New master cylinder bench bleed

[FOMOCO_1546]

I recently purchased a new master cylinder from one of our vendors. I never bench bled one before so I stuck a blunt end rod in to see how hard the plunger is to push in. It will not move. Any suggestions? Thanks

 

[m_m70]

It's not easy. Did you put the master in a vise to hold it? Are there plugs in the holes where the lines connect?

 

[Apogee]

There's a pretty stiff return spring(s) inside the MC, but you should be able to stroke it by hand with some effort (~15-20 pounds maybe?). It's stiff enough that you'll want a rag or glove or both to pad your palm as you bleed the MC with a dowel or whatever it is you're using. You didn't say what bleed method you're using, but if you're doing a plugged-port bleed, then the MC will hydraulically lock once it's been bleed and no longer has any air to compress...this is the method I use and recommend since it provides feedback as to when the unit is fully bleed and also tells you whether the internal seals are bypassing fluid or not. The other method, a recirculation bleed, uses tubes to circulate fluid from the outlet ports back into the reservoirs...this method will never hydraulically lock since the outlet ports are open to atmospheric pressure, nor can you verify that the internal seals are functioning like they should.

Tobin

 

[FOMOCO_1546]

There's a pretty stiff return spring(s) inside the MC, but you should be able to stroke it by hand with some effort (~15-20 pounds maybe?). It's stiff enough that you'll want a rag or glove or both to pad your palm as you bleed the MC with a dowel or whatever it is you're using. You didn't say what bleed method you're using, but if you're doing a plugged-port bleed, then the MC will hydraulically lock once it's been bleed and no longer has any air to compress...this is the method I use and recommend since it provides feedback as to when the unit is fully bleed and also tells you whether the internal seals are bypassing fluid or not. The other method, a recirculation bleed, uses tubes to circulate fluid from the outlet ports back into the reservoirs...this method will never hydraulically lock since the outlet ports are open to atmospheric pressure, nor can you verify that the internal seals are functioning like they should.

Tobin

I was going to do a recirculation method but now going to look into the plugged port method. Thanks to both of you for the replies. I'm sure I wasn't pushing enough to move the plunger.

 

[Speedrdr]

There's a pretty stiff return spring(s) inside the MC, but you should be able to stroke it by hand with some effort (~15-20 pounds maybe?). It's stiff enough that you'll want a rag or glove or both to pad your palm as you bleed the MC with a dowel or whatever it is you're using. You didn't say what bleed method you're using, but if you're doing a plugged-port bleed, then the MC will hydraulically lock once it's been bleed and no longer has any air to compress...this is the method I use and recommend since it provides feedback as to when the unit is fully bleed and also tells you whether the internal seals are bypassing fluid or not. The other method, a recirculation bleed, uses tubes to circulate fluid from the outlet ports back into the reservoirs...this method will never hydraulically lock since the outlet ports are open to atmospheric pressure, nor can you verify that the internal seals are functioning like they should.

Tobin

When you do the blocked port method, does the system stay “bled” when you remove the plugs and attach the brake lines? TIA

Randy

 

[ntsqd]

The m/c will stay bled while you're removing the plugs and connecting the lines until all of the fluid dribbles out of the reservoir. Depending on the reservoir volume you may need to really move to get the connections made before running out of fluid.

The only way that I've successfully avoided bench bleeding a m/c is to use my home-brew pressure bleeder, which is specifically designed to push fluid in from the bleed nipples ala how GA aircraft brakes are bled. If the system has any high points in it that are above the m/c bore where air might get trapped the pressure bleed usually isn't completely successful. If the system doesn't have that sort of "feature" then it works quite well.

 

[Apogee]

When you do the blocked port method, does the system stay “bled” when you remove the plugs and attach the brake lines? TIA

Randy

You would typically leave the plugs installed until you're ready to hook up the respective hard lines to the MC, otherwise the master cylinder will gravity bleed the fluid from the reservoirs out through the outlet ports. The goal when hooking up the hard lines should be that the respective reservoirs do not run dry, otherwise you're reintroducing air into the MC bore, which could require re-bleeding the unit. That said, if you properly bench bled it the first time with the plugged port method, any subsequent bleeds should be much easier/quicker since you shouldn't have any air trapped under the seals in the piston grooves and such.

 

[Speedrdr]

Excellent news! Been wondering about how hard it was to do a bench bleed. I’m respectively decent in unscrewing and screwing lines in.
Thanks so much for the replies and help.

Randy