Front Disc Brake Conversion - Stops terribly

[krigel]

Hi,

I've put on a front disc conversion similar to the CPP set up. I used a Mopar booster and master to keep the underhood looking stock, instead of using the GM based booster and master. I've put in an adjustable proportioning valve as well.

I've had a couple shops look at it, and no one can figure out why it is doesn't grab. I've swapped calipers, lines, still no luck.

The rotors are 11 inch. The wheels are 18 inch with 27 inch tall tires. Are the rotors just not big enough to handle these wheels?

I'm considering throwing it all in the trash and getting some big Wilwoods.

Thoughts?

Cheers,

KR

[c00nhunterjoe]

Open the proportioning valve completly.

[krigel]

I did

[Derwud]

Have you tried turning fully to both extremes?

[krigel]

Yes. It really made no difference. I had the valve checked, and it's fine.

[c00nhunterjoe]

What exactly is it not doing? A stock disc setup will stop a 10 second car with no problem.

[garner7555]

If you decide to try something diff. then look at this thread.   There is pic. of my car (14" brembo) and other's car as well.  Very cost effective.   

[krigel]

You basically have to stand on it to get the fronts to lock up. It's a bit scary.

[JB400]

Did you install the correct distribution block?

[DAY CLONA]

Hi,

I've put on a front disc conversion similar to the CPP set up. I used a Mopar booster and master to keep the underhood looking stock, instead of using the GM based booster and master. I've put in an adjustable proportioning valve as well.

I've had a couple shops look at it, and no one can figure out why it is doesn't grab. I've swapped calipers, lines, still no luck.

The rotors are 11 inch. The wheels are 18 inch with 27 inch tall tires. Are the rotors just not big enough to handle these wheels?

I'm considering throwing it all in the trash and getting some big Wilwoods.

Thoughts?

Cheers,

KR

Initially it sounds like a disc pad issue, not properly bedded or perhaps glazed? but with little info to go on it could be a variety of symptoms, how's pedal travel?, did you adjust the air gap between the booster pushrod and the master's bore?, are the brakes properly bled?, where are the bleeders located on the front calipers, at the highest point in their mounting position?....

[funknut]

Did either shop do a pressure test at all 4 corners?  What were the results?

[krigel]

Pads and Calipers were replaced, bled properly, etc. No pressure test. That would be next.

[Bronzedodge]

Hot Rod just had a quick article on brake pressure test for the hydraulics.  They used a Lamb Components gauge, LambComponents.com   

[c00nhunterjoe]

How much engine vacuum do you produce? Did you use the correct brake lever for the setup if you are using different matched components as stated in your origonal post? If you can lock them up, then they are working, its a matter of pedal effort at this point.

[toocheaptosmoke]

What bore is the master cylinder?   Did a swap one time with a larger bore master and it was about impossible to lock up the wheels, even with the power booster working correctly.

[LaOtto70Charger]

Drum or disc booster?  Is the booster good?

[krigel]

New booster, new master, but not sure of the bore.

[crj1968]

Bleed valves are pointing up correct?

[krigel]

Yes.

[Bronzedodge]

Like LaOtto70Charger said, or meant, there is a difference between disc and drum boosters.  Drum is single diaphragm, discs are dual diaphragm from the factory.   Some have used the drum booster with discs and reported no problems.  I used a factory disc booster on my disc swap after trying the drum one = much better feel and stopping. The other issue could be a pushrod that is too short.  Too long will cause the brakes to drag, too short can cause low assist.   

[68 RT]

Not enough vacuum or bad booster. If peddle is hard?

[Kern Dog]

Years back I was running  a disc/drum setup with a power booster and master cylinder from a 75 Dart. Yes, it all bolted in without any modifications.
Later I added rear discs. I was still using a disc/drum proportioning valve and it stopped fine. In 2012 I wanted to switch to a manual setup to reduce weight and clutter. I tried 4 different master cylinders with almost the same results each time. 1 1/8". a 1 1/32", a 1 inch and a 15/16". The braking force was terrible! I couldn't get it to skid at all. It felt like a power brake car with the engine off.
I put the power booster and the matching MC back on and the brakes were fine again. Later I switched to a drum/drum distribution block and the brakes got even better.
When I encounter problems like these, I make comparisons to try to make sense of it. What was so different to cause such a dramatic difference? I could only deduce that the power booster provided enough force to overcome my mistake of using a disc/drum proportioning valve. Now I am considering trying the manual setup again for the same reasons as last time.

[igozumn]

Just an FYI, but something to look at if you haven't replaced the rear flex line to the axle.  I converted my R/T from factory power 11" 4 wheel drum, to front disc using the Mopar Action article.  Factory drum power booster, Dart spindles, '73 Fury pin caliper brackets, 11.75 rotors, '70 Challenger calipers (if I recall) and I think maybe a Challenger master? with an adjustable proportioning valve and Diplomat hoses.  I'd have to double check for sure on what I used.  Been a while.   Anyway, was having a hard time getting good pedal feel, like it was under boosted or little boost assist.  Even put a big honking vacuum pump on to eliminate any "big cam" low vacuum issues that appeared to be causing my small pedal travel and disappointing braking performance.  Then I noticed the rear wheels getting hot, as if the brakes were dragging.  Found the flex line from the rear body line to the axle collapsed internally.  Replaced it and everything worked like it should have.  The flex line wasn't allowing any fluid to travel though, and since you can't compress a liquid, it was keeping the piston from moving (or moving very far) in the master.   I kinda want to hook the booster back up to the manifold and see how well it works.  The electric vacuum pump is kinda close to the radiator and if I put an electric fan on, it's gotta go.