Thread: Vacuum Problem With My Brakes

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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

Originally posted by 12seconds
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 ?
Thanks 12seconds

Check to make sure the vac. line is below the butterflies on the carb. If the line is above the butterflies you won't get enough vacuum @ idle. That might account for the good brakes going down the road (butterflies open) and not so good at idle. If you can get vacuum off the intake that would be a better source.

sounds like youve got a mess with your brakes but i can answer a couple things. you can put a proportioning valve on any brake set up that you wont and you may or may not need it. you might need it to adj. front to rear brake bal. once you get all 4 wheels woking right. on the vac. deal most of the time if you dont have enough vac. you will have a hard full paddle. if youve got a drum brake mastercylender on the disc brakes tech. is right it wont hold enough fluid, so you need to fix that first

Originally posted by techinspector1
pro, I thought of that too, but he said he was taking it off the back of the carb and there is no ported vacuum back there, only manifold vacuum.

That eliminates that thought. Sounds like he's got more than 1 issue there.

12 seconds: Check this out it might help you out!
Sounds like some one might have droped the ball
on that brake job. Now we know that you will lose
alot of vacuum with a big cam at idle. So how much
vacuum do you have at idle? And how much should
you have? This will tell you if it's a vacuum problem
or another problem your having with the brakes.
Your brakes should be set up like this diagram I'm
sending you on this site. If they are not set up like
this, that may be the problem!!!


http://www.e-z.net/~ts/hybrakes.htm

http://www.google.com/search?hl=en&l...system+diagram

~ Vegas ~