Brakes low on vacuum - Pontiac GTO Forum
User Tag List

LinkBack Thread Tools Display Modes
post #1 of 4 (permalink) Old 10-26-2014, 10:19 PM Thread Starter
Join Date: Jun 2014
Location: California
Posts: 144
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Brakes low on vacuum

I have a big cam 240/244 .605/.598 112+4 and I can push the brakes only twice before I run out of vacuum. Car is making 10inhg at idle, 18inhg at 2000rpm (my cruising rpm), and more than 25inhg after 2500rpm. I do have a vacuum reservoir and both my booster and reservoir's check valves work. I don't think the booster leaks because I can hear the vacuum escaping when I remove the check valve.
When driving on twisty roads where I keep the rpms high the brakes are ok. It recharges the vacuum fast enough before the next turn. On highway though driving between 1800 and 2000rpm I have no vacuum. Even after driving for lets say 10min , when I push the brakes there is no assistance.Its like I am loosing the vacuum on the highway (I know it sounds crazy). I already checked for vacuum leaks and there is none.
What can cause this?
How many times should I be able to push the brakes before I run out of vacuum. I would assume 4 or 5 times with the reservoir, but I can do only 2.
Can I install manual brakes with SS lines? I am using the stock brake calipers so only 6 pistons to move. My first car had manual brakes and it was perfectly fine but was only 2300lbs and had single piston front disc and drums on the rear.
Qnko is offline  
Sponsored Links
post #2 of 4 (permalink) Old 10-27-2014, 09:33 AM
Former Moderator
svede1212's Avatar
Join Date: Oct 2005
Location: Grand Rapids, Michigan
Posts: 4,081
Mentioned: 0 Post(s)
Tagged: 1 Thread(s)
Quoted: 25 Post(s)
"vacuum escaping" I know you understand it but it sounds funny. Converting your in/hg to kpa it actually shows a higher degree of vacuum that I'd expect with that cam. 10 in/hg is a MAP of 35 which sounds really low and would be a high vacuum (stock is around 40). I'm at 50 at idle with only a 10.5* overlap. You have a fairly large overlap of 18* at .005 which should make your idle kpa higher. Are you sure you're of where you think you are? Are you reading the MAP sensor or with an external gauge? Did you degree the cam when installing?

2004 Blk/Rd M6
6.3 litre LS1

Built and tuned by svede
svede1212 is offline  
post #3 of 4 (permalink) Old 10-28-2014, 12:12 AM Thread Starter
Join Date: Jun 2014
Location: California
Posts: 144
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
I know it sounds funny. I put in my post that it "sounds crazy". I took the measurements with a external gauge. And I think I did it wrong. I am looking at a log file from 2weeks ago and my idle KPA from the MAP sensor is 68. Around 2000rpm is 50kpa. I don't get it. From the MAP sensor it seems like I make most vacuum at idle (my math might be wrong, I used unit converters). The vacuum gauge says that I make the least vacuum at idle and the most at high rpms.
I will check everything again tomorrow. Its late now and my car will wake the whole neighborhood up. And no I did not degree the cam. It was the first time I do such a job. My sprockets are stock only using a katech chain (think it was c5r). I just alligned the timing marks. It was 3 years ago so I don't exactly remember but I think there was a cam locating pin and 3 positions for it. I called the place that I got the parts from and they said to use the middle one. Its been 3 years and more than 10k miles since I did it and car runs great. Only my milleage dropped a little but I blame it on the gears.
Thanks for the reply. I will recheck my vacuum levels again tomorrow and hopefully I will get the correct data this time.
Qnko is offline  
post #4 of 4 (permalink) Old 10-29-2014, 03:09 PM Thread Starter
Join Date: Jun 2014
Location: California
Posts: 144
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
I am not going to my uncles shop anymore. As soon as I got in he closed the gate and we started drinking. So anyways about the car. I was suspecting a vacuum leak because my idle was not stable and the vacuum gauge needle was jumping very fast. But could not find it myself. We hooked up the smoke machine and the leak was from my oil catch can. I looked everywhere around the manifold but didn't think off the catch can. After fixing the leak my idle is perfect now. I posted some vacuum numbers earlier that I took with my gauge that were wrong. Think it was because I hooked it up at the hose going to the booster and that's coming out from the firewall side of the manifold where the vacuum is highest. I wanted to measure it again but ended up having more beer. Now I have to go pick up my car.
Qnko is offline  
Sponsored Links

  Pontiac GTO Forum > The 2004-2006 GTO > Drive train, suspension, alignment and brakes.

Quick Reply

Register Now

In order to be able to post messages on the Pontiac GTO Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode

Similar Threads
Thread Thread Starter Forum Replies Last Post
Vacuum Hose Hookup at "Rotary Vacuum Valve" rvp986 1964-1974 Tempest, Lemans & GTO Complete Engine Compartment Discussion 3 08-29-2015 07:12 PM
Front drum brakes or new disc brakes JTS GTO 1964-1974 Tempest, Lemans & GTO Exhaust, Suspension, Alignment, Fuel Tank and Brakes. 14 01-23-2015 08:13 PM
Brakes, low vacuum? kjk990 1964-1974 Tempest, Lemans & GTO Exhaust, Suspension, Alignment, Fuel Tank and Brakes. 5 05-09-2014 05:17 PM
1965 new 389 wont idle, overheating, low vacuum torpedo51 1964-1974 Tempest, LeMans & GTO Technical and Electrical Wiring 13 06-07-2012 04:27 PM
Thumpr cam in '66 gto taking vacuum from power brakes take5 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance 7 09-03-2010 09:39 PM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome