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I have a '73 Datsun 240Z and am having some trouble with the brakes. Without the vacuum assist the brakes are VERY heavy but work fine, about like brakes should work without vacuum assist I guess.... The problem comes with the assist hooked up. The pedal is as light as a feather, it travels half way to the floor without doing anything, and then in about a 1/2 inch travel the brakes nearly lock up! Very annoying. I put a new master cylinder on have bled all of the brakes with no improvements in my problem. Any ideas out there?
 

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> I have a '73 Datsun 240Z and am having some
> trouble with the brakes. Without the vacuum
> assist the brakes are VERY heavy but work
> fine, about like brakes should work without
> vacuum assist I guess.... The problem comes
> with the assist hooked up. The pedal is as
> light as a feather, it travels half way to
> the floor without doing anything, and then
> in about a 1/2 inch travel the brakes nearly
> lock up! Very annoying. I put a new master
> cylinder on have bled all of the brakes with
> no improvements in my problem. Any ideas out
> there?

Hey there. Well I am having just about the same problem. My pedal had no feedback or resistance. When you push far enough, it almost puts you through the windshield. I changed my master cylinder and bled the system to death as well. I suspected a faulty booster, but was told by a reliable source it is probably the proportoining valve. I have not had time to try this yet. Also, make sure your brake lines are in good condition. If they are old, they may expand and cause similar problems.

Good luck,

Jason
 

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> I have a '73 Datsun 240Z and am having some
> trouble with the brakes. Without the vacuum
> assist the brakes are VERY heavy but work
> fine, about like brakes should work without
> vacuum assist I guess.... The problem comes
> with the assist hooked up. The pedal is as
> light as a feather, it travels half way to
> the floor without doing anything, and then
> in about a 1/2 inch travel the brakes nearly
> lock up! Very annoying. I put a new master
> cylinder on have bled all of the brakes with
> no improvements in my problem. Any ideas out
> there?

I had a 53 Jag sedan with the same problem, and it turned out to be a vacuum hose that was collapsing and in effect was locking up the brakes thru the boost cylinder . If the vacuum hose gets permeated with oil from the manifold, it makes to rubber soft and easy to collapse on lower vacuum. A slight release of the brake pedal would release braking. Check out that vacuum hose and never use windshield type vacuum hose. I always ure fuel hase for that application. Good luck.
 
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