C-1 Master Cylinder Bench Bleeding

[Vette66AirCoupe]

I decided to go with a new stock master cylinder for monetary reasons. I put the new master cylinder in a padded vice and bled it putting my finger over the discharge before the push rod had bottomed out each time. Pumped it about 5 times. The last two times there was no air. I refilled the reservoir. I guess the better way would have been to put a brake line on it and have it exhaust into a can of new brake fluid so that any draw back into the cylinder would be fluid and not air but I didn't have what I needed to do it that way. At any rate, I put the new master cylinder on and the pedal is hard as a rock. Considering the way brake fluid ravages paint I was thinking of foregoing bleeding the whole system again. If there is a little bit of air in the system it's all up high in the first inch of brake line and whatever might have been in the connecting fitting. My question is, if that is the case (minimal air up high), will that air work it's way out as bubbles back through the master cylinder?

 

[John Mcgraw]

Nope, once that air is in the line, it will not make it back through the master cylinder. Bite the bullet and bleed the brakes, you want to get that old fluid out of the system anyway! The glycol fluid has an affinity for moisture and will absorb moisture out of the air and let it settle to the low spots in the system where it will cause corrosion. Early cars are particularly bad since they do not have the rubber diaphram on the master cyl cap, like modern cars do to keep the moisture-laden air away from the fluid. Flushing out the old fluid by bleeding is a good practice every few years, whether you have replaced any components or not.


Regards, John McGraw

 

[IH2LOSE]

Nope, once that air is in the line, it will not make it back through the master cylinder. Bite the bullet and bleed the brakes, you want to get that old fluid out of the system anyway! The glycol fluid has an affinity for moisture and will absorb moisture out of the air and let it settle to the low spots in the system where it will cause corrosion. Early cars are particularly bad since they do not have the rubber diaphram on the master cyl cap, like modern cars do to keep the moisture-laden air away from the fluid. Flushing out the old fluid by bleeding is a good practice every few years, whether you have replaced any components or not.


Regards, John McGraw

Terry

You dont always get the answers you want to hear,But your normally get the correct answers.I agree with John 100%

But I do have to say it was a a nice try with the "not wanting to get the corrosive brake fluid on the paint"

I hate bleeding brakes also!

 

[Vette66AirCoupe]

I guess I can't get away with anything around here!
I was really surprised that the pedal wasn't spongey when I was finished and since I had replaced all of the hoses and wheel cylinders last spring and replaced the fluid by bleeding the hell out of it after siphoning the reservoir down to bare minimum I thought maybe I could slide on this one. Oh well, I'll look at the bright side. My wife and I can spend some quality time together tomorrow. Pump it up! Hold it! :L

 

[maxphunn]

I guess I can't get away with anything around here!
I was really surprised that the pedal wasn't spongey when I was finished and since I had replaced all of the hoses and wheel cylinders last spring and replaced the fluid by bleeding the hell out of it after siphoning the reservoir down to bare minimum I thought maybe I could slide on this one. Oh well, I'll look at the bright side. My wife and I can spend some quality time together tomorrow. Pump it up! Hold it! :L

I used to hate it as well so I sprung for a good vacuum bleeder. Now bleeding brakes is an easy one person job.

 

[JohnZ]

Pressure-bleeding also makes it a one-person job, and you don't have to keep an eye on the fluid level in the master cylinder reservoir

 

[Duntov-097]

Living in the Northeast rust country I've changed 100's of Master Cylinders without bleeding the brakes afterwards. Usually up here to bleed the brakes involves changing calipers and/or wheel cylinders because the bleeders do not open.:ugh Which usually leads to new brake lines because the nuts have welded themselves to the line. I used to work in the automotive repair field, owned a repair shop, and am still ASE certified in everything but Automatic transmissions so I saw this stuff on a regular basis . If my Vette was like the above I would certainly replace everything.

Most repair manuals say nothing about bleeding the entire system for a master cylinder change and I've done this on my own cars numerous times.

If you were careful taking the line off there is no reason air was introduced to the system. If you moved the line around alot and lost a lot of fluid it may be a problem. After bench bleeding the master when you attach the new line the fluid flows to where it was in the line. There is not necessarily any air in your system.

If your pedal is firm without pumping it then you don't have air in the system.

JMHO

Bob


John is there anything you do not have a picture of ?

 

[Vette66AirCoupe]

Bob,


My experience goes along with what your experience has been. I did go ahead and bleed the brakes but I did not get any air out any of the wheel cylinders. Fortunately, since I had replaced the wheel cylinders last spring, everything was still working OK. The pedal felt the same when I finished as it did before I started and a test drive indicated that everything was working very well.

 

[JohnZ]

John is there anything you do not have a picture of ?

Gotta love digital cameras - I photograph everything I do for reference (and for my monthly magazine articles and "Tech Bench" columns).