Dual master cylinder bench bleed
The Dual master cylinder I am installing on my 66 came with a kit. The master cylinder is a GM design of that time period. My question is about the additional bleeder valve ports located above where the brake lines actually screw into the side of the master cylinder. What is their purpose? I'm assuming that if I bench bleed I would connect a bleeder kit to where the brake lines actually screw into the master cylinder, not the ports located above. Am I correct? Appreciate any feedback. Thanks.
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Re: Dual master cylinder bench bleed
Yes, you would bench bled it by connecting plastic fittings and rubber hoses to where the brake lines screw into the MC. I've got the same MC you describe--whether one can actually bled the master cylinder on the car using the bleeder valves, I have no idea but that was the purpose, and even if you got the MC level (mine's a power brakes car) people report it doesn't work well. I bench bled mine when I installed it 3 years ago. -
Re: Dual master cylinder bench bleed
Paul,
I appreciate your reply to my question.
Thank You,
JeffComment
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Re: Dual master cylinder bench bleed
Those bleed valves above the exit ports eliminate the need for bench bleeding. Install the master cylinder, dry, then fill the reservoir and bleed the two master cylinder valves first, then proceed with the calipers. I prefer starting with the LF, which is the shortest path to purge air from the M/C and pipe to the junction block, and for the same reason do the LR before the RR.
The best way to ensure purging of all air is to go around at least three times with 6 to 12 strokes of the pedal depending on the length of the line - the longer the line, the more strokes. For a thorough flush, bleed at least one quart of fluid through.
DukeComment
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Re: Dual master cylinder bench bleed
So , after reading this and several other posts on the subject, I still wonder if a bench bleed is necessary on a C-2 original Dual Master Cylinder.
I have replaced the entire system on my 66 with P/B. New lines , Calipers , Booster, and rebuilt original (no bleeder valves) MC.
I read several post that say you don't need to , and others that say it is a must . One states you need to jack up the rear end at least .
Based on everything I have read , I would have bench bled it to be on the safe side , and attached it to the lines before body drop! However now that it is all installed, and Dot 3 will be used , and it is surrounded by a beautiful paint job , I am wondering if it it is indeed all that critical a step .
Surely there a others out there that have restored P/B C-2 cars without bench bleeding . How did it work out ?Comment
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Re: Dual master cylinder bench bleed
If the master cylinder does not have bleed valves above the pipe ports, you should bench bleed it. It is does have bleeder valves above the pipe ports it can be installed dry. Then follow the procedure I outlined in my previous post.
DukeComment
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Re: Dual master cylinder bench bleed
Duke,
I have no bleeders above the ports . Not what I wanted to hear , and not what I wanted to do.
But good to know what is needed, and the correct approach ahead of time !
Thanks,
JimComment
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In Appreciation of John HinckleyComment
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Re: Dual master cylinder bench bleed
Duke, I am wondering what procedure to use to bleed the master cylinder valves? I have my thoughts, but I want to be sure. I have replaced the MC with a correct version for my 70, which has the two bleed valves. I see you speak from vast experience, so I would like to know what method you use on the car with a dry MC? Thank you in advance.Comment
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Re: Dual master cylinder bench bleed
I assume you understand the two-person bleeding procedure - one pumps the pedal and the other operates the bleed valves, and you understand that the pedal operator should not release the pedal until the bleed valve is closed. If the system is empty, the pedal should be released slowly.
For a M/C that has bleed valves, install it dry, connect all pipes, fill the reservoir and first bleed the two valves on the M/C. Once air is purged, continue at the callipers.
DukeComment
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Re: Dual master cylinder bench bleed
Ralph------
No, the factory used special equipment and procedures. As I understand it, fluid was forced into the system under pressure at the master cylinder. At the same time, a vacuum was drawn at each of the caliper bleeder valves.In Appreciation of John HinckleyComment
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Re: Dual master cylinder bench bleed
...something like that. The Corvette line only ran at about 10 cars per hour, but some ran at over 50. (The early Vega line at Lordstown was supposedly designed for 100/hour, but I don't think they ever actually achieved that rate.) At that rate you can't spend 10-20 minutes filling/blending the brake system.
It has to be done in a manner of seconds.
You can buy pressure bleeding equipment for field use, and most professional mechanics who do brake fluid flush jobs regularly do it that way, but I only do a brake flush job about once a year, and if not in regular use, pressure bleeding equipment should be thoroughly cleaned and stored properly in a low humidity environment, which reduces the time advantage.
I had a friend who bought pressure bleeding equipment, and the first time he used it he applied too much pressure and blew the adapter off the master cylinder. It made a hell of a mess as you might imagine. Don't use more than 10 psi, and preferably start out with less.
Given that I find it easier to do it the shade tree way. It's works fine, but requires a helper and more time, but there's no equipment to clean and store other than the clear vinyl bleed hoses and the empty margarine tubs I use to collect the fluid at each wheel and under the clutch master cylinder, if applicable.
Since glycol-based fluid is water soluble, water, maybe with a little soap, is all you need for cleaning. Never use any petroleum-based solvent to clean brake equipment.
According to my local fire department hazmat guy, it's okay to dispose of the old fluid in the toilet because my house is connected to a state-of-the-art sewage treatment facility that breaks down the glycol and leaches out any metal ions.
DukeComment
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Re: Dual master cylinder bench bleed
Thinking about how the brake system was filled and bleed in the old days got me to thinking, they are still doing it on the new cars. Wonder if anyone notice this operation at one of the plant tours? It needs to be a very efficient operation.Comment
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