Master cylinder/brake booster leak - help!

[elijahcole92]

Hey all,

So I recently replaced my master cylinder as I could tell it was leaking as the paint on the brake booster was peeling under it. However I just went to install my BBK and while bleeding I noticed it is wet between the master and booster yet again. The master cylinder has less than 5k miles on it, is it bad or is there anything else that could be causing this problem?

 

[Old Guy]

Did you buy one of the kits complete with reservoir or did you buy one of the remans where you have to transfer your exisiting reservoir to the new MC? If the latter, you might have a problem with the reservoir - MC connection leaking and allowing fluid to run down along the bottom of the MC to the base of the MC where it connects to the booster. Easy enough to check. If your replacement came with a reservoir, then all I can suggest is that you have incredibly bad luck.

 

[elijahcole92]

Did you buy one of the kits complete with reservoir or did you buy one of the remans where you have to transfer your exisiting reservoir to the new MC? If the latter, you might have a problem with the reservoir - MC connection leaking and allowing fluid to run down along the bottom of the MC to the base of the MC where it connects to the booster. Easy enough to check. If your replacement came with a reservoir, then all I can suggest is that you have incredibly bad luck.

I honestly don't remeber. I think it was a new reservoir, but the rubber seal and inside of Mc both are wet

 

[Old Guy]

Then it does not sound like a leak from the reservoir. When you installed the new MC, did you bleed it by pumping the pedal or did you pressure bleed? On a new MC, pumping should not risk damaging the piston seals as there should be no ridge in the MC bore. However, if you purchased a reman and they did a poor job of removing the cylinder wear ridge, pumping could have led to damage of the piston seals and subsequent leaking. My personal preference is to pressure bleed whenever possible.

The rod that comes out of the booster and presses on the MC piston is supposed to have a specified clearance to the MC piston (I think 0.002 " or less). Unfortunately, to check this clearance you need a special Honda tool unless some wizard has come up with an alternate method of doing the measurement. All the cars that I have worked on (I haven't done this on the NSX - yet) have some method for adjusting this clearance. However, I have found that as long as I don't mess with the adjustment of the rod, I have never had issues with doing an R&R on the MC without checking the clearance. If the rod adjustment got messed with, you might have negative clearance which is forcing the piston too far into the MC bore and possibly damaging the seals ( I don't know whether that is even possible). If the rod adjustment didn't get messed with, I return to my original diagnosis of bad luck.

 

[elijahcole92]

Then it does not sound like a leak from the reservoir. When you installed the new MC, did you bleed it by pumping the pedal or did you pressure bleed? On a new MC, pumping should not risk damaging the piston seals as there should be no ridge in the MC bore. However, if you purchased a reman and they did a poor job of removing the cylinder wear ridge, pumping could have led to damage of the piston seals and subsequent leaking. My personal preference is to pressure bleed whenever possible.

The rod that comes out of the booster and presses on the MC piston is supposed to have a specified clearance to the MC piston (I think 0.002 " or less). Unfortunately, to check this clearance you need a special Honda tool unless some wizard has come up with an alternate method of doing the measurement. All the cars that I have worked on (I haven't done this on the NSX - yet) have some method for adjusting this clearance. However, I have found that as long as I don't mess with the adjustment of the rod, I have never had issues with doing an R&R on the MC without checking the clearance. If the rod adjustment got messed with, you might have negative clearance which is forcing the piston too far into the MC bore and possibly damaging the seals ( I don't know whether that is even possible). If the rod adjustment didn't get messed with, I return to my original diagnosis of bad luck.

Thanks for the help! On the new MC i just bled it by putting fluid in it with it clamped in a vice and just pushed the piston in and out with my hand before putting it in the car. Then i just bled the brakes as normal. I did not remove the center plug or bleed anything there, was i supposed to?

 

[Old Guy]

I have only done the semi regular fluid flush of my system using the pressure bleeder. I have never had to bleed air out of the system. That said, I don't think an error in the bleeding technique would contribute to failure of the MC seals, just leave you with mushy brakes.

 

[Larry Bastanza]

Thanks for the help! On the new MC i just bled it by putting fluid in it with it clamped in a vice and just pushed the piston in and out with my hand before putting it in the car. Then i just bled the brakes as normal. I did not remove the center plug or bleed anything there, was i supposed to?

So what does "Normal" mean?