.

I'm still not clear about the difference between doing this, and doing on a bench (other than the lack of a mess).

I'm also not clear about return tubes versus plugs. Maybe return tubes until almost all of the air appears to be gone, then replace the fittings with the plugs and pump until little pedal travel?

Any updates would be appreciated.

Thanks,

Jeff

 

[needabronco]

Jeff, my personal experience is that its faster and easier to just install the master cylinder and bleed it. Why spend the time bench bleeding it, then take your bleeding apparatus off, install the master while trying not to drop brake fluid everywhere, and the bleed it all over again?

 

[Apogee]

My $.02, plugged port bench bleed method. Clean, fast, easy and definitive as to when the unit is fully bleed. For all of the threads on this site alone regarding brake issues that more often than not end up being bleeding issues, bench bleeding is a good idea IMHO.

As for you guys spilling fluid everywhere, I'm not sure what that's about, but isopropyl alcohol is great for cleaning up brake fluid.

 

[suthernboy]

My $.02, plugged port bench bleed method. Clean, fast, easy and definitive as to when the unit is fully bleed. For all of the threads on this site alone regarding brake issues that more often than not end up being bleeding issues, bench bleeding is a good idea IMHO.

As for you guys spilling fluid everywhere, I'm not sure what that's about, but isopropyl alcohol is great for cleaning up brake fluid.

See!?! ANOTHER good use for alcohol!! Dang!....It's like a MIRACLE liquid!