low pedal

-
Mopar-Matt

Mopar-Matt

Member
Joined
Jan 3, 2014
Messages
8
Reaction score
0
Location
Milwaukee, Wi
I have a 68 barracuda and it had a LBP manual system. My last 68 had power and when I sold it, the new owner wanted manual so I swapped it for him. That left me with a power setup sitting. This year I took the car out and one of the calipers stuck and it was enough of a ride from storage to the house to make the rotor questionable. I replaced the rotors, bearings, calipers, hoses and than swapped to the power master and booster. I had to put in new lines to the distribution block as well.

The lines run below the MC and have a loop on the way down to the distribution block.

When I started it the brake pedal sunk to the floor immediately so I knew I had air. I bought one of the motive forward bleeders that sit on top of the MC and wow the amount of air I had even after doing the vacuum pump method was a lot. I did this a couple times and believed I was free of air as I saw no bubbles. The pedal seemed okay before I started the car, but once I started it, the pedal went down some again. Not to the floor but some.

It was hard for the car to move in either direction without throttle so I know I have drag in the system. The pedal will work the brakes, but it is still not right.

I am ordering a new MC and will bench bleed it as I did the last one, but what other items might be my source for drag and low pedal?

Do I still have air or is the used MC likely my issue?
 
MC has been ordered and should be at the house before the weekend. I guess in the long run it is better to get a new one and eliminate the question of if it might an issue when I NEED brakes. Any opinions on if I still have air in the lines?
 
The pedal going down some more when you start the car while holding the pedal down with a power brake booster is normal; it is always going to do that some. (And BTW, that is a good test to tell you that the booster is working.)

As for the air, if the pedal is too low, then it certainly could be the case. It is easy to test for this: with the engine off, pump the brakes 5-10 times to bleed out all of the vacuum in the booster; you will know this is done when the pedal position rises. At this point, you will have non-boosted brakes. Let the brakes 'rest' for 10 seconds or so. Then rapidly pump the brakes 5-6 times with the same foot pressure applied each time. If the point at which the pedal stops rises with the first few pumps, then you have air in the system.

One issue with bleeding is that any rapid movement of fluid, either pumped by the MC or pushed/pulled with a bleeding aid, can cativate the fluid and entrap tiny air bubbles anywhere from the MC to the calipers or wheel cylinders. So, I like to bleed from the MC with a 2nd person pushing the pedal, and move the pedal slowly. I get perfectly hard brakes every time. BTW, make sure your new MC is perfectly level when you bench bleed it.

As for the drag, you'll just have to jack it up and spin each wheel and find where it is.
 
So I had to give up and get some pro help. The booster I have I thought was from a later a body based on the profile. More short and tall over the 68 more long and narrow diameter. It seems that it is getting bound up as it goes higher in the bore hole and the master is more centered. Any ideas which master would be the right one? Maybe a 76 over a 73? Grasping at straws for help.
 
Mopar-Matt said:
... getting bound up as it goes higher in the bore hole and the master is more centered. ...
Click to expand...
You mean the MC piston seemed to be hanging in the bore? The booster rod output to the MC is a linear motion, w/ no change in angle that I have noticed.
Re what will fit, even a later 2-bolt aluminum MC will bolt to a 60-70's Mopar booster if you use a 2-4 adapter plate ($30 ebay). I did so on my 65 Newport. Easiest are ones w/ 2 ports (usually ABS) and not the newer "bubble flare" (~2000+).
Generally a power MC will have a smaller bore than a manual MC, though some years used the same bore (~1"). Check rockauto. I know some say "smaller is backwards", but makes sense when you consider the lever ratio in the power booster brackets.
 
I have the same question as Bill: what does the 'bound up' description mean in more detail?

BTW Bill, as a point of interest, some of the 70's PB MC's had a larger bore than the manual; seems backwards, I know.
 
Well turns out, it was indeed a problem with the wrench actuator (Me). When I got the step up second hand, the linkages from the pedal to the booster were in pieces. I cleaned everything and worked off pictures to get it back together. I did everything correct except that how I connected the booster to the linkage was upside down.

This caused for the angle of the linkage rod to be moving upward which made the booster rod move up. As it went up into the master it would bind. This would not allow for full brake force on the way in and not enough spring pressure to push it all the way back. For whatever reason the right front was getting pressure first/last which was the source of the drag.
 
If you don't have 'wrench actuator' problems from time to time, it means you aren't doing anything LOL. Good to hear it is resolved.
 
You must log in or register to reply here.
-

Back
Top