pedal to booster ratio mechanism
 

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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

Re: pedal to booster ratio mechanism
 

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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

Re: pedal to booster ratio mechanism
 

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.

Re: pedal to booster ratio mechanism
 

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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

Re: pedal to booster ratio mechanism
 

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

Re: pedal to booster ratio mechanism
 

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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

Re: pedal to booster ratio mechanism
 

The conditions you are describing sounds like your engine is not producing enough vacuum for multiple brake pedal applications or you need a check valve in your brake booster vacuum line from the engine to hold the vacuum in the brake booster.

Also, If you're running an aftermarket camshaft with significant lobe duration or narrow lobe separation these can reduce your engine vacuum.

Re: pedal to booster ratio mechanism
 

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I just put a gauge on it. It's a fairly steady 13 to 14 inches mercury plumbed directly to the intake.

The check valve tests good. It allows airflow only out of the booster. When I pulled it out of the booster I got the distinct sucking sound of the booster filling its vacuum chamber.

-Kevin

Re: pedal to booster ratio mechanism
 

Not quite enough vacuum at 13-14 to sustain more than one hard pump. Ideally you need 17-18. You might need a vacuum can for more storage or maybe a vacuum pump.
As for your pedal ratio decision it’s easier to just drill a new hole in the pedal arm about 1 inch lower to test.
As your braking is now can you lock up all the brakes?
And did you check vacuum in neutral or in gear?

Re: pedal to booster ratio mechanism
 

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Good point on the vacuum test. I did it in neutral/park and on cold start so I need to redo that at operating temp and in gear (auto trans). This booster and the same amount of vacuum available worked well for years before this recent trouble had me replacing all these parts…I could lock up the tires repeatedly without any fail or fade in pedal.

My plan is to replace the booster with something similar sans bellcrank. Then add the 2nd pedal arm hole later if necessary. I removed the vacuum line to booster completely last night and found a potential leak at the intake. The line at the port as cracked and failed when I went to disconnect it. Trimmed off the bad and put it back in. Today the brakes feel better so it’s possible that line had a leak. Tonight, I was able to panick stop successfully multiple times in a row at speed. They did feel a bit stiff at idle still though.

Thanks for your input.

-Kevin

Re: pedal to booster ratio mechanism
 

Easier to add a second hole first. Might not need any other changes.