Replaced Brake Booster Now Pedal Goes To The Floor

[Totty101]

Hello-

Title says it all.  

One day I was driving and I lost a lot of brake power and it started hissing when I braked.  Figured easy fix, swop booster out for with a new one I'm good.  Right?  Not the case.  Installed the new one and pedal goes an 1" or 2" from the floor.  Have brakes but now real soft/spongy.  I've tried adjusting the push rod from the pedal to the booster with no luck.  There is no adjustment from push rod to master cylinder so I'm out of ideas on what the deal is.  Do I need to unscrew the pedal push rod out more?  If I go to much further out I'm thinking I'm not going to be able to get it hooked up to the pedal again.  I rebuilt the rear drum brakes, completely bled the system and tested before I started on this booster project, which worked fine minus the booster issue.  I tried one other new booster to see if maybe the auto parts store gave me the wrong one but had the exact same issue.   I can't imagine my master cylinder went bad.  It needs a new cover seal but it still works like normal if I put the old booster back on.  Thought about bleeding the brakes again but didn't want to start that again if I didn't need to.  It is an 84 GMC C2500 Sierra High

Any ideas?

 

Old booster+old master cylinder=have brakes just not "power brakes" 

new booser+old master cylinder=have brake but goes to floor and are spongy

[SteveBBB]

Sounds to me like you need to bleed the brakes again.

[Totty101]

why would i need to again if i did already and the brakes work fine when old booster is installed minus the no power in the brakes.  

[muddkatt]

Which engine, Diesel or Gas?

What brake code is in the glove box JB6 or JB7 for gas engines or JD6 or JD7 for diesel? and did you get the right booster for that code?

[Totty101]

Already checked that but that service part label isn't there.  From all the research I've done I am pretty confident I have a JB7 brake system.  

Yes, according to the auto parts store this the booster I have is for JB6 and JB7.  I even tried the one that is supposed to be for JB5 and that didn't work.

 

I have a 3/4 ton with front disc and rear 13" drums with vacuum booster 8600lbs rating

 

[Totty101]

oh and it's gas

 

[davester]

You might consider comparing the old and new boosters.  I know for GMT800 trucks, different models use booster's where the shaft going into the master cylinder have different lengths.  Maybe something similar is happening for you?

[muddkatt]

Since you don't have the tag, send your vin number on a PM to forum member "newdude". He will be able to tell you what brake system you have if they still have records that old.

 

http://www.gm-trucks.com/forums/profile/41978-newdude/

[Totty101]

Davester-

Yes, the push rod from booster to master cylinder is a little longer than the replacement one.  Only problem with that is I already tried swapping them out, same results.  I went as far today as to replace the m/c to see if that was the problem.  Same results there too.  Auto parts guy told me today that the proportioning valve might be the issue but if that was the case why will my old booster and m/c work fine minus the power and why does the new set up not work?

 

I'm going to bench bleed the new m/c, pull the booster back out, adjust the pedal to booster push rod, put it all back together and then completely re-bleed the system.  Boy I hope that works, I'm out of options.

 

Anyboday have any experience with those proportioning valves?  I called all the local parts stores, looked online and even called one of our local GM dealers but can't seem to find one for my truck.

 

1984 GMC C2500 High Sierra

V8 350 5.7L 2WD 3/4 ton

13" rear drum/front disc

vacuum booster

GVWR 8600

believe it's a JB7 braking system