I did a 3/4 tons axle(10 bolt 8 lug and 9.5 14 bolt)swap on my 85 K10.  I couldn't get any brakes peddle went to floor. Replace rear cylinders, and new calipers on front.  All lines are tight no leaks or breaks. I can get some break if rolling and pump fast and hold to floor but not much. I did do a little reading, is it true that I need to upgrade the master cylinder, with the 3/4 ton axles? And if so what can it come out of? Please let me know any suggestions!
Thanks!  

7purdy:
I did a 3/4 tons axle(10 bolt 8 lug and 9.5 14 bolt)swap on my 85 K10.  I couldn't get any brakes peddle went to floor. Replace rear cylinders, and new calipers on front.  All lines are tight no leaks or breaks. I can get some break if rolling and pump fast and hold to floor but not much. I did do a little reading, is it true that I need to upgrade the master cylinder, with the 3/4 ton axles? And if so what can it come out of? Please let me know any suggestions!
Thanks!

Yes, the bigger brakes in rear have a bigger wheel cylinder which displaces more fluid. You need a master cylinder that matches this. Sorry to see that you wasted time and money of calipers and wheel cylinders when all you needed was a new master cylinder of proper size. Find one out of a application that has that size rear brakes.  

theres no reason the master cylinder won't work on the 3/4t axles, the wheel cylinders aren't that much different in size and the calipers are the same unless theyr'e off of a 8600gvw diesel.

sounds to me like they only need ot be bled or the porportion vavle reset.

the mc won't pump up air and make the brakes work, if it's pumping up theres fluid there.  

mudb8-.:
theres no reason the master cylinder won't work on the 3/4t axles, the wheel cylinders aren't that much different in size and the calipers are the same unless theyr'e off of a 8600gvw diesel.

sounds to me like they only need ot be bled or the porportion vavle reset.

the mc won't pump up air and make the brakes work, if it's pumping up theres fluid there.

Guess you are not to well versed here. There is every reason it will not work. Bigger brakes have bigger wheel cylinders which takes more fluid to expand. Master cylinder is sized to displace "X" amount of fluid to rear brakes when you depress pedal. If this is not enough then you get into the "pump up" routine as it tries to get extra fluid beyond master cylinder displacement back there to build pressure. Sometimes you can get by if you adjust brakes up somewhat snug with drum brakes and wheel cylinders do not have to expand much but then when shoes wear a bit and travel increases you can have low or no pedal. It is really simple physics here. You need a master cylinder that displaces more fluid to rear brake when pedal is depressed.  

I'm very well versed, as I know the smaller mc works fine. yer right the larger mc moves more volume,

the brakes have to be severely out of adjustment to make a spongy pedal using a car mc on a 3/4 truck.

I've used mismatched psrts out of neccesity and know full well what works and what doesn't work. I have a mc off of a olds delta 88 on a 1 ton diesel. works perfectly. I coulndn't even count the amount of axle swap i've done either....

there is another factor causing the guy's problem besidess the stock 1/2t master cylinder.

your advise here is to spend another 80 bucks on a part that will only fix the problem if the wxisting mc is actually bad.... not cause it's the wrong part.  

Maybe you have had some luck but to suggest that you do not need to adjust master cylinder size with bigger wheel cylinders tells that you have been shoe stringing some of your mods in past. There is a correct way to do which is to adjust the master cylinder size when they is a big change in displace in rear wheel cylinders or try to use old one and hope for the best. You seem to promote the later. This is not the best advise and I am kinda surprised to hear it from you too as I thought you were more on top of this. Also some 9.5's have bigger brakes than others and with them even bigger wheel cylinders. Again, do it right or wing it, the choice is yours.  

I was once young and had to make do with what whas at hand....

theres no winging involved there, point is the stock cylinder works without issues.

the early 9.5 have 1.5x 13" and after 84 they are 2.5x12"
same cylinders and are only slightly larger in diameter, the 10 bolt half ton cylinders also work in those axles along with the brakes and drums from a 10 bolt.  

Once again, bigger wheel cylinder takes more fluid and master cylinder is fixed in displacement. I guess Detroit has it all wrong and they could have saved maybe 100's of millions of dollars the last 30 or 40 years using one master cylinder for all apps. There are many different master cylinders for this very reason. Generally I have a lot of respect for your advise and rarely challenge it but you are wrong on this one.  

I'm not advising to use the 1/2t master cylinder, point is it will work. changing it at this point won't solve the problem.  

okay ive got a question how come when i swapped my axles in, 8 lug d44 and 10.5 14 bolt full floater, my 1/2 ton blazer why didn't i have any brake problems without changing master cylinder?, after proper bleeding the truck stops better than before im guessing becuase of the bigger rear brakes but the only problem i have is the rear tires will lock up if i stomp on the brake pedal suddenly  

I is possible that the shoes were adjusted up a lot tighter on that axle to begin with so wheel cylinders did not have to displace much fluid to work. Unlike disc brakes which float pads just above surface and take little additional fluid (for their displacement) to engage,  a drum brake retracts shoes when not braking and how much fluid is required to engage them depends on the static level that the shoes are adjusted too which relates to how much fluid is required to expand wheel cylinders to the point that the brakes work.  

I am going to agree wit mud, although a bigger master cylinder is better a smaller one will work. When i swaped to 3/4 ton running gear in my stepside i blrd the brakes several times and now the truck stop pretty quickly for as big as it is...(8" lift, 38" tsl's) thats alot of rolling mass and it comes to a stop quick without the pedal moving more than an inch or so. I would say you might want to ajust and check your porportioning valve.  

Tyler__820:
I am going to agree wit mud, although a bigger master cylinder is better a smaller one will work. When i swaped to 3/4 ton running gear in my stepside i blrd the brakes several times and now the truck stop pretty quickly for as big as it is...(8" lift, 38" tsl's) thats alot of rolling mass and it comes to a stop quick without the pedal moving more than an inch or so. I would say you might want to ajust and check your porportioning valve.

Like I said, there is a right way and a wrong way and this is the wrong way here. Sure sometimes you can make it work but myself brakes are far too important to try to cut corners with.  

Great advise guys. I want to thank you all.  I have 39.5 tsl's so although I probably could use the orginal MS. I will probably go ahead and get the larger MS. I only replaced the the wheel cylinders and caliper because the ones on the axles were not operational...and now I know I got new stuff that works...I have not cut any corners on this project, and not saying using the orginal MS would be cutting but I would rather be safe than sorry.
Thanks all!  

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