I have a 1969 Mach 1 with a 351 W, power disc brakes on the front and drum on the back. I just rebuilt the motor that now puts out 440 HP and almost 500 torque. My problem is now my power brakes don't work as well as they should. When the motor was pulled I replaced all the brake lines, rotors, calipers, drums and hardware and rebuilt the combination valve and porportioning valve. My vacum is 13. From what I read on the internet you need a minimum of 16 to 20 on the vacum for the brakes to work to their fullest capacity.I can stop the car, but I have to really stand on the brake pedal. And I can't lock the tires up no matter how hard I stand on the brake pedal.Anybody have any suggestions how to bring the vacum up? Without changing the timing and ect because the engine runs very well the way it is.
lets try a short test on the power brake booster...warm up the engine so it starts quick... shut it off..pump the brake pedal till there is no booster vacuum left... while holding moderate pressure on the brake pedal... start the engine.. the pedal should drop slightly almost instantly....let you foot off the brake pedal... rev the engine a few times to build max vacuum in the booster...shut the engine off.. wait 15 seconds... see how many boosted applications you get.. usually one and a half.. you could have a bad check valve.. you could have a blown power brake booster diaphragm..you could have a bad vacuum hose.. or perhaps some kind of restriction in the fitting or passage in the intake.. there are power brake vacuum reservoirs that can be hidden in a wheel well.. using a vacuum gauge to monitor the vacuum in the reservoir ... so you know how much braking force you are going to be assisted with...is there any chance that the pedal push rod got attached to the wrong pivot hole in the pedal.. probably not..OPGI or classic industries has a neat brake pressure gauge set that screws into bleeder screws...but then it might be easier to get some of the electric brake tees used on pickup trucks when an electric brake controller is added... so you can tee off the lines right at the master.. or even better off the lines at the combo valve.. you might also be able to create your own vacuum reservoir out of galvanized water pipe. with a cap on one end.. a tee and a plug on the end of the tee the hose fitting out of the side.. this could be hidden again up high in the fender well.. . i have a 12 inch length of 2 1/2" pipe with bell reducers in both ends.. bushing reducers in those.. with a hose barb in both ends.. i can hang that on a shock tower with a loop of wire to supply fresh fuel to the fuel pump for testing... when i have the tank and lines out of a car. works great.. lastly... in the rear shoes... there are short linings.. and long linings on the shoes.. short linings go toward the front of the car.. i know you got this right.. but i had to ask.. as this reduces the braking force because of the reduced braking friction apply area..
oh.. there are electric brake booster vacuum pumps available that increase the vacuum 4 to 6 inches.. some manufacturers have better versions..
I fixed the problem. I purchased a electric vacum pump from Jegs. The vacum is now above 18 and the brakes work.