doonjumper
1/2 ton status
I forgot to mention single diaphragm or double. Seems to me that a double dphm would give a smoother "push" with larger brake system than a single would. Just my .02.......could be misinterpreting the basics of the system. 
CK5
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4wheel discs, FOUND PERFECT MASTER CYLINDER!!
- Thread starter muddybuddy
- Start date
your 1/2-1" shorter means like 3" at the pedal, that's a complete "pulled out of the ass guess" but there is some ratio calculating required between that rod and the distance it takes a 12" piece of steel (pedal) to push it.
doonjumper
1/2 ton status
Well I see some modification in my future 
doonjumper
1/2 ton status
Ok so I picked up the new booster from Napa part # 54-71069. Also need to pick up 2 3/8 metric nuts to secure the MC to the Booster. As you can see the new stock 84' Booster rod is about 1" shorter than the 73'-75' stock booster. 
The threads are also different.
The threads are also different.
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doonjumper
1/2 ton status
I decided to make a new rod using parts I already had on hand 
I welded a support over the joint to be safe as Brakes are kinda important
I welded a support over the joint to be safe as Brakes are kinda important
doonjumper
1/2 ton status
Final product took me bout three hours counting driving time. I probably coulda found one, but it was fun making it 
cool, nice mod, whatever gets it done and is safe 
74kay5
1/2 ton status
going to give the raybestos brand version master cylinder MC390278 a try and see if it works. My MC is leaking, so I figured I would try and upgrade. These are super cheap on ebay right now. I ordered it for 45 shipped.
chevysmithz71
1/2 ton status
Well I see some modification in my future
Way back to top on this one. Wanted to know how yours is doing jumper? I read the whole thread and everyone seems to own an 80's truck and this mod is bolt on for them. Now, for the 70's guys anything new? What you guys using?
chevysmithz71
1/2 ton status
Anything?
I tried to do this mod on my last truck ('83 K20 and 14FF w/disc brake conversion). The master cylinder I purchased did not work on hydroboost brakes. So, make sure you specify hydroboost when buying the new one if you need it.
This thread is about a 4 wheel disc master cylinder for vacuum boost applications. Folks using the '99 C3500 master cylinder should NOT specify hydroboost.
Unless you don't have a vacuum boost application like a diesel truck...
What I am saying is there is a difference between the parts for vacuum and hydro boost applications. Don't blindly buy the part numbers listed in this thread.
I believe the different size mounting hole break is between 80/81. So you'd want an 81 or later for the larger diameter mounting hole. It looks like in the compatibility charts, all of the boosters from 81-86 are the same, then went to burbs and k5's up till the 91. 87+ pu's have different mounts to the firewall, but the same mc bore mount.
This may not be 100% but I'm pretty sure it's correct.
This may not be 100% but I'm pretty sure it's correct.
not sure what I am missing here. I have four wheel disc and tons along with 38's. My brakes feel great with the factory master and brake booster. I have the vacuum connected and the other day had a MAJOR panic stop (was about to destroy a minivan that decided to stop in the middle of the road) and I was able to lock up all four wheels pretty successfully.
I put a d60 on w/ jb8 brakes. My factory mc worked, pedal gets hard in the last inch or so of travel. Not enough volume for the bigger brakes. Putting AAM 10.5 on back wont do it any favors either.
Are you running the jb7 or jb8 fronts? And what rears?
Are you running the jb7 or jb8 fronts? And what rears?
I have no idea. I replaced the fronts with K30 calipers and the rears are 3/4 ton.
But your story and many others is the reason this mod keeps getting done. There is no support from the people who sell the kits, which is frustrating. I tell everyone thinking about doing this DONT DO IT, save the money for alcohol or different parts cause the failure stories are also numerous. Not worth the apparent "works/doesn't work" coin flip.
consider yourself lucky
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The long pedal gets you into trouble, the brake pedal gets you out. And the stakes are high when that brake pedal doesn't work.
