Which brake booster?

[gunrac]

I have an 85' 3/4 ton 4x4 gas 5.7.......GVWR range is 6,100 - 7,000
Parts store is showing to different units it could have been used that year.
I swapped booster out a year ago w/an 86' 2 wheel drive 1/2 ton. It didn't do any better. It is still bolted up. The $#@% is, I tossed the original, so nothing to compair with the new one. Both the new units looked the same to me on the outside, I don't know, they didn't seem to know either.
Since then I have replaced pads an shoes, new calipers, wheel cylinders and master cylinder.
Parts store wants to know whether it is a J-6 or a J-7......said the numbers should be on glove box stamp......mine does not. They said they would run VIN # at the GM dealer tomorrow. After checking out my #'s on 73-87chevytrucks.com.......I still can't come up w/any J#'s they want......any suggestions?
THX, Ron

[Irish_Alley]

if you have a plastic master cylinder you have the jb6 if its the metal one its the jb7. whats the problem? could your master cylinder be bad?

[Irish_Alley]

this might also help
http://forum.73-87chevytrucks.com/smforum/index.php?topic=28592.0

[bd]

What problem are you trying to solve?  What drove you to replace all of those parts?

[gunrac]

LOL......I'm not sure wat the problem is. The original Master Cylinder, did have the steel lid. Now the 86' swap had the plastic top, but since then I put the oridginal cyl.. back on w/o changing out booster it came with..........I did just change out master cyl. w/a steel cap, but not the booster.......hoping this will be the last fix to get proper brakes........You make it sound to simple........will pass this info onto the store an get the correct 1.......THX Ron

[gunrac]

What problem are you trying to solve?  What drove you to replace all of those parts?

I've always had brake issues w/it. With all that has been done, an yes, it needed it..........I will often get a had peddle w/little brake. Seems to be worse when I am towing my wood chipper w/it.....

[Irish_Alley]

a little more info if you click on that link about the master cylinders
JB5 BRAKE, POWER, DISC/DRUM, 6400 LBS
JB6 BRAKE, POWER, DISC, DRUM 7200 LBS

[bd]

...hoping this will be the last fix to get proper brakes....

Thanks Ron.  So, the vehicle won't stop, or it travels too far before stopping.  Does the brake warning lamp in the dash illuminate when you first turn on the ignition or with the park brake set?

I've always had brake issues w/it.  ...I will often get a hard peddle w/little brake....

Have you replaced the vacuum booster supply hose and/or verified a strong vacuum signal to the booster?  Booster hose is a special, thick-wall, 11/32" hose that is DOT approved specifically for vacuum booster service.

[gunrac]

I will go ahead an replace hose, but at any rate, i still have the wrong booster. I will replace it also.........Thx for the help. I'll let you guys know the outcome.
Ron

[bd]

That isn't quite where I was going, Ron.  Check the vacuum booster hose to make sure it isn't restricted with carbon buildup or collapsing under vacuum.  Does the Brake warning lamp work and is the tan wire connected to the combination valve at the front crossmember?  If you are set on replacing the booster again, use a dual diaphragm booster.

[gunrac]

That isn't quite where I was going, Ron.  Check the vacuum booster hose to make sure it isn't restricted with carbon buildup or collapsing under vacuum.  Does the Brake warning lamp work and is the tan wire connected to the combination valve at the front crossmember?  If you are set on replacing the booster again, use a dual diaphragm booster.

I replaced the filter and check valve last week. Rubber line was not heavily gunked up. I could only blow thru 1 end of the old check valve. Thinking it may have been still good, but replaced anyways.
When running the truck, it draws very good when I plug it off w/my thumb, idol also increases a bit.  After I put check valve on, run it a few min. then shut off, pull check valve out of booster, I get a good shot of air.
The brake indicator lite does not come on w/the key on, even after applying emergency brake........I found the wire on the combination valve, it was connected. Are you thinking corrosion? Will need to get it off from the frame to pull wire. How is the wire  connected? May have room to pop off, if it is a pop off type. Didn't want to force until I was sure.
The new master cylinder was benched bleed. Followed up by bleeding all for corners......all had good flow when finished. I'm sure your thinking something....please share..........thx Ron

[bd]

IMO, pulling the vacuum hose off the booster with the engine running should create a large enough vacuum leak to nearly kill the engine.  Just make sure that neither the fitting nor hose are restricted at all.  If you need to replace the vacuum hose, be sure to specify 11/32" DOT approved booster hose.  There is an overwhelming trend by unknowledgeable countermen to substitute 3/8" fuel/vacuum/emission hose.  Don't accept the substitution.

The tan wire that connects directly to the combination valve should be a short jumper about 1' long.  The opposite end of the jumper should connect to a single-wire frame mounted junction.  Temporarily ground the tan wire to see whether the dash Brake warning lamp illuminates.  You are attempting to determine whether the combination valve possibly has tripped and locked out 1/2 of the brake system.  If the valve is tripped, but the dash light doesn't work, it could be easy to overlook.

Just trying to be sure the basics have been covered and nothing simple and innocent overlooked, such as a false assumption made that's returning to bite. 

[gunrac]

done wat u suggested, good ground , still no lite. I did tag it to positive side of battery w/a test lite.......it lit right up, telling me i did have a good ground.
As mentioned, i had no inside lite using emergency brake either.....
changed out bulb also......rear brakes are fully adjusted......there is no full lock up at anytime unless on gravel.....in reverse only, and just the front brakes......if this helps any.......Ron

[bd]

So are you telling me the Brake warning lamp in the dash simply doesn't work even with a new bulb?

It also sounds like you inserted a test light between battery positive and the combination valve with the wire running to the dash disconnected, and the test light illuminated.  If this is correct, the combination valve is tripped, locking out the rear brakes.  The valve will need to be recentered or replaced to regain brake application at all four wheels.

[gunrac]

So are you telling me the Brake warning lamp in the dash simply doesn't work even with a new bulb?

It also sounds like you inserted a test light between battery positive and the combination valve with the wire running to the dash disconnected, and the test light illuminated.  If this is correct, the combination valve is tripped, locking out the rear brakes.  The valve will need to be recentered or replaced to regain brake application at all four wheels.

this is original .....i'm sure........hard part will b gettin lines off is my best guess......is a new outfit good to go.......or wat do i need to do?.......all new to me........but i'm old enough...should i replace rear end mech. hardware.....?