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The 100-Six Forum

2 pump brakes

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newtoit Shannon Allison
Mandeville, LA, USA   USA
Have a 100-6, BN4 which I have been restoring. Placed new master cylinder (3/4 inch bore) and front disc brakes. Takes 2 pumps to get brakes. Replaced the master cylinder and still have the same issue. Have bled and rebled with no significant changes.
A mechanic said it might be that the disc brakes require more fluid to be pushed through to set them. If this is the case, is there a larger master cylinder available to push more fluid>
Thanks

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NaDaDawgRacer John Jones
Waxhaw, NC, USA   USA
On the contrary, disk brakes actually take less fluid to actuate but they do require more pressure which is why the cars fitted originally had a 5/8 mc. You will likely find a very heavy peddle. But back to your problem, there is likely still some trapped air somewhere. I have fought this thru several bottles of fluid in the past without getting the air out. A tip was given on this forum I think that finally solved the problem. Pump the brakes up and block the pedal down in the on position overnight. The next day when you release it odds are that everything will be good.
John

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sliproc Kevin Quistberg E
Long Beach, CA, USA   USA
Shannon,

I'm not an expert on Healey disc brakes(my 100-6 has drums all the way around)but it sounds like a trapped air bubble in the system, probably in a wheel cylinder. Healeys are known for this. I'm not sure about the disc calipers tendency to trap air but I know the wheel cylinders on the drum system do.

The problem is a large air bubble trapped in a cylinder, no matter how much you bleed the system it won't push the bubble out, the brake fluid just flows around the bubble. With the system closed you can vigorously pump the brakes trying to break up the bubble into smaller bubbles that can be bled, sometimes it works or you can manually compress your wheel cylinders while bleeding to push the bubble out.

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newtoit Shannon Allison
Mandeville, LA, USA   USA
Thanks! Will pump the heck out of the brakes to see if bubbles break up and then block over night to see what happens.
Appreciate the input.

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sliproc Kevin Quistberg E
Long Beach, CA, USA   USA
Shannon,

John and I have two different techniques to rid your system of air, it probably won't work mixing the two. It sounds like John's idea involves coaxing the bubble out slowly by using constant pressure over a period of time. My method involves breaking up the bubble through vigorous brake pump action into smaller bubbles which can be passed when bled. When using my method you want to bleed ASAP before the smaller bubbles can recombine back into a big bubble. I'd try both.

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Rengle Roger Engle
Lawrence, MI, USA   USA
I had the same problem with my bn4 and went thru the same list of solutions only to notice that the shaft on the master cylinder was shorter than the original--a little fabrication a the problem went away.

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