What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Which 1-ton master cylinder for K5?

Mastiff

Mastiff

1/2 ton status
 Premium
Joined
Jul 21, 2002
Posts
3,263
Reaction score
265
Location
Tucson, AZ
I know some people are getting by with their original master cylinder after a 1-ton swap, but since mine is 30 years old anyway, I'd like to go ahead and swap it out. Looking at K30 cylinders for my same year, will they pretty much just bolt on? I'm thinking about this one:

RAYBESTOS Part # MC39435 {#2539435} Professional Grade; Bore Size=1 5/16"
Master Cylinder Stamped 1925; 13" X 3-1/2" Rear Drum Brakes

Professional Grade; Bore Size=1 5/16"
Raybestos hydraulic brake master cylinders use aluminum and iron castings for an exact match to the design of your vehicle's original equipment. Technically advanced components ensure safe and reliable braking.

Feature: Castings match Original Equipment Design; Aluminum per O.E.
Benefit: Reduced weight does not stress the brake booster mounting

Feature: Like metals used
Benefit: Prevents chemical reaction that leads to corrosion and early failure

Feature: Ultra-smooth cylinder bore finish
Benefit: Longer service life
Material Type: Cast Iron
Number of Outlets: 2
Primary Outlet Size: 9/16"x18
Secondary Outlet Size: 1/2"x20
Click to expand...

Is this correct?
 
what I did on my old truck .

85k10 with 1tons and disk rear swap .

swaped in 85 k30 hydroboost unit with stock master . then used a inline adjustable proportioning valve and got rid of the stock valve.

few test drives it stoped great and was a huge difference over reg setup .

don't forget the brake pedal is different pin hole for hydrobost .
 
That sounds like the slick setup, but not sure I want to bite off that project right now. Any idea if I can plug in the 1 ton MC to my existing vacuum setup?
 
Square body 1 tons will have hydro boost so the m/c is different.

If you have rear disc brakes - I'm using a vacuum m/c from a '99 C3500 (4 wheel disc application) on my K5.

(Edit: made my posted more understandable :doah: )
 
Last edited:
nvrenuf said:
Square body 1 tons will have hydro boost so the m/c is different.

If you have rear disc brakes a vacuum m/c from a '99 C3500 (4 wheel disc application) on my K5.
Click to expand...

Is that right? Every 80's K30 has hydroboost? I always thought it was a diesel thing. I was also under the impression that some people swapped in 1-ton MCs with their 1-ton swaps, but not necessarily a whole hydroboost setup.

I still have drums BTW.
 
I'm pretty sure most if not all 1 tons before '87 had hydroboost--I remember even a C-30 we had at the junkyard with a 292 six had hydroboost brakes..

I think there are some P-30 step vans that had 4 wheel discs factory,and that someone here used the master cylinder off one for their 4 wheel disc brake conversion..
 
I'm keeping the rear drums for now.

Looking over Rock Auto, it does look like there are no K30 vacuum brake boosters available. I'm looking at 1983, the year of my K5.

So, is there any "easy" option for this I wonder? And what are people talking about when they say they swapped in a 1-ton MC? I saw the thread about the disc/disc version but I don't think that applies since I have drums. :dunno:
 
Afaik, any square body with a 6.2 (1 tons included) got hydroboost. Without a doubt all '81+ square body 1 tons were also hydroboost and I assume this to be true of all '73-'80 30 series trucks.
 
If you're keeping the rear drums just get a 3/4 ton master cylinder, it'll have a bigger reservoir than a 1/2 ton.
 
nvrenuf said:
If you're keeping the rear drums just get a 3/4 ton master cylinder, it'll have a bigger reservoir than a 1/2 ton.
Click to expand...

As I understand it, the issue is pushing enough fluid for the D60. Did K20's have D60's without hydroboost?

Another question, possibly stupid, is whether it matters to the master cylinder whether it's vacuum or hydro? Maybe I can just plug on the 1-ton MC to my vacuum booster.
 
No K20's w/ 60's.

I don't know for sure but I'd bet that the pin depth on the back of the master cylinder is different between vac & hydro.
 
I'm not saying it's right but...

On my old '80 K10 I had a typical 60 / 14 bolt (rear disc) and used an '80 K20 master cylinder and it worked great. This was a disc/drum m/c but it moved plenty of fluid for my 60 so I'd assume (careful :doah: ) any 3/4 ton m/c would work for a 60.
 
nvrenuf said:
No K20's w/ 60's.

I don't know for sure but I'd bet that the pin depth on the back of the master cylinder is different between vac & hydro.
Click to expand...

Not on my 82 K2500 diesel with hydroboost,,I put a master on it from a 77 GMC with a gas engine/vacuum booster and its the same depth pin...works fine...but I'd still check to be sure,they may not all be like mine!..
 
watch out as some bigger masters have a bigger rear body were it meats up to the booster and wont fit the smaller stuff.

prob best to buy or at least look at these masters at local parts store so you arn't mail mail ordering a lot .
 
paratrooper307 said:
After my swap I put on a 3/4 master for the JB7 brakes. Works great. I still have drums in the back.
Click to expand...

Good info. Did you have issues with the original MC?

Would a K20 have significantly bigger front brakes than a K5? I was thinking the 3/4 tons just had D44 fronts most of the time, but maybe the brakes are different?
 
I have multiple vehicles with Dana 60's and stock half ton master cylinders. No brake problems at all.

Martin
 
Mastiff said:
Good info. Did you have issues with the original MC?

Would a K20 have significantly bigger front brakes than a K5? I was thinking the 3/4 tons just had D44 fronts most of the time, but maybe the brakes are different?
Click to expand...

I didn't have any issues, but the JB7 brakes from the K20 axles I put on were significantly larger than the K5 brakes, I figured it was just cheap insurance, since I had the largest brakes available for the axle configuration I have.
 
The main difference on the 3/4 tons was the rear wheel cylinder piston sizes were larger on the ones with 13" drums,the 11" ones were the same as the 1/2 tons ...they used a master with a larger bore for the rear brake portion to mate up with the larger wheel cylinders--but I have seen many trucks use the 1/2 ton master with 3/4 or 1 ton axles with the stock drums and not notice any real difference in street driving...

I may be wrong but I think drum brakes have a check valve in the master to retain a bit of line pressure so there wont be a lag when applying the brakes,and the discs are plumbed direct to the master with no check valve..
But it may not amount to much--my brother has a 66 Hillman that has a 2.5 AMC engine,a 60's dodge dart rear axle with drums and he converted the front brakes to discs,but still uses the stock mopar drum brake master cylinder with no issues with dragging ,etc..
 
You must log in or register to reply here.

Latest Posts

Top Bottom