# Can't get good brakes



## xkeots (May 16, 2009)

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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## Goat Roper (Oct 28, 2014)

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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## xkeots (May 16, 2009)

Thanks but how do I drive it if it will not stop?


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## Goat Roper (Oct 28, 2014)

xkeots said:


> 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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## xkeots (May 16, 2009)

My proportioning valve is self adjusting. It's a PCC PV2.


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## xkeots (May 16, 2009)

Sorry it's a CPP PV2 self adjusting.


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## Goat Roper (Oct 28, 2014)

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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## xkeots (May 16, 2009)

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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## Goat Roper (Oct 28, 2014)

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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## 05GTO (Oct 6, 2004)

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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## xkeots (May 16, 2009)

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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## Goat Roper (Oct 28, 2014)

Well I am stumped, from everything you have written there is no reason your brakes shouldn't work.


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## PontiacJim (Dec 29, 2012)

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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## Goat Roper (Oct 28, 2014)

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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## Rukee (Feb 8, 2007)

Calipers on the right sides?? Bleeders should be on top or above the brake hose.


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## ALKYGTO (Mar 29, 2010)

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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## Pinion head (Jan 3, 2015)

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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## xkeots (May 16, 2009)

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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## xkeots (May 16, 2009)

PS: it's SSBC, typo in post


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## xkeots (May 16, 2009)

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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## geeteeohguy (Feb 2, 2008)

What size is the bore in the master cylinder? You may have no air in the system, but insufficient line pressure out at the wheels. There are very expensive brake pressure gauges available to check this specific issue. My gut tells me you have insufficient line pressure at the wheels.


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## Goat Roper (Oct 28, 2014)

_I bought it from OPGI and they could not help me._

Then send it all back to them and buy another system.


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## PontiacJim (Dec 29, 2012)

OK, a few more basic checks.

***Silly question, but, are your lines connected correctly? The rear well of the master cylinder operates the front brakes. The front well, operates the rear brakes. It would make sense, if you were not a "car guy", that the front well would be for the front brakes and the back well would be for the back brakes. Just sayin'.

***What is the pedal height from the floor? The '68 chassis manual states it should be 7 1/8" as measured from the underside of the standard pedal pad to floor panel. Your '67 may be different, but getting that dimension may be helpful.

***With the master cylinder cover off & fluid set on the low fill side so you can observe this- When you depress the brake pedal do you see any air bubbles or brake fluid spurt out of the compensating holes located on the floor of the master cylinder wells? You should see a tiny hole at the bottom of each well. If you do not see any bubbles (which you should not if it is bled) or if the fluid does not spurt up into the wells, your pushrod which comes out of the power booster and goes into the back of the master cylinder is too long and needs to be screwed in slightly. 

***I read the installation instructions for the SSBC Kit. It has *the small 9" booster* and requires at least 16 inches of vacuum at idle. Do you have 16 inches or more? Do you have the correct minimum outside dia 11/32" vacuum rated hose? Was you car originally power drum brakes or power disc brakes?-you may have the incorrect pedal pushrod as noted by ALKYGTO.

***Here is another check I might do, but it will require great caution! I would try to see if I could isolate which part of the braking system is giving me my problem - if at all.

*First, make sure your emergency brake is working well, because you may need to use it.*

I would pull the line at the master cylinder that goes to your rear brakes, drain the master cyl well first. Then disconnect the line. This should make your rear brakes inoperative and still allow your front discs to work - just as if you had lost a line or the rubber hose at the rear axle let go on you going down the road. The front brakes should still operate so as to stop you, thus the role of the dual master cyl.

Ensuring that you have the car located in a place where you can now test drive the car at low speed, I would get the car rolling and apply the brakes. If the car seems to stop OK with just the front brakes, then your problem could be in the rear brakes. If the car does not stop well, then there may be a problem with the front discs.

Repeat the same test, but with only the back brakes working.  Same thing, test the rear braking as measured against the front braking. Does it stop? Does it stop better? Does it stop the same?

If either the front or rear seem to stop way better, then the problem may be in the brake system that is not stopping as well. If they both seem to stop the same and that is not too good, then it has to be something other than the brakes themselves, ie not enough engine vacuum, a bad/incorrect master cyl, bad/incorrect brake booster, bad proportioning valve, etc.. What you are doing is both eliminating and narrowing down your problem as to where to look for the problem.

:thumbsup:


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