I didn't know such a condition could exist, but can it?

The new booster that was put in my 300 seems to allow the fronts to be REALLY "touchy" to brake pedal pressure, and there is a slight hiss heard from the under dash area (which was there before when the original power drum set up was still in place....not sure if it is booster or vacuum leak from the A/C switch).

Since it's been a decade *at least* since I've done brake work/had to use diagnosis on a brake system, does it seem reasonable that the "new" booster is faulty?

In the good news column, swapping out the rebuilt OE AFB to the reman 750 Edelbrock AFB has eliminated the stumble when the throttle is goosed !

hmmmmm. i assume you do not have a adjustable proportioning valve, so did you convert from drums to
your current disc brake setup?

Yes, and a hold off valve was also installed into the system.

The MC is for a '73 T&C disc/drum. The booster is from Rock Auto, a rebuilt unit that they spec is for '68 C-body even though it is visually different from what they show (the B-body Hemi car/Disc brake unit

vs what I received


There is about 1/4" take-up before the brakes apply, but the fronts apply HARD and NOW when I tried them doing a forward and backward brake test at VERY low speed.

So it is an all of a sudden grab?
If you let off a little bit, do the brakes let off as well? Or do you still feel them slowing you down?
Basically what I would check for is to see if they are just grabby, so if you let off back up into what you think is the take up 1/4 pedal and you have no brakes like when you were pushing down, then I might agree.

If however when you let off, the car still slows down and the brakes are engaged, then my bet is the interal valve that "breaks" when you push the pedal down is sticky and suddenly gives causing what feels like grabby brakes.

The brakes do release when the pedal is let up, and I don't think there is any major dragging.

I'm wondering if there isn't a problem with the booster rod being too long, since I assume there is some adjustment that can be made there. I'll see what I can find for a FSM online since I left mine at the shop. The other thing I can do is measure the length of the old rod from the drum booster assembly since that is in the back of my Armada.

I just found this nugget on the Online Imperial Club's site;
Question from Don (1968):

My problem is the front brakes don't release-I don't know what the rear brakes are doing but definitely the fronts won't release. Today I parked the car in my driveway which has a substantial slant to it. I put the car in NEUTRAL and I could not budge it. I put it back in PARK, separated the master cylinder from the booster and put the car in NEUTRAL and it started to roll down the incline unassisted. I put it back in PARK, reconnected the master to the booster and put it in NEUTRAL one more time-it would not move! So, is there a push rod adjustment somewhere or am I looking at this illogically?

Replies:

From Larry:

Yes there is an adjustment to solve the problem you described. On most cars the rod from the brake pedal to the booster (it goes through the firewall) is adjustable. Some cars also have an adjustable rod between the booster & master cylinder. There should be a little free play when the pedal is all the way up. If these adjustments don't work, you may have some mismatched parts. I have seen the wrong master cylinder mounted to a correct booster and the wrong rod at the pedal.

I HOPE it is an adjustment and NOT a parts mismatch!

I don't think that is your problem, but it doesn't hurt to check.
What I did was opened up the top of the master and looked into the holes with a flash light.
Loosen the master cylinder and pull it back some on the studs. Then push it back into place and see if the piston moves inside the master when you do that.
If so the rod is too long. They make adustible ones. Or you may just have the wrong master cylinder, as the piston has a hole in it that can be a different depth.

My 66 300 was the same after installing a new booster. I suspect yours is worse as the booster in the pic looks like a dual diaphragm unit which could account for your very touchy brakes. The hissing you hear is probably the booster sucking air through the filter. The back side of the diaphragm(s) has to be exposed to the atmospheric air pressure to work properly.

Well crappacino....it's not the rod adjustment, but it is the booster, probably a weak spring or something inside.

Time to call Rock Auto