Hey Crew!

My brake booster went bad on the ol 73 the other week so I purchased a new one from Rock Auto. This booster and master cylinder is identical to my old. Same look, shape, etc. But now the problem is reversed... instead of my brakes going to the floor like the worn out booster did; my brakes now are insanely too sensitive. It's so bad I won't let anybody drive it but me in fear of them putting their face through the windshield.

There is about 2" of play in the pedal before you feel any resistance on your foot. The brakes lock up well before then. The truck will violently stop locking up all four wheels. Testing in gravel it seemed the front would lock up first while in reverse and the rear would lock of first going forward.

ive bled the brakes. A bunch of crap brown came out but now they are bled clear and that hasn't help the issue.

I'm not sure if the old master cylinder had two (front and rear) reservoirs but this new one does.

anyone have any insight? Thanks in advance!

 

If it has drum brakes all around it sound like they need tightened up. Other than that I don't understand how something like that could be going on.

Just read again and the forward to reverse sounds like you have discs only on front. It's normal for drums not to work as good in reverse.

Maybe the rod is adjusted out too far and it's already applying the brakes some before you actually touch the pedal?

 

Do you have the original master/booster combo? If yes, separate them and pull the pushrod out of the booster. Then separate the new master and booster and remove that pushrod as well. Compare them side by side. Very willing to bet the new rod is adjusted out too far from the factory.

I have a thread on here from some time ago titled "brakes are not fun". In it, someone pointed me to a site that had the exact procedure for measuring that pushrods length in relation to the bore, etc.

 

If your brake pedal went to the floor, your problem wasn't the booster, it most likely was the master cylinder. It the booster went bad, the pedal would have been hard as a rock, and almost no braking power.

Like mentioned already, the rod that goes from the brake pedal itself needs to be adjusted some to make it shorter. I'd turn it in an inch or even two and see what the difference is.
Also, with the drum brakes, are you sure that the brake shoes are on properly? The shoe with the most brake material should face the back of the vehicle.

Unk Bob

 

Be very careful pulling that push rod out of that booster. There is a rubber reaction disk behind that pushrod and if it is dislodged it will do exactly what you are saying yours is doing. If you check the manual you will find a warning “A dislodged disc may cause excessive pedal travel and extreme operation sensitivity.”

You can pull the pushrod and look inside if you see silver instead of black on the seat where the pushrod fits into the disc is missing. If you got it from Rock Auto like that you need to call them. If you pulled the pushrod and dislodged it yourself you will have a bitch of a time finding it in there and putting it back into position. I can tell you from hard learned experience. NEVER PULL THAT PUSHROD OUT.

 

Procedure for setting the booster output rod length.

http://www.maximummotorsports.com/as.../MMBAK-1r1.pdf

The input and output rods of the booster are attached inside the booster housing and should not just freely slide out. If either one of them does, it has problems and the booster needs to be turned in as a core on a newly rebuilt unit.

 

Here is the proper procedure for adjusting the pushrod and yes the pushrod can be pulled out of the booster unit with no problem. I use to pull mine out and use the calipers on it to set the length accurately until one day I accidentally pulled it out before releasing the vacuum. It popped when I pulled it out and it dislodged the rubber reaction disc. I had the same symptoms as Rynoride has and I spent a week going through everything until I decided that it had to be the power booster. I got the manual out for the power booster and studied it and discovered the warning about the reaction disc. I pulled the pushrod back out and sure enough the disc was missing. I couldn’t retrieve it and had to buy a new unit. That is why I cautioned about pulling the pushrod out.

 

Wow Guys! Awesome responses. Thanks!

so, 8 learned yesterday that no matter how far in or out the push rod is adjusted, it makes no difference in the braking. With the rod adjusted all the way out the brake was rock hard (like you'd expect). With it adjusted all the way in I had about 2"-3" of pedal travel and then "WHAM" full brakes. I'm so at a loss here and, like most of you, I do t want to take it in. I want to dial her in myself; but I'm at a loss.

one of you mentioned before about the pedal rod needing adjustment? The main rod on the cab side of the booster? How do you go about adjusting that?

 

The booster output rod should be adjusted to within .005"-.010" short of the tip touching the piston of the MC, when the brakes are not being applied. The gap should be small but, it should not be touching with the pedal in the static position.

Unless the eyelet on the booster input rod between the brake pedal and the back of the booster is threaded, it's not adjustable.

 

If the input rod from the brake pedal to the booster looks like this, it's non-adjustable.



If the rod looks like this, it is adjustable.

 

Ok. By these pics mine is not adjustable. I pulled the vacuum line out to eliminate the booster and my pedal was very stiff as it took decent leg pressure to stop the truck. (I basically eliminated the booster.) I then put the vacuum line back on and it's back to its old habits. If you were going 60mph and reached down to the pedal and pressed it with your finger you'd stop the truck on a dime. It's like this booster doesn't know how to give 20%, 30%, or 70%. It's 100% booster power in an instant.

i have a couple of other friends helping via Facebook but we still haven't found a conclusion. :/

 

If the MC doesn't have a smaller bore diameter than what you originally had (mix ups in boxing products happens), the other two possibilities of having sensitive brakes is most likely the booster output rod is adjusted too far out or, the reaction valve inside the booster is bad. (see symptoms of 'Brakes Too Sensitive' below in the following troubleshooting guide).



The booster you have may be a new (rebuilt) replacement but, that doesn't mean it isn't defective too.