Riven02

I recently bought a 65 mustang with stainless steel disk brakes on the front, but I noticed the braking was pretty poor and the pedal was fairly hard. So in the pocess of tracking the problem down I checked the Vacuum of the motor. At idel the vacuum rapidly fluctuated 11-14 but when I gave it a little gas the vacuum would lock on to 18 and was completly steady. I read that that could be worn valve guides but the engine was rebuild about 3000 miles ago and I called the shop that did it (it was before i bought it) and they showed they had done a valve job. The car does have a non-stock cam but I do not know the exact numbers on it. The idle of the car is a little rough but not to bad. The brakes work perfectly if when i go to stop I hit neutral and rev to motor some to get that vacuum up. So if i can address this vacuum issue it will fix my brakes.

Scott H.

Welcome to the forums!
Nice to see you tossed a vacuum gauge on there. Rapid fluctuation, as you stated is a valve train issue, but under the right circumstances could also be a head gasket/crack issue. Along with a loose valve guide, it could also be a lifter pumping up, or a valve sticking, or even one of the umbrella seals on an intake valve that has moved up the stem or possibly a bent push rod, or stud pulling loose. I'd do a wet and dry compression test just to make sure all the valves are seated correctly, and that you have no issue with cracks or gasket failure.

You can also pull the plug wires one by one to isolate the cylinder(s) that are having issues.

If you can give the shop some more information (like from the compression test) any chance they would be willing to look it over for you? Any warranty left?

If not, once you isolate the problem to the specific cylinder(s) you can pull the valve cover and do a static visual. I've got a set of clips that go on the rockers and keep oil from flying all over the place while you set your valve lash, I'm sure someone still makes them. This will give you an idea of where the problem lies with it running. A flat mirror or piece of glass is all that is needed to check the pushrods for straightness.

Hope to hear it is something real simple, like a fouled plug!

Riven02

I did do a compression test and all the cylinders have very close to the same compression around 160. I also have a water system pressure gauge and there was no rapid increase in pressure when running. So the head gasket isnt letting any exhaust back into the system. The water system also holds pressure when i pump it up.

68COOP

What rpm are you at when it is at idle??? I'm thinking you could go as high as 800/850 at idle, and still not do hard shifts from Park to Reverse to Drive. Should bring up the vacuum up some too.

Riven02

Yeah i thought about that but i dont have a tach ATM. I have the idle as slow as possible to still run with the AC on and in gear which due to the cam that is in there is a little faster then i would like at fast idle because it seems to still go into reverse a little hard when it is first started.

Riven02

would a vacuum canister help with the brakes? or would does the vacuum in the can equalize with the motor to fast?

Scott H.

Vacuum cannister will only allow a few extra stops before you have the same thing all over. If you are in stop and go, or the 4th guy behind the stop sign you'll end up with a hard pedal before you get a chance to replenish the vacuum supply.

If your cam is that radical, the only real choices are go with a non-power brake master cylinder setup, use an external vacuum pump, or change your booster to a hydraulic power assist (if you have power steering).