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Discussion Starter #1
I have been ill since April but able to get into my 67 GTO last week. What was wrong is not important. I installed an SSBC front disc brake conversion kit with new master and vacuum assist. With that I used a CPP model PV-2 two disc proportion valve so the front discs and rear drums work correctly. Everything was installed to the book. Master bench bled and I had to first gravity bleed the front and rears then bled them with someone on the pedal. I have a great pedal inside the car but it stops like crap. If I am going back and forth on my driveway, all is good. I took the car down my street and it slows down then stops. Not safe. I am at wits end. I called the makers and they tell me I didn't bleed the master when I did. I even used a Motive pressure bleeder, no bubbles in any wheels. I have done in my past, easily 100 brake systems. But never a conversion. Always stock swap outs. Anyone out there have experience with this? I am almost stumped. It makes no sense having pedal pressure, no air bubbles but not stopping like discs should.
Thanks
Jax
1967 GTO
 

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You have to drive it and seat the pads, mine did the same thing after the install.
After they were seated I cranked the proportioning valve to the point the back brakes locked up under hard braking then backed it off 2 clicks.
Now the car squats under hard braking instead of diving.
 

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Thanks but how do I drive it if it will not stop?
You have to yell "Whoa Goaty" when you hit the brakes.
:)
Is your proportioning valve centered?
Turn it all the way till it stops then turn it back the other way until it stops counting the clicks or turns then go halfway.
When I did mine it didn't want to stop when I pulled it out of the shop but they started to grab after braking hard a few times.
When you brake hard do the rear drums lock up and skid?
 

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Maybe the valve is defective, if you have a solid pedal and no air in the lines and the calipers are functioning there is no reason the car shouldn't stop.
Can you feel the power assist in the pedal from the booster?
You may not have enough manifold vacuum.
 

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Discussion Starter #8
I am getting at least 18psi to the booster. When running the pedal feels fine. Just not stopping as it should. If I am doing 25mph and I hold the brake, it slows down for about 200 ft then stops.
Not cool. Master was bled and lines have no air bubbles.
 

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How about the linkage that goes from the pedal into the master?
Is it adjustable and is it bottoming out before it gets full stroke to the master?
 

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I would also check the brake booster check valve, if the check valve is not working or leaking at idle you'll have good vacuum and while driving the vacuum in the booster goes to zero. If your not storing vacuum that may explain why you can't stop. Also, is your cam stock?
 

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Discussion Starter #11
The booster is fine, when stomping on pedal, doing 20 mph the pedal goes to center of travel, not close to floor.
 

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Things to consider.

Size of the vacuum booster. Larger requires 17" of vacuum or more while the smaller boosters may actually require up to 22" of vacuum to work correctly. Here are some tips on testing the power booster, push rod length, and lever ratio of the pedal: http://www.jegs.com/installationinstructions/500/555/555-631010-116.pdf

What size bore is the master cylinder. The size of the bore has bearing on the amount of pressure applied at the pedal to stop. My '68 factory manual states that the drum brake system used a 1" bore while the disc brake set-up used a 1.125" bore. From online, " If the master cylinder bore is too large, the driver will feel a very hard pedal feel with minimal pedal travel, and will have a difficult time generating the necessary pressure to effectively operate the brake system. If the master cylinder bore size is too small, the driver will be able to generate a lot of pressure, but the pedal may have an excessively long stroke or a very spongy feel. And if the bore size is way too small, the pedal or master cylinder may bottom out before enough volume is pumped to operate the calipers. The best advice would be to consult with the manufacturer of the brake system you will be using and work with them to pick the right size for your application."

Proportioning Valve: Here is an article on the valve AND a brake gauge kit that might be a great tool to identify/verify your line pressure: Install a Brake Proportioning Valve - Tech Article - Chevy High Performance Magazine

How does your emergency brake work by itself? This will give you an indication of your rear brakes. I'd do a slow roll and see how they do and then I'd put the e-brake on and see how much force it takes for the engine to overcome the brakes from a stop- nothing harsh, just test them.

What happens when you apply your brake pedal and try and take off? The brakes should hold pretty good. If they don't hold at low engine RPM's and are easily overpowered, then I would think that there is not enough pressure going to the brakes to hold them clamped.

Also, assume your car was originally drums and you are not doing a disc brake update to an existing set-up on the fronts? Just in case, this website shows the different metering blocks and factory proportioning valves. I would think that if you had a factory proportioning valve (if it was discs or you are doing an upgrade to an existing system) that this might cause a problem using the factory proportioning valve with your aftermarket one. Tech Center - GM Classic Car Valve Configurations


OK, that should take you into this weekend to check everything out and do your testing. If none of this reveals the problem, then my guess would be you have a defective part in the system and I would call the brake kit supplier with all that you have done and they may be better able to diagnose or replace a defective part. :thumbsup:
 

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Interesting read Jim, maybe the kit he bought is mismatched and their isn't enough volume to apply full pressure to the calipers.
 

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Pushrod length? I used an adjustable pushrod on mine when I went from power to manual.

Also, there are two holes on the brake pedal arm, one for manual and one for power.
 

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Nearly every A body reproduction booster will cause the brake pads to drag, the solution to the poor mfging is to shim on the studs between the booster and the firewall with washers. Mention that, though don't see this being the problem here. I hate playing plug and play but am thinking it would eliminate the master if you installed a different disc brake master cylinder, bench bleed, re-bleed, then see if problem still exists.
 

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Discussion Starter #18
Thanks,
The front discs, master cyl., booster and proportioning valve are a kit from OPGI made especially for the GTO with 14" wheels. I spoke to them and they had no clue what's wrong. SBBC told me to rebleed the master and CCP just sold me a tool to hold the valve in the proportioning unit in the middle so I can get a correct bleed. When the tool comes, I will re bleed and see what happens. If not better I will call SBBC again and speak to tech. I am totally stumped as this is not close to my first brake dance. My first conversion, yes. Thanks.
If any more thoughts, my eyes are open to read your thoughts. The front wheels are 80% of the stopping power, I don't get it.
 

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Discussion Starter #20
Got to the point that I brought it into a shop that works on classic muscle. I fave them the centering tool for the CCP proportioning valve. They couldn't get. Good brake either. Driving the brake feels fine till it has to stop. Then it feels like non power manual brakes that won't stop right feeling like I need to use 2 feet to push on the brake pedal.
I spoke to SSBC and CCP and I got nowhere. I have been Googling my issue and I am not the only one with this problem. Nobody posted an answer. Even here, I saw a post from a shop with this system on a 67
GTO with the same problems.. I cannot drive it like this. I bought it from OPGI and they could not help me.
Help!
Thanks
Jax
1967 GTO
 
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