VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 Bleeding new master cylinder

There is a port near the outlet end of the master cylinder which seems to be one to use in bleeding. Is this to be used for bench bleeding or to be initally used to bleed before brake bleeding so residual air in the master cylinder does not enter the brake lines? I was able to get the bench bleeding parts: plugs and hose at NAPA. Great resource the forums! Thanks everyone who replies to these questions. Howard

To the best of my knowledge, master cylinders, unlike wheel cylinders, don't need no outside help for bleeding. They are self bleeding. At least, on a 41.
Charlie

Are you talking about the threaded hole near the end of the casting? That is where the hydraulic switch for the brake lights is mounted.

I always bled the system at each wheel, starting at the closest wheel, then proceding to the rest of the wheels. I never had a problem doing it that way.

Hi While your description of "self bleeding" could be better, at first I was amazed that the traditional bench bleeding was not necessary but desirable, but the more I think of it, I think you hit the nail on the head. That port is near the outlet end of the mc (5/8" from end of casting to c/l of port,a threaded hole and plug,about 5/16 in diameter and located up but tilted 45 deg towards the driver side) and "must" be a prebleeder for the mc before bleeding the brakes, so no bench fill is needed. This apparently allows air to escape there instead of being pushed all the way to the first brake bleed, if the bench fill is not done. And the introduction of air while connecting up the brake line to the mc if it has been bench bled is eliminated. To add another piece to the puzzle: I wondered why the center of the refill cap under the floor pan hole was displaced about an inch or so forward. This must be so the mc bleed is more easily available from the inside of the car, allowing one person to do this. As the mc vender lists this mc as for 40-52, and there were over a million cars produced in '41, this was not unique. So given time, someone in the forum group will respond to confirm this theory. The Chevy shop manual I have nowhere references this mc valve or show it in any diagram, only discusses the wheel bleed. Another issue: a note on the package the mc came in "When bleeding this mc, do not bottom out the piston as the rear seal is beyond the filler port in the reservoir and this is commonly misinterpreted as a defective mc".

I always bleed the wheel cylinder that is the farthest away from the master cylinder first. The hole in the master cylinder cap is a vent.

I agree. Bleed the furthermost wheel cylinder first.

Ray Please read my latest reply this morning (about 7 AM). This seems to be a much different problem from that you've assumed. And yes, in the final brake bleed, do the furtherest brake cylinder first. Humer

My comment was in reference to the post by donsbigtrucks. I don't understand your post.


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Antique Mechanic figured it out. The previous mc replacement person put the brake switch in the brass block, and not where it should be, on the side at 45 deg. So that clears up everything for me. Thanks for all your suggetions, however. Howard Huemmler humer

Some have said, over time, that they bleed the right rear wheel cylinder first. If you are to bleed the farthest brake cylinder first, owing to the routing of the lines, wouldn't that be the left rear? That's the one I've always bleed first. Then the right rear, left front, and last the right front (with it being the closest).

Anyone think I'm doing it wrong? I'll appreciate your rational for doing it differently.

Thanks,
Charlie

Charlie
The sequence I use is the longest line first finishing on the shortest. The longest is determined by where the pipe is mounted onto the rear axle, if on the left rear bleed the right rear first then left rear, with your upside down cars the right front will be the next then left front.
Tony

Thanks Charlie and Tonyw. That idea about working upside down is the best I've heard for easiest access. However isn't that hard on the roof and the guy inside pumping being upside down -you may have a hard time getting a volunter for this!! But seriously Charlie you are right: my 41 exits the mc and crosses over to the passenger side and there splits with one line going to the front and the other to the rear. After connecting to the RR and the RF, they both cross over to the driver's side, the rear across the back on the axle to the LR and the front crosses on the frame to the LF. So both the LR and LF are about both the longest. However, for many, many years I've heard longest first, but I would like to understand the rationale for this. Any ideas why? Thanks for all your comments Humer

Tony,
I agree with you, I think.

On left-hand drive (non-British influenced world) then it would be LR, RR. LF and RF. One must remember that the line from the MS crosses over to the right side of the frame and then proceeds to the wheel cylinders with the longest route being in order as described. Actually, I'm not so sure that the LF is not a longer route than the RR but why quibble.

Still with me?

Charlie

Question: On right-hand vehicles does the routing from the MS cross over to the left side of the frame?

Howard,

I think the reasoning is that it helps evacuate air from the other parts of the lines generally leaving behind only air trapped in the other wheel cylinders themselves (until you bleed them in turn).

I didn't say the above in the most polished and easy to understand way but I think you get the picture.

Charlie

As anyone thats been to the '48 section knows I have a '48. Just bleed my brakes yesterday. According to the '48 manual, it states to start with LR, then LF, then RR, then RF. I also have always heard furtherest first, but???

On hindsight, I would think that may just be the farthest in descending order of length - through the lines - from the MS which is what one would be seeking. It, of course, does not describe the closest direct linear distance from the MS to the wheel cylinders which is what one would NOT be seeking.

Charlie

I've bled a lot of brakes on cars 51-59 and never could see enough difference in where you started and where you ended up to make any difference. Maybe it was because I never saw a Chevrolet Shop manual until I was over 50 years old! I just got the job done.

I also have found as MrMack that following the service manual bleeding order doesn't matter much, I have found that it takes longer and pushing more fluid through the system to get a good pedal when doing it in the wrong order.

Well, I may have been doing it all incorrectly for many years. And I may have been using more brake fluid than necessary. But at least I know that I have replaced all the old brake fluid in the system with fresh new fluid that really does not cost that much. And I know that all the air is out of the system.
Is it really that big of a deal as long that you have good braking action?
Just my opinion, To each his own.

When you consider that any air in the system is already in a line or cylinder, it really shouldn't make much difference which order you use to clear them. I was taught RR-LR-RF-LF, but the most air will be in the line or cylinder that has been opened for some reason or problem, and that would be the best place to start. If the master cylinder is the source of the air, it would probably be best to bleed the closest wheel cylinder first, as you would then be forcing most of the air to the closest exit point.