Technical Performance cam causing poor braking?

The engine is a GM 350 290 hp crate motor. Engine runs fine, has a nice lope, but really have to stand on the brake to stop. Master cyl is 1 " bore with a booster, proportioning valves, disk in the front and drum in the back,I checked the linkage,all good, very light car a Model A , scary to drive!!
Was told years ago a performance cam can cause vacuum issues resulting in poor braking. The cam I have is not radical ! What gives ? Gene in Mn

 

Again, leaving important details out..!

And...your idle vacuum is.........., your cruse vacuum is............

No one can make informative recommendations without...the details..!

Mike

 

That's crazy talk!

 

Put a vacuum gauge on it. What are you pulling?

With a vacuum booster, if you’re not making much vacuum, you’re running a badly set up non-boosted brake system.



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Why do you feel you need to use a booster in a "very light car a Model A"? That 1" bore mc will stop it easily with the proper rod pedal ratio.

 

Also you may need to put more initial timing in it. Carb adjustment means a lot. Put a gauge on it and see what you got now and then play with the timing and then readjust your carbs idle mixture screws. Watch the gauge with each change.

 

Additional info that would be useful. What are the origin and sizes of the brake components at the wheels? What size/diameter wheels/tires? Just because they are discs and drums really means very little.

On a light car like you describe, you shouldn't need any booster or proportioning valve. Correctly sized front rotors and calipers, matched to a suitable bore master cylinder and correct pedal ratio, should take care of the front end. The rear does a very low percentage of braking and does not need large brakes....unless you have huge rear tires too.

I know there are no recommended specs in the above commentary, but without more info I would just be guessing and that hasn't gotten good brake results so far.

But, as an example, about ten years ago I had a '47 Ford Sedan Coupe that I was updating the brakes, suspension, etc. As to the brakes, I used a Speedway front disc kit that had F-100/F150 12" rotors and I added 'BIG GM calipers (used on full size '71 to '76 models before down sizing. Later pickups use the same caliper). Kept the stock Ford floor brake pedal assembly but added a '67/'72 Mustang M/C. Rear end was swapped out to an 8" Ford with 10" x 1.75" shoes/drums. I included residual pressure valves in front and rear 2 psi/10 psi, respectively. The 'ol Ford had great brakes, a nice firm pedal with moderate pressure and great brake modulation with varying pedal pressure. Tires were 205/70-15 front and 235/70-15 rear (oh the horror..they were radials).

My point is to illustrate how giving some thought to the specific combination of parts selected to do the job on a specific car...taking into account the whole system from pedal ratio to tire size and the car's weight, can produce the desired results without including the automatic ubiquitous 'go to fixes', unless they are really needed.

Ray

 

Sorry ,will check the vacuum

 

Thanks for the replys. Yup, will check the timing ,reset the carb w. the the use of a ( vacuum gauge) which I will now have to re purchase borrowed mine out and was never returned
Freshly finished car with many small bugs to iron out. gene

 

There is a tool to take the bugs out of the paint.........
You may need to use a vacuum boost canister that connects between the booster and the vacuum source.
Make sure you have your hose connected to at least a 3/8" vacuum port, not necked down to 1/8".

 

7/8" master and dump the booster. 3500# stops great. But to me it sounds like a booster problem. As you are slowing down your vacuum goes up and a booster system should work great. I do use 4 piston Wilwood calipers but your car is a lot lighter.

 

I thought it was supposed to be 2psi disc/10 psi drum?

 

You are correct, of course........I wish it was my only mistake today....


Edit:
Hey, I just checked on that......it’s posted correctly.....

 

You do have a booster check valve don't you? Does the booster work for a least one time after the engine is off?

 

That engine is known to have low vacuum, I know I have one, I bought an electric vacuum pump and hated it every day (loud) but it worked. After the electric pump quit I got a 9"dual booster and the brakes are not bad. Could be better but it is a heavy truck. (1966 c10)

 

If someone's 90 year old grandmother will be driving it, power brakes are a must!

 

How many pre 65 hot rods had power brakes?