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Discussion Starter #1
On my dual booster, floor pedal '69 spider I am having trouble getting the brakes back in order. They worked fine when I pulled the motor this winter. I was going to rebuild the MC but when I took it apart it looked in great shape so I just cleaned it and put it back together. Pistons moved freely with air.

But after bleeding the brakes a couple of times I still have little pedal pressure. I am aware that this system is a bear to bleed. I don't mind buying a power bleeder but my mind keeps going back to the MC and wondering if I missed something.

My question is this: When the MC fails, what does exactly does it do? How is it different from just having air in the system? Does the pedal drop to the floor? Not able to build pressure?

Also, I seem to recall someone saying that you need to bleed the MC on this system but I haven't found any info on how this is done (so obviously I haven't done it). Is that the smoking gun?

Thanks in advance.
 
J

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You might try bleeding the brakes w/engine running or w/vacuum source to boosters; this is a hydro-vacuum booster system.
 

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When the MC failed:( in my '69, it gave me a VERY low, just about to the floor, pedal. As far as I know, when the booster fails, you'll have a rock hard, or much harder, pedal.
 

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Discussion Starter #4 (Edited)
What a pain! I decided that the MC must be bad so I removed it from the car and opened it up. I really don't see any problems. The seals are all in great shape as is the barrel. I did find a bit of crud under the rear primary seal that is held in place with the spring. So I cleaned that out replaced all the seals and put all back together.

Man, what a pain putting it back in the car. Getting the four hardlines to line up just about killed me and in the end I ended up removing most of the lines before I got them all back.

Unfortunately it was for not - the brakes are still not working correctly. They are initially stiff but if I continue to hold down the pedal it will drift to the floor. They will pump up (which makes me think there is still air in the lines) but they won't hold as the pedal will again drift to the floor under foot pressure. When I do pump them up they initially drag (something they didn't do before). So I am tired and frustrated and resigned to the fact that I won't have the car ready for the national next week. It is back to the drawing board.

Does this make sense:

If the pedal goes to the floor but pumping them brings the pedal and stiffness back, then there is air in the lines. The process of pumping compresses the air and allows fluid to increase pressure. Pressure will be maintained until the pedal is released and then the compressed air is allowed to expand.

If the pedal is initially stiff but will drift to the floor, then the MC is bad. In this case a proper seal is not maintained and fluid is allowed to flow past the seal and back to the resevour. It is initially stiff because the fluid cannot accelerate as quickly as the piston. This is akin to the way a damper in a shock absorber works.

I know from previous threads that there are compensating ports that can get clogged but as far as I could tell everything was clean and clear when I rebuilt it. There is one other think I need to check: The length of the pushrod. If it is too long I think it can improperly position the pistons. But right now it's time to walk away and decompress a bit. Fight another day...
 

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What a pain! I decided that the MC must be bad so I removed it from the car and opened it up. I really don't see any problems. The seals are all in great shape as is the barrel. I did find a bit of crud under the rear primary seal that is held in place with the spring. So I cleaned that out replaced all the seals and put all back together.

Man, what a pain putting it back in the car. Getting the four hardlines to line up just about killed me and in the end I ended up removing most of the lines before I got them all back.

Unfortunately it was for not - the brakes are still not working correctly. They are initially stiff but if I continue to hold down the pedal it will drift to the floor. They will pump up (which makes me think there is still air in the lines) but they won't hold as the pedal will again drift to the floor under foot pressure. When I do pump them up they initially drag (something they didn't do before). So I am tired and frustrated and resigned to the fact that I won't have the car ready for the national next week. It is back to the drawing board.
BaD MC. Just replace with a new one. Easier. Rebuilts don't last that long.
Soft pedal is a bad MC or bad bleed job.
Proper way to bleed is to start with the farthest away have someone pump the pedal. Then while pedal is up, open the bleeder, have the helper press the pedal down, when it hits the bottom, have the helper keep it ther while you close the bleeder. Do that until air is out of the system. Right rear, left rear, right front, left front. The hand pumps are no good. This is the best way to bleed. You cannot get all the air out with those pumps or any other way.

Bad booster would give you hard pedal BTW.

Try that, if that does not work, replace the MC. I have a few in-stock if needed. I can let it go for a good price. I can get it to you asap, so you can make it to the meet. Replace and bleed and I am sure that will cure your problem. I am 99% sure it is the MC. You have all the symptoms of a bad MC or air in the system.
Good luck.

Ciao!
Jason
 

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Discussion Starter #6
Jason:

I have bled the brakes as you describe (with speed bleeders no less) so I am really leaning toward the MC. I did measure the extended arm length and found it to be nearly an inch too long. Looking at Papajam's great diagrams it looks like if it is that far out, the piston could be forward of the compensating ports. I am going to adjust it Monday and see what it does. But I'm not too optimistic at this point. So I may take you up on your MC offer...
 

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Did you bench bleed the M/C after you reassembled it? It's very difficult to get all of the air out of the master cylinder any other way. This is usually done with plastic adapters that fit into each of the brake line fittings on the M/C. Those adapters then have hoses to route to the reservoir to keep fluid circulating. Then use a large screwdriver or similar tool to repeatedly depress the piston until no more air bubbles up in the reservoir. This may be the problem an solution.
 

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If the pedal is initially stiff but will drift to the floor, then the MC is bad. In this case a proper seal is not maintained and fluid is allowed to flow past the seal and back to the resevour.
Assuming no external fluid leaks, yes, a primary piston seal is bad. Except that the fluid will not go back to the reservoir. It goes past the seal into the rear chamber of the cylinder. When the pedal is released, the fluid then flows through the piston bypass ports and reed valve back to the front chamber.

What seals were used for the rebuild?

If need be, I could next day air to you a rebuilt, ready to install M/C so you can make it to the Convention.
Hint on starting the brakelines. Start one line and tighten the fitting about halfway. Then start a second line. Then a third, etc. When all four lines are started, THEN bolt the M/C to the body.

Jason,
I'm interested in purchasing your floormount tandem M/Cs. Please PM me with pricing and such.
 

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Go and replace the MC with a new one as soon as possible. If not the brake fluid leaking will ruin your booster diaphragm too!
 

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Discussion Starter #10
Well, I've have made a bit of progress today. I adjusted the length of the pushrod (it was nearly an inch too long). The brakes now work (as in the car stops) but it is far from correct. The brake pedal goes down about half way before engaging and then the front dips a bit. This leads me to think that the rear brake piston (which is actually the forward piston in the stack) has air in it. When I press the pedal, nothing happens until the forward piston spring starts to move the rearward piston (which activates the front brakes) and then we get some action. I have bled the brakes but will work on them some more.

At this point, I think I am not going to need a new MC. I am starting to button up the spider and have now a sliver of hope that she will make it to the nationals.

Thanks for all your help!
 

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Be (very) careful when you adjust the length of the pushrod. In another car I have, I once told the "mechanic":rolleyes:that I thought the pedal went too close to the floor and that it should be higher. He "adjusted":rolleyes: the pushrod out to make it longer. Well, I did get a higher pedal:cool: but also as the car warmed up, I (guess because of expansion of parts) lost ALL free play and the car came to a complete stop.:( I had to adjust it out to get going again. Last time I went to that "mechanic."
 

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Discussion Starter #12
Good point. On the diagram that I believe Osso posted, it shows a dimension of 150mm +/- .5mm from the centerline of the clevis pin to the centerline of the forward mounting hole. I measured nearly 175mm. With it back to 150, it is actually operational if less than perfect...
 
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