I have a 1956 Chevy 1/2 ton Apache with a fading power brake situation. The truck is running a 1967 350 SBC from an SS Camero, and a Saginaw 4 speed. The booster that was on the truck when I bought it is an 11 inch after market unit. The vacuum line runs from the carb (newer 650 cfm Holly Demon with vacuum secondaries) not the intake manifold. I suspect this vacuum source was used due to carb interference with the intake manifold source (elbow). The engine produces roughly 18-20 inches of vacuum at speed with a bit less at idle. The problem is that, when I apply the brakes, the vacuum bleeds down quickly, and does not recover beyond about 12-16 inches, unless I rev the engine again. I added a vacuum tank to the system but still the same issues. I suspect the booster is bad, and may be only a single diaphragm unit. It is too old and painted over to see any markings, so it will be replaced. The truck is currently running the OE drum brakes up front, with a 68 drum brake 12 bolt at the back. I would also prefer to return the vacuum source to the intake manifold rather than the carb. My question is what is the best way to go here? I lean towards a new dual diaphragm booster. I suspect these firewall mount boosters from that era are a universal bolt pattern for Chevys. I'm just not sure if this will fit up to my existing pedal and brackets. Have done some research online but still not sure of the best way to go here. Would like to order all necessary parts as a set as these days, things are too expensive to buy with just my ***umptions as a basis. If you have some. experience with this issue, any advice will be appreciated. Thanks
The firewall mounted brake pedal is not original...the MC came under the floor. If it were my truck I'd put the original type MC under the floor with the original pedal, and just not worry about that power ****. But if you want help figuring out what you have, you probably ought to post some pictures, inside the cab and under the hood, so we can see what you're dealing with. Otherwise, we're guessing blind.
Jim Thanks for your reply. The truck has a compressor tank mounted under the floor where the OE MC would have been. The previous owner had an air horn installed. I removed the horn, but left the tank & compressor in place, in case I ever went to airbags. BTW, this truck has a one piece fibergl*** front clip, so there is not an issue with space under the hood. The system that is there now works, it just needs updating. I'll get some photos posted in a bit. Tom
Make sure your vacuum check valve at the booster is still good, and take a hard look at the grommet it fits into. I had a very similar issue and it turned out to be a bad grommet at the booster that wasn't at all obvious. The next problem is it turned out to be an odd size (aftermarket booster also), took visits to several parts house to turn one up that fit.
A newer booster might help, but if you only have 12 inches or less at idle no booster will work well.
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