Classic/Antique Car Repair: brakes, Power brake problems

Question
i have a very rare 57 Pontiac made in canada.its based off a 57 chevy and looks like one.i put on a 7 inch chrome booster and ever since brake problems now.brakes lock up over and over,all new parts keep going on this but the pads don't open all the way sometimes.there was a guy at a car show one night with elect brakes because he had same 7 inch booster and problems but he said he was taking the elect brake unit off because it made alot of noise and he was putting something else on and now i cant remember the name of it..well of course i haven't seen him now to ask the name or where to get one..do you know something about that or how to get my problem fix because its costing me alot for guys to do over and over      thanks

Answer
Hello George,

I'm assuming your brakes are within specs. and worked properly before the new booster was installed. If not, check your entire system, especially wheel cylinders, for leaks, weak springs, delaminated shoes and deteriorated lines, etc. You've got to have a known baseline prior to troubleshooting the booster.

Next, hopefully you saved the old booster, or, if an upgrade, can compare/confirm that the new booster is correct for your car. Once you've confirmed the part is correct, check the installation. Since power brakes have additional boost, they require less mechanical actuation travel than regular brakes. In English, this means you need to be sure you have the proper length actuator push rod as manual brakes can require a longer rod than power brakes. So, if you use a manual rod in a power application, the result can be the overboosted condition you describe.

Another method used on cars of this era was to use the same pushrod but have different power and manual brake locations through the firewall to adjust for length in this way. Since your car is a Canadian Poncho, I can't advise you for sure which method is used without a shop manual. But what I can say is I think that everything else being equal, the level of boost is the likely cause of your problem.

Good luck with this fine car!!

Bob