Clutch master cylinder air bubble

Stoopid Jeep

YJ Enthusiast
Joined
Dec 25, 2021
Messages
328
Location
North Idaho
I recently replaced the clutch master cylinder.
After bench bleeding the master and installing it, the clutch disengages just fine, but the pedal is noticeably softer.
There is almost certainly an air bubble where the line plugs into the master, I did not bleed the system any further.
I was considering hooking a Mighty-vac vacuum pump to the cap on the master cylinder and attempting to pull the air up and out.
Does this seem reasonable?
Is there a better way?
 
I dont think thats gonna get it. There is a bleeder on the slave and personally thats where Id be hooking the M Vac and flushing the system as well as getting the air out. Odd as this might sound on some of the bikes/atv's Ive had it was near impossible to get the air out of the system. I read a blurd where the Poster said get it as good as you can then pump the brake lever till you get a bit of pressure and zip tie it back till the next day. Cut the zip tie to release the brake lever and see what you get. Sure enough the brakes were good as new. Ive never tried this on a clutch hyd system but keeping the clutch pedal down using a 2x4 or? would accomplish the same thing. Id also try rapping on the MC lightly to try moving that air bubble up and out of the MC. Ive had this help on calipers.
 
I dont think thats gonna get it. There is a bleeder on the slave and personally thats where Id be hooking the M Vac and flushing the system as well as getting the air out. Odd as this might sound on some of the bikes/atv's Ive had it was near impossible to get the air out of the system. I read a blurd where the Poster said get it as good as you can then pump the brake lever till you get a bit of pressure and zip tie it back till the next day. Cut the zip tie to release the brake lever and see what you get. Sure enough the brakes were good as new. Ive never tried this on a clutch hyd system but keeping the clutch pedal down using a 2x4 or? would accomplish the same thing. Id also try rapping on the MC lightly to try moving that air bubble up and out of the MC. Ive had this help on calipers.
Makes sense, easy enough to try, I'll give it a shot.

It is working fine, but there's about a 100% chance there's a small air bubble in there.

I'll go pump it up and wedge the pedal down right now, come back in an hour and give it some love taps.



"If you can't fix it with a hammer, you got an electrical problem"
 
I'm wondering if you have an internal slave or external? I know it's the master in question.

I've changed or rebuilt 6 or so masters/slaves over my years and bled dozens, some worked much better, others the same, and some needed extra bleeding. How is your transmission shifting? If it's not hard to shift or get in gear and shifts smoothly then you might be able to sit back and enjoy your new clutch feel.

As a side note for others who may read this, I've seen people install a new external slave cylinder upside down with the bleeder screw on the bottom and not the top and that will never bleed out all the air.
 
  • Like
Reactions: Stoopid Jeep
I'm wondering if you have an internal slave or external? I know it's the master in question.

I've changed or rebuilt 6 or so masters/slaves over my years and bled dozens, some worked much better, others the same, and some needed extra bleeding. How is your transmission shifting? If it's not hard to shift or get in gear and shifts smoothly then you might be able to sit back and enjoy your new clutch feel.

As a side note for others who may read this, I've seen people install a new external slave cylinder upside down with the bleeder screw on the bottom and not the top and that will never bleed out all the air.
It's external. I replaced both pieces with a pre-bled unit and the master failed after 15 miles.
That was the last complete unit the parts house had available, so they just warrantied the master.
The bleed screw is on the top side of the slave, but i'm not even gonna try to bleed it. it is working fine, I guess.

...

I'll go pump it up and wedge the pedal down right now, come back in an hour and give it some love taps....
no change, no worries.
 
  • Like
Reactions: Flyer58
These pre-bled units shouldn't need any bleeding. Install and go. A lot of them seem like junk anymore though and I read stories like this all the time of failure very quickly. The one you have is it all plastic? Just curiuos. When I did my auto to manual swap (external slave) I ordered a AMS Automotive unit from Rockauto. It was very similar to the OEM style with the components mostly metal. So far, I'm at almost 1200 miles since rebuild and it's been great.
 
Brand? so we can avoid what Im gonna guess is Napa junk. :cool:
Don't remember the brand, but it's not Napa. I got it at the local Federated Auto Parts store
These pre-bled units shouldn't need any bleeding. Install and go. A lot of them seem like junk anymore though and I read stories like this all the time of failure very quickly. The one you have is it all plastic? Just curiuos. When I did my auto to manual swap (external slave) I ordered a AMS Automotive unit from Rockauto. It was very similar to the OEM style with the components mostly metal. So far, I'm at almost 1200 miles since rebuild and it's been great.
the Master cylinder is aluminum (slave too)
Everything seems to be working properly so far.
 
  • Like
Reactions: Gilaguy23