Reducing power brake assist?

[Matt Harwood]

So I bought my wife this lovely 1966 Mustang convertible, and the only flaw on it is this aftermarket power brake system someone installed in the past. It uses a 7-inch dual diaphragm booster for clearance reasons and a modern 2-reservoir master cylinder. The upgrades are a good idea for safety. However, the brakes are EXTREMELY sensitive now and I'll wager that I could probably stop the car by merely blowing on the brake pedal. Seriously, it takes little more than a tickle to stop the car, it's insanely over-boosted.

Now ordinarily this wouldn't be a problem, but if you're familiar with how Mustang brake lights work, you know that there's a switch on the brake pedal assembly that uses the force on the pedal to trigger the lights. I just installed a new switch with the lightest possible spring on it hoping that it would work, but unless I'm in full lockup, there are no brake lights. Sadly, it doesn't seem to be adjustable, it just needs X amount of force on the pedal to trigger the light, and with this booster, it simply doesn't happen. When I've stopped, I stomp the pedal and the lights come on so nobody rams me, but just slowing down in traffic, there are never any brake lights.

I drove the car today with the booster disconnected (no power brakes) and I find it acceptable, but I'm putting my wife and kids in it and I'd really rather have power brakes. With power brakes, it's WAY too sensitive, but without it takes about 30% more effort than I think she would be comfortable with.

So my next thought is trying to get rid of some of the assist. Any thoughts on how I might do that? I don't want to change the geometry of the pedal or change to a smaller master cylinder, I just want less vacuum assist.

Thanks!

[Rusty_OToole]

Some cars use 2 different pedals, the manual brakes have more leverage. Are there 2 holes in the pedal where the rod goes on or is there a different pedal?

[jrbartlett]

Another factor might be the adjustment on the end of the rod extending from the booster into the master cylinder. I replaced the booster on a '66 Riviera once and had the same crazy overboost issue. I turned the adjustment out to lengthen the rod so it started applying pressure to the master cylinder before the booster really kicked in. It solved the problem. Had the reverse issue on an '86 Mercury Sable. It had a really hard pedal that the mechanic couldn't fix. I asked about the same adjustment, and he said that those had been done away with years before. But when I removed the master cylinder myself, there on the end of the rod was a similar adjusting screw. This time I turned it in a turn or two -- so the master cylinder was more energized when the brakes were applied. Again it solved the problem. I don't know what the system is like on your car, but thought it might be helpful to hear my experience. There might be several possibilities in your case.