I have a 351 Cleveland Engine built with a High Rise Manifold with a High lift Crowler Cam , 1971 Mach1 Mustang .

I have great brakes at running speeds but coming up to a stop, I loose almost all of my brake pedal . The pedal is at the floor and real spongy . I had the mastercylinder checked and they told me that it was ok ..
I have a feeling that I need more vacuum . I have an after market cannister 2-3 liter mounted at the firewall on the passenger side with a vacuum line running from the back of the holley 650 cfm carburetor and the larger feed line from the cannister to the brake booster . There is another outlet from the back of the weind intake manifold which is plugged off, but makes me wonder if the vaccum feed line needs to come off of it rather than the carburetor ?

One other interesting thing is that the rear brakes have been converted to disk, but retaining the original brake master cylinder . Someone appears to have put a crossover brake line under the master cylinder between the front and rear brake lines which is interesting & dangerous . There is a valve mounted close to the rear axle which may be a pressure regulator type valve , but not sure ? it does have an adjustment knob .

Does anyone have any suggestions on what direction I need to go in , to solve my problem . I just purchased the mustang and have not found anyone to help at the local garages .

mustang runs good, just won't stop , any comments or suggestions welcomed . I had a nationally known brake company work on it for 3 hours but they could not solve this issue .

Thanks 12seconds