Weird momentary brake loss problem...

[madmike]

I've got a weird, occasional, momentary loss of brake problem that I can't seem to pin down.  My car is a '68 Charger, it was a factory manual drum brake car.  I have converted the front brakes to manual disc, using a Scarebird kit that uses GM calipers on 11 3/4" Chrysler rotors, and a master cylinder from an '85 Chrysler Le Baron (I think I recall).  I purchased the kit used from a fellow board member, who didn't seem to have any problems with the kit.  I am unable to reach 'Scarebird' at all, for any help.  I have retained the factory metal brake lines, and the original distribution block.

Anyway, occasionally when turning the front wheels lock to lock (like in a tight parking lot manuever), or in reverse with the front wheels turned, my brake pedal will go to the floor!  A few quick pumps brings the pedal right back up to normal, but this scares the crap outta me!  My first thought was that maybe I had the flexible brake lines routed to the calipers poorly, and that maybe they were somehow getting pinched.  I removed the banjo bolts, rotated/routed the lines a bit differently, and now I'm positive that the lines are not getting pinched, but I still loose the brakes occasionally doing this manuever.  I have also discovered that the banjo bolt contacts the lower shock body.  In normal, straight driving, or around regular turns, at speed or going slow, the brakes feel fine, and work very well.  Other than the parking lot style slow speed/turns, the brakes seem fine.  I have probably gone through gallons of brake fluid, trying to make sure I don't have air in the system, and I don't think I do.

What can be causing this momentary loss of brakes?  I'm afraid if I don't figure this out, my brakes might go out at the most inopportune time...  Thanks!

[cdr]

it sounds like when turning lock to lock the caliper is getting pushed on ,so the piston gets pushed back in,then you have to pump it back out to get your pedal back,THE CALIPER SHOULD NEVER HIT ANYTHING

[Charger RT]

I agree with cdr. It sounds like the quick pumping is just pushing the piston back out into position. Check for something pushing the caliper in and check it with the suspension compressed and hanging and anywhere in between. Also make sure the wheel bearings are tight.
Tim

[mickelsdogs]

well with motor running AT IDLE PARK BRAKE ON.      NO POSSIBLE WAY 4 CAR TO MOVE.        turn wheels all the way till u feel it push back go both ways  a few times.  LOOK AT YOUR LOWER CONTROL ARMS.    FRONT    If there is a WITNESS MARK shiny spot. Your calipers possibly the brake hose end banjo style is what is causing this problem.        EASY FIX on the lower ball joint u will  c where it should HIT ON  THE LOWER CONTROL ARM. THERE IS A STOP protruding from balljoint is this making a mark? If not  THEN U Need a pair of 9/16 nuts or so to reduce your turning radius. This will PREVEN the most wildest WRECKS OF YOUR LIFE. From trial and error.  mike

[John_Kunkel]

I can't imagine that the physical contact of the caliper/frame could overcome the hydraulic force in the caliper. Even with light pedal pressure there's several hundred pounds of force applied to the calipers.

[madmike]

When I first assembled the front brakes, I did not even realize that the banjo bolt head on the caliper was contacting the lower shock body.  It wasn't until after a drive or two later that I even realized I had a problem.  I have turned the wheels lock to lock, with the weight of the car on the ground, and looked for marks indicating inappropriate contact.  this is when I saw the banjo bolt hitting the shocks.  I'm guessing that since these are very large GM calipers retro-fitted to a MoPar, I'm usiing parts that weren't intended to originally function together.

I will have to fabricate some sort of 'bumper' or steering stop, so that the contact no longer happens.  However, I also think that just turning the wheels lock to lock and making contact would NOT cause the piston in the caliper to create my brake problem.

I'm wondering if I need some sort of 'residual pressure' valve, or a proportioning valve?  Right now, I only have the drum brake distribution block.  The car seems to stop fabulous, in normal driving conditions, when the steering wheel isn't at full lock.  When I use the brake, I only need to depress the pedal an inch or so, and the brakes begin working almost immediately.

Any other opinions?

[cdr]

IF YOUr FOOT IS ON THE BRAKE VERY LIGHTLY WHEN YOUR CALIPER HITS THE SHOCK the piston will push in & it WILL cause you to loose pedal..why not fix the known problem you have & then see if that fixes it,instead of assuming that is not the problem 

[Dino]

This is not your problem but since you asked I would get rid of the drum distribution block and get a combo valve with hold off and prop valve built in.  Your master cylinder should have a residual valve built in but double check.  If not, stick a 2# valve in the rear line.