70 Charger-Converted to Disk Brakes, but may have air in Brake calipers???

[johnjb]

I just finished converting from a front drum brakes to disk brakes on my 70 Charger.  New MC, rebuilt booster, new front brakes lines, new calipers, new rotors, and etc.  I installed adjustable prop valve and 10psi residual value for rear brakes lines.

Here is problem--I have bleed the brakes serveral times, no air bubbles left.  But when I start the car the brakes go the floor, and do not pump up.  I have heard you can get air bubbles trapped in the calipers if the bleed hole is at the bottom?  I used new 72-72 Challenger calipers with new brake hoses which attach to the bottom of the caliper.  Anyone experienced this???

[mikepmcs]

did you bleed the master cylinder as well before install?
i'm guessing this is power brakes? you said when you start the car the pedal goes down.  how about when the car is off, do they pump up??
it's most likely still air in the lines but could be a vac issue as well.  how many bleed screws on each caliper?
are you using a vac pump to bleed them?

just trying to get more info

not picking on you but did you install the prop valve  and the 10# Press valve correctly(in and out going the right way)

v/r
Mike

[is_it_EVER_done?]

I have heard you can get air bubbles trapped in the calipers if the bleed hole is at the bottom? 

If the bleeders are at the bottom, the calipers are on the wrong sides. The bleeders HAVE to be at the top or it's impossible to get the air out.   Post a pic of your setup.

[johnjb]

The master cly. has been bleed three  time and looks good--no air bubbles.  In fact I tried two new MC one with a 1 1/8 in bore and one with a 1 1/32 inch bore--not much difference in either.

When the car is off--no power booster--the brakes do pump up.  When car is running with the booster, brakes will not pump up.  I am using the drum booster which has been rebuilt. 

I tried a vac pump, but finally did the bleeding the old school way with a plastic tube and container.  no air bubbles and then tighten bleed screw.  I even adjusted the rear brakes until theywere locked up and the brake pedal still goes to the bottom.  The front disk brakes do engage when the pedal is bottomed out, so it still seems like air in the line.

I double and triple check to make sure the prop and 10psi valve, they connected correctlly for the rear brakes.

I NEED TO CORRECT MY SELF--THE CALIPERS HAVE THE BRAKE HOSE AT THE BOTTOM, BUT THE BLEED SCREW IS ON THE TOP BACKSIDE--SO THE CALIPERS ARE MOUNTED CORRECLTY.

ANY HELP WOULD BE APPRECIATED--I HAVE SPENT TOO MUCH TIME ON WHAT SHOULD BE A SIMPLE PROBLEM.--

I have adjusted the booster push rod at .9 to 1 inch for engaging the MC.  seem to be right???

[is_it_EVER_done?]

Assuming that you have the system bled properly (sounds like you do), and since you have tried two different M/C (mostly rules out a M/C problem), the problem has to be either the power booster is bad, or the booster-M/C pushrod is adjusted to short.

I believe that the pushrod is out of adjustment, as you state that you have tried it at .9 to 1 inch of length. A 1/10th inch difference is huge, and should be the difference of the pedal going to the floor, or the brakes hanging up from the inability of the M/C to release pressure.

You need to measure the depth of the M/C pistons push rod receptor from the bottom of its hole to the M/C mounting surface. A depth gauge is best, but even a blunted pencil and a straight edge will do for a rough measurement (OOPS! there I go giving away trade secrets  ). Then make sure the booster pushrod is adjusted to the same length (+ a turn or two in order to assure that it is making contact) -- DO NOT DRIVE THE CAR THIS WAY, but test the brakes with the engine running. You should have good pedal, if so, you need to readjust the booster pushrod so that it is NOT making contact in the M/C bore (by about a half turn). You should now have normal brakes. If not, and you are certain that everything else is in good working order, then the fault lies in your booster.

[mikepmcs]

ok, i got a question then.  if the brakes pump up when the car is off how does that push rod come in to play there.  wouldn't it be the same if it was out of adjustment with the car on or off.  i don't know that is why i'm asking.  i definitely want to try your adjustment trick, although i gotta read it about 100 more times to understand it(i'm slow and need to see things done)
anyways if there should be no difference with the car on or off on the rod thing, then it's air or the booster, i agree.
v/r
Mike

[is_it_EVER_done?]

If the brakes pump up with the engine off, the problem is most likely in the booster itself, however it can be very difficult to determine if the brakes are actually pumping up, or if the non energized booster is causing so much resistance that it feels like they are pumping up. The only way you can tell (and I don't recommend this) is to check the brakes on the same stretch of deserted road, same speed, with the car running, then shut off -(in neutral).

I would have guessed that your rear brakes were out of adjustment, but you said you adjusted them to the point of lock-up, and it sure sounds like you have made sure that there is no air in the system, plus you tried another M/C (hopfully a known good one), so it has to be a bad booster, or pushrod that is out of adjustment. I suggest a pushrod adjustment first since you said you have adjusted it substantially, so it could be far out of adjustment.

Sorry for not being able to make myself clearer, but since my digital camera went to heaven (or hell), I can't provide pics of what I'm talking about, but the idea is to make sure that the boosters pushrod is adjusted to just slightly longer (about 1/16th of an inch) than the M/C hole is deep. If this provides good pedal, you will need to properly adjust the booster pushrod before driving the car, but if everything is the same, you will need a new booster --- If you know that everything else is right, which it sounds like you have done/checked.

Do you have a factory service manual? If not, I would highly recommend buying one as they are a wealth of information.

[dodge freak]

It sounds like all the air is out, but I had a car that no matter how many times I try to bleed the brakes the back would not work. The only way it work was to open the bleeder screw and have the MC cap off, wait 5 mins or so and the bleeder screw would start to drip, I let it drip for a few min. and then close it and do the other side same way. After that my back brakes would work again. You might want to try that on all 4 screws , open 1 at a time and let it drip for a few min. , do all 4 and then refill the MC , make sure all screws are tight and recheck it.

[johnjb]

I really appreciate all the help.  Here is the latest on the brakes.

I adjusted the push rod until the front brakes were locked when bolted to the MC.  The backed the push rod off (3 times) until front brakes turned freely.

I re-bleed all brakes manually--no air.  With the brake booster hooked up the brakes will stop for a very slow speed at around 20mph--both the front and rear brakes engage--but not well. 

If I take the vaccum hose off and plug both the manifold and input to the booster the brakes are very hard and do not work!  If I up-plug the connection to the booster, the brakes will work poorly again.

On other cars I have had you could un-plug the booster and plug the manifold and booster input and the brakes would still work--just harder brake pedal.

If the brakes were working correctly, I would think the front disk (all new) would be enough to provide good stopping and brake pedal feel.

It still seem like air in the lines,  I guess I could rebuild the rear, but they seem to be in good shape and not leaking.