Bad Brakes - Low Vacuum/Cam?

[Big Block Bob]

My brakes work good when the rpm's are up (highway driving).
But when I am in heavy traffic or coming up to a stop sign I only get one good pump and then the pedal goes real hard about half way down.
I have replaced everything with new parts; master, booster, front hoses, calipers, rear cylinders and lines, even the engine is new but no chnage. Actually the new engine did make a little difference as the old one had very low vacuum.
It seems like I loose vacuum at low rpm (under 1500), it has a Comp 268 cam, not too lumpy, would that cause this problem? Any other ideas?
Thanks

 

[Robert1320]

Re: Bad Brakes - Low Vacum/Cam?

You need to add a vacuum cam to your booster.

 

[Big Block Bob]

Re: Bad Brakes - Low Vacum/Cam?

You need to add a vacuum cam to your booster.

I think that might work, I was going to buy an electric vacuum pump, but this is a cheaper route.
Thanks for the info.

 

[LS6 Tommy]

Before spending money on a vacuum pump, replace the check valve on the booster vacuum line. I bet it's stuck open. a 268 hyrdaulic cam should have plenty of vacuum at idle.

Tommy

 

[78maliburat]

WHAT TOMMY SAID... 268 COMP SHOULD HAVE NO VAC ISSUES AT ALL .. START CAR UP AND HAVE SOME ONE PUMP BRAKES SEVERAL TIMES AND LISTEN FOR AIR..... BOOSTER MAY BE BLEEDING OFF ALSO

 

[Big Block Bob]

Installed vacuum can and it has new check valves, no change.
After the one or two pumps, you can quickly pump the pedal and it makes a taping sound like it is bottomed out on something under the dash and it's only about half way down? Could someone before me have changed the rod, is it too long? Could the back brakes be cranked out too much?

 

[bowtie81]

What transmission do you have? the vacuum line fell off the transmission on my t-350 and it gave me the samething problem you are describing.

 

[Robert1320]

Installed vacuum can and it has new check valves, no change.
After the one or two pumps, you can quickly pump the pedal and it makes a taping sound like it is bottomed out on something under the dash and it's only about half way down? Could someone before me have changed the rod, is it too long? Could the back brakes be cranked out too much?

Why do you need to pump the brakes to stop?

 

[Big Block Bob]

Installed vacuum can and it has new check valves, no change.
After the one or two pumps, you can quickly pump the pedal and it makes a taping sound like it is bottomed out on something under the dash and it's only about half way down? Could someone before me have changed the rod, is it too long? Could the back brakes be cranked out too much?

Why do you need to pump the brakes to stop?

I don't need to pump them to stop, but they aren't great either.
It feels like I only have half the pedal travel I should. You can push the pedal down hard (when at a light or in park) and it feels like it stops or botoms out on something 1/2 way down. Then if you do quick short pumps it makes a tapping sound and won't go down any further. Daily driving when the rev's are up you don't notice it, just at a stop or slow traffic when rpm's are low. In short it feels like I only have 1/2 pedal travel and it is does affect my braking at low rpm's. If any of that makes any sense .....

 

[Robert1320]

Could be the MC. I street drove a .7xxx lift motor with a can and had brakes.
The MC should have good feel with good pad and shoes and vacuum to pull the brakes.
1/2-2/3 pedal is too much travel. Even for a GM.

Do you have vacuum gauge? If so place it between the can and the booster.
You will be able to measure vacuum at the boost this way.
also
Double check the pedal, linkage, you could even pull the MC (don't disconnect the lines) and check for leaking fluid or mech issues.

If you have vacuum.
Mech linkage is good.
MC could be bad.