Lose RPMs when I hit the brakes...help?

[iamadonut]

Hey guys,

I have a 70 with power discs/rear drums. The car idles fine around 1k in N, but when I hit the brakes the rpms drop by 200-300 to the point where the car is idling real rough, around 600-700 rpms.

Is this the sign of a vacuum leak? I had a leaking master cylinder that I replaced, and I'm not sure if fluid got into the PB or not. How can I check for that?

Any ideas as to what this problem could be and how I can fix it? I'm a noob so I don't really know how to start diagnosing.

As always, thank you for all your help.
Ian

 

[iamadonut]

PS: The car stalls dead when I hold the brakes down and rev.

Is my car not producing enough vacuum? Should I just set the idle higher?

 

[XStang302x]

Are you stopped when you have the brakes applied and revving? Is it everytime you hit the brakes or just when you are on them hard?

-Frank

 

[iamadonut]

It happens everytime I hit the brakes. Every stoplight, etc.

The car stalls when I'm stopped and rev. (Brake torquing? Torque braking? Something like that. )

 

[iamadonut]

PS: NICE BIKE!!!

 

[XStang302x]

Thanks man

Ah ok...just wanted to make sure it wasn't float or needle valve oriented. I've no experience with power brakes...have to leave that to some of the experts here hehe.

-Frank

 

[2nd Mustang]

Remove the vacuum hose from the intake manifold to the power booster and cap the connection on the intake manifold/carb, then try to run the engine and press on the brake to see if the same thing happens. If no, it's in the booster, if yes, keep looking. What is your vacuum reading at idle, do you have a hot cam with a lumpy idle, what is your idle rpm?

 

[skywalker]

Are you stopped when you have the brakes applied and revving? Is it everytime you hit the brakes or just when you are on them hard?

-Frank

OT: MArietta? You're about 20 minutes from me and I do my drill weekends near you.

On topic: Sounds to me like a possible vacuum leak.

 

[iamadonut]

2nd Mustang: Previous owner did the engine work, but I don't think I have too hot of a cam. The engine idles smoothly right when I start it, at around 1k. According to the previous owner in his description: Comp. Cam. Mild. Comp cams double roller and Comp cams lifters.

When I remove the hose, do I remove the hose at the power booster, or do I remove the intake hose from the vacuum tree?

I'll have to pick up a vacuum gauge (and figure out how it works. )

Thanks for the help...keep it comin! Hopefully I can tackle this today.

 

[pabear89]

Try testing the Booster first.
With eng off pump brakes to empty the booster of vac.
With your foot firmly on brakes start eng and run rpms up over 1500,
the brake petal should drop down if the booster is working properly.
If it goes slowly down then there is a internal leak of the diaphram.


PB

 

[2nd Mustang]

2nd Mustang: Previous owner did the engine work, but I don't think I have too hot of a cam. The engine idles smoothly right when I start it, at around 1k. According to the previous owner in his description: Comp. Cam. Mild. Comp cams double roller and Comp cams lifters.

When I remove the hose, do I remove the hose at the power booster, or do I remove the intake hose from the vacuum tree?

I'll have to pick up a vacuum gauge (and figure out how it works. )

Thanks for the help...keep it comin! Hopefully I can tackle this today.

I have a "mild" Crane hydraulic roller cam and the best vacuum at idle is only 14 inches of vacuum, just barely enough to keep my booster working.

Remove the vacuum hose from the tree and cap the tree.

A vacuum gauge is worth it's weight in gold and more. I have two, one on the gauge pod inside and one in the engine compartment, helps when tuning.

 

[iamadonut]

Thanks for the help guys -- I'll pick up a vacuum gauge this week and try to diagnose the problem ASAP.

I'll keep you updated.

If i'm not producing enough vacuum for the PB, will a vacuum canister help me?