Hard stopping after rebuild

As part of my engine rebuild I changed the cam and intake on my 350 Skylark Custom with factory power front disc brakes. As a result of the rebuild the brakes are noticeably less efficient, especially on startup. I added an aftermarket vacuum canister but it hasn't totally corrected the problem. Any ideas??

 

What kind of vacuum are you pulling? And do you know what it was before?

 

I used to have VERY big cam in my car and the brakes sucked! Try this trick to see if you can generate some vacuum. Coast down hill and down shift, that should cause the motor to pull more vacuum. If that works, you simply have too little vacuum. If that doesn't work, you may have an issue with your vacuum lines, the routing or the booster may be failing. You can spray WD-40 around the vacuum lines to find a leak. If the engine stutters when you shoot the WD-40 at the line and fittings, you are on the leak.

 

Don't remember the vacuum numbers off hand but I do know the cam brought it down enough to require the canister which helped a bit. Once driving all is OK but if I start off or sit for awhile the drop off is noticeable.

 

If you rev it up, do your brakes get better? Just idle it up a bit more to see if they improve, too!

 

Put a vacuum guage on it and see what you have at idle. Until then you're just guessing.

 

Where would the most accurate reading come from? Or does it not make a difference?

 

From below the throttle plate (aka non-ported).

 

Thanks

 

15 inches of vacuum is getting down there for reliable power assisted brakes. As someone else stated your guessing until you have checked vacuum.

Your booster may also not be performing well. a quick check is to have the engine shut off , depress the brake pedal several times to deplete any remaining vacuum, then with your foot on the brake pedal, with gentle amount of pressure, start the engine. If your booster is functioning the pedal should soften under your foot a small amount. If its hard you have vacuum or booster issues.

 

I have 15 inches of vacuum; power brakes work awesome. Firm pedal, can brake right up to the traction threshold with total control.

 

With 15 inches of vacuum your at the bottom range of properly functioning power brake. I have customers with less than that and the power assist reliability drops off considerably after 15 inches. Lower vacuum will give some power assist but pump the pedal a few times and it will become hard. Vendors such as Master Power recommend more than 15 inches. Heres a link.

http://www.mpbrakes.com/technical-support/troubleshooting-drum-02.cfm.

I've had more than one customer who thought that he would brave installing power brakes knowing from the start that he might have insufficient vacuum. I offer no guarantee to a customer his brakes work with less than 15 inches. They are on their own if they want to proceed ! No gaurantee means no refunds or exchange !

Also, all boosters dont perform eqaully. Some provide better assist than others. especially used or unrebuilt units. One way to compensate for low vacuum and a hard pedal is to install a smaller bore master cylinder. Most GM units are 1 1/8 inch bore. Installing a 1 inch bore can help easing the hard pedal situation. Just my 2 bits. Marcel.

 

Installing a smaller bore master cylinder will indeed reduce pedal effort, at the cost of more pedal travel. Cheaper than a new cam...

 

Haven't heard from the owner what his vacuum reading is. Lots of guess work until that happens...

 

I'm getting there! Couldn't find my vacuum guage. Need to buy one but with Christmas here I forgot! LOL

Merry Christmas to all!!!!!!!!!!!!!!!!!!

 

Don't worry you'll find it after you buy the new one.
Jamie

 

Ain't that the truth! But only after he loses the receipt and/or the packaging. Been there and done that. It's a good thing sanding blocks are cheap. I have 5 of 'em.