pedal to booster ratio mechanism

[weq92f]

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My 67 has been updated to power Disc/Drum using what appears to be a CPP upgrade kit. The only thing left of that system is the booster and its connection through the firewall to the pedal. I'm thinking of replacing this final component to help with my current brake issue: not enough brake force is being applied to the MC to lock up the wheels. I've replaced pads/rotors/calipers/MC already.

If I drive along for a few minutes, the first stomp on the brake pedal feels pretty good and does provide a LOT of brake force. However, if I let off the pedal and reapply pressure once or twice in quick succession, the pedal becomes low and stiff and does NOT provide enough clamping force to the calipers...If I use extraordinary force on the pedal at this moment, I can get more clamping.

The booster installed does not have any ratio modifying mechanism between it and the firewall...it's just a straight horizontal rod going all the way to the pedal. I've noticed both types of boosters available in the aftermarket...my question here is do I need this ratio modifying mechanism? It appears to be a lever between the pedal rod and the booster rod that increases the plunger push over what just the brake rod would deliver alone...

EDIT: pictures are for reference only.

Thanks,

-Kevin

[weq92f]

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After some additional research I've found the mechanism is called a bell crank and it actually reduces the pedal to rod movement ratio. My understanding now is that manual brakes need 6:1 and power brakes need 4:1 for the proper "feel" or clamping response to a given pedal movement. If a conversion from manual to power is done and no corresponding change in this ratio is dialed into the system, the pedal and brakes will be touchy/grabby but otherwise function well safely.

Since this conversion on my 67 did not include a new hole in the pedal arm (one lower down by an inch or so) my brakes should be a bit touchy/grabby and that incorporating a bell crank into this system or modifying the brake pedal with a lower mount hole would alleviate this...but the brakes should still reach max braking force during the pedal stroke regardless.

If I'm grasping this correctly, I should be able to install either type of booster and still get safe brakes...one may "feel" better than the other due to the ratio but this shouldn't affect overall maximum brake force available at the pedal.

Are these conclusions inline with your understanding?

Thanks,

-Kevin

[Rust_never_sleeps]

Leverage is constant, but you're getting diminishing returns for the same "work".
That doesn't sound like a problem with your linkage.
Could be your booster isn't holding vacuum, as a first guess.

[weq92f]

Originally Posted by Rust_never_sleeps

Leverage is constant, but you're getting diminishing returns for the same "work".
That doesn't sound like a problem with your linkage.
Could be your booster isn't holding vacuum, as a first guess.

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I do suspect the booster however it passes the tests:

Check valve is good
Vacuum to unit feels strong
It holds the pedal after engine shutdown
Eng off, pump away any vacuum hold pressure on pedal start engine pedal drops slightly

Just trying to figure out whether my replacement booster needs that bell crank or not.


-Kevin

[Rust_never_sleeps]

I'm unclear on what the "two types of booster" are that you mention

[weq92f]

Originally Posted by Rust_never_sleeps

I'm unclear on what the "two types of booster" are that you mention

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One booster has the bellcrank mechanism embedded in the bracket between it and the firewall.

Another does not have the bellcrank, instead it has straight rod to the brake pedal arm.

Hth,

-Kevin