Can't Get My Brakes Right!

[ashley_j]

I am stuck! I am trying to get my Challenger brakes to firm up but I am having no luck.  Everything is new (changed master cylinder (3 times-have a Bendix now), booster (2 times), proportioning valve, calipers, pads, shoes, wheel cylinders, drums, rotors and all related hardware).  The new valve is from In-Line and I have talked to them to make sure I was bleeding the brakes correctly thru the valve. I have bled and rebled at least 6 times.  I have adjusted the rear shoes and even over tighened them once to see if I get any change, which it didn't.  I am getting good fluid flow to all the wheels.   I have tried a vacuum canister to get more vacuum but I am getting the same (10"-12'hg) with or without the canister. I don't know where to go from here. 

I have a 383ci with 284/484 MP camshaft. Edelbrock carb and intake and the engine builder figures I am getting 10.5:1 compression. I know cams can affect the vacuum but I have to pump the brakes to get any pedal.

Am I missing something? This is not the first time I have adjusted brakes but I am stumped!   
Any Suggestions?

Jeff
 

[Chryco Psycho]

is your pushrod length long enough ?

[ashley_j]

It is the rod that came with the booster/master cylinder combo from the original replacement during my rebuild.  What is the correct rod?

 

[ashley_j]

Actually isn't there 2 rods in the booster system? One from the pedal  into the booster and one from the booster into the master cylinder. I know when I mounted the new Bendix master cylinder I did not feel any resistance as I torqued down the 4 mounting nuts to the booster. Is there suppose to be any resistance or a snug fit of the rod into the master cylinder? 

 

[Cooter]

At this point anything will be a guess at best here. I would assume the wheels cylinders are new, and not "Rebuilt". I would try replacing them again. If that doesn't work, you amy need to install a couple of those 2 LB residual pressure valves in the rear brakes. This has firmed up a couple pedals for me, albeit not STOCK systems, but firmed up none the less...

[Bullitt-]

Actually isn't there 2 rods in the booster system? One from the pedal  into the booster and one from the booster into the master cylinder. I know when I mounted the new Bendix master cylinder I did not feel any resistance as I torqued down the 4 mounting nuts to the booster. Is there suppose to be any resistance or a snug fit of the rod into the master cylinder? 

 

I've been studying the booster rod adjustment for another vehicle. I cannot find info on this adjustment in the '73 Service Manual but have read that the booster rod should just contact the plunger in the M/C.  I have read here that the rod length is adjustable with the M/C removed, I'm thinking I will use vernier caliper to measure depth and adjust the rod to match.

[ashley_j]

I did pull the rod as far forward as possible while I had the M/C off but it would not come completely out nor did it change position. It just slid back into the seat. I did not work with it to long as I could not readily see an adjustment. I could feel the seat  in the booster but I am not sure how an adjustment could be made to the booster-M/C rod. If you look at a parts breakdown for 1973 the booster rod has a "cone" that the rod slides into. I have not ever taken a booster apart so I am not sure what the cone does. It may also be different since this is an aftermarket booster.  I am trying to add a parts breakdown.

 

[ashley_j]

I also should say that the 'cover, filter, hub screw' arrangement is not present in aftermarket setup I have. The pushrod is rounded as to seat into the M/C. I am not sure if this arrangement is not a holdover from previous years and 'not quite' what was installed. However, I still do not see where an adjustment is possible. 

 

[72cudamaan]

Did you bench bleed the master cylinder? If not,you could pump forever and not get all the  air out of the system.

[Jesus H Chrysler]

Did you bench bleed the master cylinder? If not,you could pump forever and not get all the  air out of the system.

That was my first thought.  Mounting a M/C without bench bleeding it first is useless.  The booster will never be able to push the M/C through it's full stroke to get all the air out.  If you don't push the M/C piston through it's full stroke with fluid in it there will always be an air bubble trapped in there causing a spongy pedal.  Remember air is compressible, fluid is not.  If the pedal is spongy or can be pumped up, there is air somewhere.  My advice is to take the M/C off, get a couple of fittings and bend them back around from the outlets to the reservoir and mount it in a vice by the booster flange.  Then top off the fluid making sure the lines are now submerged.  Take a pencil and push the piston slowly in all the way and release.  Doing this a few times should release those deep bubbles and allow the M/C to be fully filled with fluid.  Then replace and start bleeding the wheels again.

Also on some boosters the "screw" at the end of the rod that fits into the M/C is more like a nut.  This can be adjusted a little bit but not nearly enough to fix this issue.  Good luck!

[ashley_j]

Now I did that while the M/C was mounted  and used a bench bleeder kit (fittings and lines back into the reservoir) to pump out the air. Do you think the method described would be better, JHC?

 

[Jesus H Chrysler]

Not to be a jerk, but that's why they call it bench bleeding, you can't do it effectively in the car.  Sometimes you get lucky if the M/C releases the bubble through the reservoir, but in most cases the bubble is trapped deep and you have to force it out by pushing the piston past it's normal operating range.  I'd say if you've tried everything else, this is one (free) thing to do to make SURE all the air is out before you go through another round of parts replacement.  If you are using the plastic and hose bench bleeder kit that came with the M/C just make sure the hoses are tight and kept submerged while bench bleeding.  Just like on the car, you don't want it sucking air back in on the return stroke.

FYI as a general rule if calipers or wheel cylinders aren't stuck or leaking they're good.

[ashley_j]

Pointing out my shortcomings is not being a jerk.....just ask my wife.  I realized after I said it that if the rod was to short obviously the bleed would not be effective.  Is there a better way to make sure that you don't let air back in the M/C after the bench bleed?


It has been a long day already here in TN. Yet another storm after one this morning. 

And Thanks for the tips everyone. 


 

[Jesus H Chrysler]

Not shortcoming, its simply lack of experience.  Everything I explain was learned either by asking questions or the good old fashioned hard way!

The rod in the booster is not necessarily too short.  It can be 100% stock factory parts and not do what you want it to do.  The rod only has so many inches of travel.  When you push the brake pedal from top engagement to the floor is only so many inches.  The M/C is designed to have more than that so there is no physical damage in case there is bad adjustment somewhere.  What happens is when you bleed it in the car the piston in the M/C travels only so far.  The plungers push fluid and air bubbles escape.  Well if one gets trapped behind the plunger in the "extra" space, it's not going to come out easily.  Since air compresses you feel you have a pedal when you pump the brakes.  What you are really doing is compressing the air until it's up to enough PSI to start pushing fluid through the system.  When you stop pumping the pressure is off and the air re expands.  In a perfect system with zero air, your pedal should engage early and come nowhere near the floor.  Certainly never any pumping.  That's the goal!

As far as the bench bleed goes, I just keep the bleeder lines on the M/C until it's all put back on the car then connect them one at a time to minimize exposure to air.  I've done many many brake jobs in my day so I made a set of hard lines for bench bleeding.  I got a pair of M/C threaded lines and bent them back in a C shape so they are about halfway in the reservoir.  That way I don't have to hold the plastic hoses in place and can carry the whole unit back to the car while minimizing air sucking in.  If you're only doing this once or twice, it may not be worth it.  Just be as careful as you can.  But the real important part is to get full stroke on the M/C piston.  Once that pesky bubble is out it's unlikely to return unless you bleed the system dry.

[burdar]

The rod that comes out of the booster should be 1/16" from bottoming out in the master cylinder bore.  I adjusted mine to 0 clearance measuring from the mounting face of the booster.  The gasket between the master and the booster added roughly the required 1/16"  If you are having to pump the brakes to get any pedal, this may be your problem.

The Inline Tube prop valves are known to have issues.  The last thing I heard was that the whole lot was bad and they weren't able to correct the problem.

There are plenty of people who bleed the master on the car who don't have any problems...however since you are having issues, that is one thing I would look at. 

What fluid are you using?  If you are using silicon fluid, you need to let it sit out in the sun to reliece the air bubbles before putting it in the system.