Brake Conversion Troubleshooting Woes! - Chevelle Tech
Brakes, Suspension & Steering Conversion questions & more.

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post #1 of 13 (permalink) Old Aug 16th, 19, 5:44 PM Thread Starter
Dave
 
Join Date: Jun 2015
Location: Augusta, GA
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Brake Conversion Troubleshooting Woes!

Hello all!


So I will give you all the knowledge about my car and you can tell me what I am doing wrong...OR...what somebody (PO) else did wrong!


I have a 1970 Chevelle SS that started life as a 307 Malibu sport coupe according to the VIN. The car currently has a 454 from something, Turbo 400, and 10 bolt (323 maybe) with an LSD. I recently put a stage 2 BMR full suspension kit under the beast to solve a lot of neglect in the suspension. The one aspect that I was not concerned about was the brakes since I got the car with an SSBC 4 wheel disc kit preinstalled. When I was able to drive the car in a bit more spirited manner with the upgraded suspension I found out that the brakes were not up to par.


The SSBC kit was the basic 4 wheel disc conversion, nothing special, seems to be off the shelf GM parts if I had to guess. Big single piston calipers up front, single piston calipers with PB provisions at the rear. The kit came with a master cylinder, brake booster, and adjustable proportioning valve.


Here is what I have done so far...


I have replaced the SSBC master cylinder since it was contaminated with water. I went back with a stock 1.25" bore from AZ for the time being. Both front calipers have been rebuilt with new seals. Rear calipers seem to be fine....but more on that later...The whole system has been bleed SEVERAL times (2 gal brake fluid) with a power bleeder and no air the last three times.



Here are the two problems that I can't get around.


Something is still not right when you hit the pedal. The brakes will stop you, but not even close in an "emergency" situation. I only have about 2inches of pedal travel before I seem to hit a "stop" halfway to the floor. It literally feels like I am hitting something with the pedal.


The second problem, to compound the first, is no brakes to the rear axle. I am getting fluid back there with a power bleed but you can touch the disc with your fingertip after a panic stop and still keep your fingerprint! Panic stop today......front discs 450...back discs 101....it was 97 in Augusta today.


The original install of the brake kit was ****ty at best.....where should I go from here?


I know this raises more questions than answers so please hit me up for more info if needed.


Thanks in advance...you guys have always been a wealth of information!
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post #2 of 13 (permalink) Old Aug 16th, 19, 8:42 PM
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Rame
 
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Re: Brake Conversion Troubleshooting Woes!

Did you replace the combo valve? I seen/read about issues with pretty much any brand of combo valves. Also do you have the right length rod on your brake pedal to master?

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post #3 of 13 (permalink) Old Aug 17th, 19, 6:20 AM Thread Starter
Dave
 
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Re: Brake Conversion Troubleshooting Woes!

Thanks for the reply sampo.

Not sure what you mean by the Combo valve. The car does still have the stock proportioning valve on the frame under the MC/booster. There is an adjustable Proportioning valve inline behind it, and that is wide open right now.

I have very little knowledge of any changes to the pedal, rod, or booster internals for that matter. Any suggestions on where I could find more info about what I might be looking for?
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post #4 of 13 (permalink) Old Aug 17th, 19, 8:10 AM Thread Starter
Dave
 
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Re: Brake Conversion Troubleshooting Woes!

I have been doing some research this morning and I have stumbled across two different "stock" proportioning valves. One is for disc/drum the other is for disc/disc. My car was originally disc/drum, so could the original proportioning valve be causing my rear brake issues?
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post #5 of 13 (permalink) Old Aug 17th, 19, 8:48 AM
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Re: Brake Conversion Troubleshooting Woes!

If the pedal is firm when you slam on the brakes and doesn’t start stopping at one point and drop during braking, I’m guessing you’ve got a brake rod length issue. You’re getting front brakes and no rears.
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post #6 of 13 (permalink) Old Aug 17th, 19, 9:14 AM
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Re: Brake Conversion Troubleshooting Woes!

Quote:
Originally Posted by daveholiday View Post
Thanks for the reply sampo.

Not sure what you mean by the Combo valve. The car does still have the stock proportioning valve on the frame under the MC/booster. There is an adjustable Proportioning valve inline behind it, and that is wide open right now.

I have very little knowledge of any changes to the pedal, rod, or booster internals for that matter. Any suggestions on where I could find more info about what I might be looking for?
Sorry about that, I'm from the south, but yeah I meant the proportion valve. But yeah that could be your problem. Also try what Alan said about slamming the brakes.
Brake rod info:

https://techtalk.mpbrakes.com/how-to...r-cylinder-gap
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post #7 of 13 (permalink) Old Aug 17th, 19, 5:50 PM Thread Starter
Dave
 
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Re: Brake Conversion Troubleshooting Woes!

Quote:
Originally Posted by Alan F View Post
If the pedal is firm when you slam on the brakes and doesn’t start stopping at one point and drop during braking, I’m guessing you’ve got a brake rod length issue. You’re getting front brakes and no rears.



So the pedal is firm, and the brakes engage as soon as the pedal is depressed. But as you push the pedal harder you get a "stopping" point where the pedal won't depress anymore, and the braking power doesn't increase.


Here is a theoretical question....


Could it be that I am bottoming out the master cylinder with the pedal? And with such a short pedal travel could it be that the master cylinder is not fully back to its "resting" position due to improper linkage. That being said....if the master cylinder was partially engaged when bolted to the booster, and I bleed the system in that position could it be that I am not getting front brake performance and lack of rear brake engagement because of this?
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post #8 of 13 (permalink) Old Aug 17th, 19, 6:43 PM
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Re: Brake Conversion Troubleshooting Woes!

Quote:
Originally Posted by daveholiday View Post
So the pedal is firm, and the brakes engage as soon as the pedal is depressed. But as you push the pedal harder you get a "stopping" point where the pedal won't depress anymore, and the braking power doesn't increase.


Here is a theoretical question....


Could it be that I am bottoming out the master cylinder with the pedal? And with such a short pedal travel could it be that the master cylinder is not fully back to its "resting" position due to improper linkage. That being said....if the master cylinder was partially engaged when bolted to the booster, and I bleed the system in that position could it be that I am not getting front brake performance and lack of rear brake engagement because of this?
Did you try super hard braking? Try to kick the brakes through the floor, in park of course, that will cycle the prop valve, I had trouble on a 72 with the rear drums not working (still bleed though) and that fixed the problem. If that don't work you need to look further into the brake system, lots of vendors sell these upgrade kits but they don't give you everything needed for the system smaller brake lines, prop valve, etc. Thats why the stuff at wilwood and baer is so darn expensive cause it comes with the parts and tech support. You can make this work though gather up part numbers on the components used, brake line size, hose etc and post what you got. Sorry for the long rant but yeah that could cause a problem.

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post #9 of 13 (permalink) Old Aug 17th, 19, 7:05 PM
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Re: Brake Conversion Troubleshooting Woes!

I would ditch the proportioning valve and just run the rear adjustable prop valve; line goes out of master cyl to a T then each front brake, other line goes master cyl to adj prop valve to rear brakes.
Next thing, was this car originally power brakes? If not, the rod that goes to the pedal may be in the wrong spot. And/or that rod may need shortened because it’s holding the brakes partially engaged.

No car yet; currently looking for 66-69 project/driver.
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post #10 of 13 (permalink) Old Aug 18th, 19, 6:39 AM Thread Starter
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Re: Brake Conversion Troubleshooting Woes!

Quote:
Originally Posted by sampoerna415 View Post
Did you try super hard braking? Try to kick the brakes through the floor, in park of course, that will cycle the prop valve.

Yeah, kick them hard and you still end up feeling like you are hitting something solid about halfway down! I think I am going to start looking closer at the booster and linkage involved out of curiosity sake!
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post #11 of 13 (permalink) Old Aug 18th, 19, 6:46 AM Thread Starter
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Re: Brake Conversion Troubleshooting Woes!

Quote:
Originally Posted by skreddy View Post
I would ditch the proportioning valve and just run the rear adjustable prop valve; line goes out of master cyl to a T then each front brake, other line goes master cyl to adj prop valve to rear brakes.
Next thing, was this car originally power brakes? If not, the rod that goes to the pedal may be in the wrong spot. And/or that rod may need shortened because it’s holding the brakes partially engaged.

I had already planned on bypassing the original prop valve...In hindsight I am sure that due to the contamination of the rear chamber of the previous MC the prop valve is full of crap as well!


Next step bypass orig prop valve and inspect all the linkage associated with the pedal.


All I have is a VIN to go on....any chance that would give me a clue as to was brake setup the car had originally? Given the factory prop valve in place would it have been a disc/drum car?
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post #12 of 13 (permalink) Old Aug 18th, 19, 9:08 AM
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Re: Brake Conversion Troubleshooting Woes!

All cars had a distribution block. The disk brake cars also received the golf ball hold off valve and a delay valve under the car near the rear axle. Anything added on a disk brake system would dampen the rear drum brakes to work with the disks.

The first thing I would do, is attempt to witness that the rear brakes are receiving pressure at the master cylinder. Loosen the line at the master cylinder and have some one pump the brakes once and hold the pedal down until you retighten the line or it will suck air in to the system at that point. There should be a solid blast of fluid. If it does, I’d keep working each line towards the rear to see if you’re losing pressure from the system. If you do not have a solid blast of fluid, your brake pedal and brake pushrod have some problem.

Mencken: “The whole aim of practical politics is to keep the populace alarmed by menacing it with an endless series of hobgoblins, all of them imaginary.”
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post #13 of 13 (permalink) Old Aug 18th, 19, 9:50 AM
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Re: Brake Conversion Troubleshooting Woes!

I would get the brake pedal/rod sorted out first and go from there, then I would look into what car your brake kit is sourced from (usually camaro, sometimes caddy or s10) and look at the differences in the system compared to yours line size, master bore, etc. and while you are there try using the prop valve from that car (if it even has one). If all else fails throw some money at if https://www.summitracing.com/parts/w...SABEgIgdPD_BwE

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